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Reflectance measurements of aluminium surfaces using integrating spheres
Optics and Lasers in Engineering 32 (2000) 419}435
Re#ectance measurements of aluminium surfaces using integrating spheres I. Lindseth *, A. Bardal, R. Spooren Department of Physics, Norwegian University of Science and Technology, 7491 Trondheim, Norway SINTEF Materials Technology, 7465 Trondheim, Norway Received 16 November 1999; accepted 11 January 2000
Abstract In the investigation of the considerable absorption of visible light in industrially rolled aluminium surfaces, a thorough knowledge of the total re#ectance measurement method is required. In this paper a general introduction to the integrating sphere method is given, with emphasis on the current understanding of instrumental artefacts and ways of correcting them. Selected aluminium surfaces were measured employing two spheres; a single-beam instrument equipped with a white-light source and a Si-photoelement detector, and a double-beam sphere, which measures re#ectance properties with spectral resolution. It was found necessary to take precautions concerning the orientation of rolled samples relative instrument geometry, to avoid arti"cial losses from the sphere. The use of a specular reference standard is assumed to minimise the e!ect of several sphere artefacts, since it produces similar angular distribution of re#ected light as the rather glossy aluminium samples. Measurements with spectral resolution show that the total re#ectance of aluminium is somewhat red shifted after rolling, a tendency that cannot be revealed in ordinary white-light measurements. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Aluminium; Integrating spheres; Optical re#ectance; Rolling
1. Introduction Aluminium does in general have a very high optical re#ectivity. A `cleana surface of pure aluminium re#ects more than 90% of incident light for most visible wavelengths.
* Corresponding author.Tel.: #47-73-59-81-31; fax: #47-73-59-77-10. E-mail address: [email protected] (I. Lindseth). 0143-8166/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 0 ) 0 0 0 1 0 - 5
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This makes aluminium suitable to use in many products requiring high total (hemispherical) re#ectance for visible light. By total re#ectance we mean the sum of specularly re#ected and di!usely scattered light. One group of products demanding high total re#ectance is light re#ectors, which are often produced from industrially rolled aluminium materials. Measurements show, however, that the total re#ectance of aluminium decreases in the rolling process [1]. Some rolled materials exhibit absorption of more than 15%, even after thorough degreasing to remove lubricants from the surface. In the investigation of this phenomenon a thorough understanding of the total re#ectance measurement method is essential. It is common in the lighting industry to use commercial integrating spheres for determining the total re#ectance of aluminium materials. The use of such spheres is considered to be straightforward, and the method is well standardised. Our experience is, however, that in order to avoid incorrect measurements from aluminium materials, certain precautions may be necessary to take, and a basic knowledge of the measurement method is essential. This paper gives an overview of the integrating sphere method, particularly with respect to total re#ectance measurements of aluminium surfaces. A general introduction to the method is given. Based on literature, the current understanding of sources of error and ways of correcting them are outlined. Experimental results from total re#ectance measurements of aluminium materials are presented and discussed in light of this. The measurements have been made employing two di!erent integrating spheres; one measures total re#ectance of white light, whereas the other measures with spectral resolution. Results from the two spheres are compared and discussed with respect to the details of the instruments and their limitations and artefacts. Measurements with spectral resolution revealed tendencies in the re#ectance properties of rolled surfaces that cannot be revealed in ordinary white-light measurements. All the presented results are measurements from aluminium materials of di!erent surface "nish, and restricted to the wavelength range of visible light. Even though this investigation is limited with respect to choice of integrating spheres, samples, and wavelengths, the observations and conclusions most probably apply to other measurement geometries and samples, as well as to a wider spectral range. The outline and observations presented in this paper emphasise that the measured reduction in total re#ectance of aluminium upon rolling by no means can be related to the measurement method and companion artefacts. The phenomenon will therefore be discussed in relation to material structure and topography elsewhere [2,3].
2. Methods 2.1. Introduction to the integrating sphere method Integrating spheres, also known as Ulbricht's spheres, have the ability of collecting and spatially integrating optical radiation, and this makes them useful for a wide range of applications [4,5]. The luminous #ux originating from lamps and lasers can be measured. An integrating sphere can also serve as large area source of uniform,
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di!use radiation. Their most important application, however, is in re#ectance and transmittance measurements of di!erent materials. An integrating sphere is a closed sphere with a few port openings, such as for the incoming light, the sample, and the detector. The inside of the sphere has a coating, which ideally should re#ect 100% of the incident light, and be uniform and ideally di!use. (A Lambertian or ideal di!user is one whose radiance is independent of the angle of view under any condition of illumination.) The total port area should in general be as low as possible, and as a rule of thumb constitute less than 5% of the total sphere surface. When doing measurements, a radiation balance should be established inside the sphere after as few internal re#ections as possible. The amount of light hitting the detector then corresponds to the average light intensity inside the sphere. However, light re#ected from the sample should not pass directly to the detector. This is because the re#ectance of the wall coating in real spheres is less than 100%, and radiation is therefore attenuated whenever undergoing multiple re#ections from the walls. Radiation direct from the sample is not attenuated in this way and becomes overweighed with respect to the rest of the re#ected light. This error can be avoided by mounting ba%es in the sphere wall to block the detector's view of incident #ux coming directly from the sample. The ba%es should be coated with the same material as the sphere wall. There are di!erent sphere geometries available. In a substitution sphere there is only one port for the sample and the reference standard, and the two surfaces have to be measured subsequently. When changing from the sample to the standard, the average re#ectance of the sphere is altered. This makes it hard to determine the sample re#ectance with high accuracy. To avoid this problem comparison spheres have been designed. They have two ports for sample and reference standard, so that the average sphere re#ectance is kept constant. There are two types of comparison spheres. In single-beam instruments the position of the sample and the reference standard is alternated, so that each of them is placed in the incoming beam subsequently. In double-beam instruments there are two equivalent optical paths, one for the sample and one for the reference standard. These two paths are then usually chopped alternately and synchronised with the detector system. Schematic illustrations of a single-beam sphere and a double-beam sphere are shown in Figs. 1 and 2, respectively. Many integrating spheres are equipped with some sort of standardised white-light source, which may be similar to daylight. They may also be equipped with detectors that are modulated to replicate the eye's light sensitivity (photopic vision). In this way the choice of light source and detector re#ects our interest in a certain type of optical re#ectance, strongly connected to the visual appearance of the surface, and to the functional properties that are essential for a speci"c application. Total re#ectance measurements are often made in accordance with some sort of standardisation. The lighting industry often refers to the German standard DIN 5036 part 3 [6], which treats di!erent methods for measuring hemispherical parameters like di!use and total re#ectance and transmittance. The DIN standard gives a rather detailed speci"cation regarding the design of integrating spheres, the choice of light source and detector, and how to make re#ectance measurements. DIN 5036 integrating spheres contain "ve ports in addition to a light source and a detector. The
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Fig. 1. Schematic cross-sections of a single-beam sphere following the standard DIN 5036, part 3. A horizontal cross-section is shown in (a), and a vertical cross-section in (b). Ports 1 and 2 are sample and reference standard ports. The light enters the sphere through port 3. Port 5 is the specular exit port, which should be open when measuring di!use re#ectance, and closed when measuring total re#ectance. Port 4 is used only when measuring transmission properties. The position of port 2 may depart from what is shown in the "gure.
Fig. 2. Schematic illustration of the Beckman double-beam sphere geometry. Ports 1 and 2 are reference and sample beam entrance ports. Port 3 is for the sample, and 4 is for the reference standard. Port 5 is the specular exit port, whereas 6 and 7 are the PbS and the PM detector, respectively. When measuring di!use re#ectance, port 5 has to be covered with a beam dump. For total re#ectance measurements, it must be closed with a BaSO reference plate.
geometry of the sphere is shown in Fig. 1. The light source is a standardised "lament lamp (type A) giving a relatively white light, and the light's angle of incidence on the sample is 83. The detector is usually a spectrophotometer or a Si-photoelement. In the latter case the detector sensitivity is adjusted to spectrally simulate the light sensitivity of the eye. The detector is positioned behind a di!using screen in the sphere wall, shown in the vertical cross section of the sphere in Fig. 1b. The coating of the inside of
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the sphere should ideally re#ect 100% of the incident light, but a total re#ectance as low as 80% is satisfactory according to the standard. The sphere inside should be perfectly di!use and wavelength non-selective. For further details we refer to the standard itself [6]. ISO 6719 [7] standardises the use of integrating spheres for measurements of re#ectance characteristics of aluminium surfaces. The main contents are the same as for the DIN-standard, but the ISO standard is not as detailed and covers a wider range of integrating sphere geometries. When it comes to calibration standards, ISO 6719 speci"es three reference samples. A certain black surface should be used for the stray-light measurements or dark current compensation. In addition to a white di!use calibration standard, a specular surface should be used as working-standard. According to the ISO standard, each sample should be measured in three di!erent orientations. The exact orientation is given by the angle between the machining direction of the sample and the optical plane of the instrument, and the three orientations are 0, 45 and 903. Three re#ectance measurements in each orientation should be made and an average value obtained. We argue later that this way of averaging measurements from di!erent orientations not necessarily gives the most correct re#ectance values for directional samples. 2.2. Integrating sphere theory, known artefacts, and ways of correction The "rst theories of integrating spheres considered the case of a perfect sphere assuming the apertures to be too small to have any signi"cant e!ect on the results. The reference standard, the sample and the sphere inside were considered to be perfectly di!use. These assumptions are in general not ful"lled in real integrating spheres, and several authors have estimated the e!ect of sphere imperfections. Jacquez and Kuppenheim made calculations both for a perfect sphere, and including e!ects like #at sample and standard, and a specular instead of a di!use sample surface [8]. The authors considered both substitution and comparison types of spheres. One of their conclusions was that errors related to the substitution method are larger than for the comparison method. Goebel developed a general equation for the e$ciency of an integrating sphere with a non-uniform coating [9]. Finkel made a simpli"ed approach to the integrating sphere theory, and thereby found expressions for the e!ect of angular dependence in the detector, port losses, and sample #atness [10]. Clarke and Compton have given a full review of possible errors in re#ectance measurements caused by the integrating sphere itself, and how to correct them [11]. They divide the errors in several groups, and those of greatest relevance to our measurement systems and samples are summarised below. The general integrating error is a term collecting all e!ects departing the sphere from ideal conditions, e.g. non-uniform coatings, losses through ports, internal screening, ageing e!ects, and deposition of dirt. If radiation passes directly from the sample to the detector without any re#ections from the sphere wall, we get a diwuse reyectance-screening error. In most modern spheres this is avoided by internal screening. A similar source of error exists only for rather glossy samples. Since they create a strong specular (or regular) component of re#ected light, the detector should preferably be screened from view of
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where the patch of this radiation falls. Without such screening, some radiation from the specularly component may pass directly onto the detector without being attenuated by multiple re#ections. The specular part is then overweighed with respect to the di!use component, giving the regular-reyectance screening error. Most spheres lack adequate screening for glossy samples. When measuring total re#ectance, the specular component of light usually falls onto a white cap covering the exit port. It is important that this cap has a re#ectance equal to the average sphere wall re#ectance, but this is di$cult to achieve. Re#ectance deviations of the cap may cause a regular reyectance-coating error. Clarke and Compton state that any of these or the rest of the groups of errors could be the largest experienced in a particular instrument, and in some cases the errors may compound additively. However, they have shown how each of them to a greater or lesser extent can be determined and compensated for. When it comes to errors related to the specularly re#ected component (regularre#ectance coating-uniformity error and regular-re#ectance screening error), they can be determined by measuring a mirror with known re#ectance relative an ordinary di!use reference standard. Clarke and Compton show examples of errors of 7}10% for Al and Cr mirrors measured relative a matt standard in an ordinary sphere. According to them, the majority of the error is due to lack of screening between the specularly re#ected-component patch and the detector port. Such e!ects make it necessary to derive corrections for measurements of glossy samples, as long as a matt reference standard is used. The larger the specular component of re#ected light from the sample, the stronger the correction. However, valid results can be obtained for mirror samples by replacing the di!use reference standard with a specular one. The errors are then of the same proportion for both sample and standard, and thereby cancel out. In 1988 Roos, Ribbing and Bergkvist reported how experiments of structured samples showed a relationship between directional properties of the re#ectance and port imperfections in the integrating sphere [12]. A steel sample polished in only one direction was rotated stepwise in the sample port, and measurements were made at each orientation. The re#ectance varied signi"cantly as function of orientation. This could only be explained by considering the strong directionality in the sample surface, causing the light to be scattered in a long, disk-shaped pattern perpendicular to the polishing grooves. For a certain range of orientations this disk-shaped pattern fell onto the entrance port, causing a signi"cantly reduced re#ectance. Another artefact occurred for a small range of orientations as light directly from the sample fell on the inside of the detector collar, causing an erroneously high re#ectance value. Such anomalies should be taken into consideration whenever measuring samples with strong directionality in the surface (e.g. rolled samples). Roos and Ribbing published also in 1988 a paper on interpretation of integrating sphere output for samples that are not ideally di!use [13]. They show through calculations and experiments that when estimating correction factors to integrating sphere results, the common approach of dividing the total re#ectance into a specular and a di!use component is su$cient only in certain cases. Generally the di!use component must be further divided into a fully Lambertian and a more specularly
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di!use component. In 1994 RoK nnow and Roos extended the model to take stray light into consideration [14]. Hanssen has studied the e!ects of non-Lambertian surface coatings on integrating sphere measurements, using Monte Carlo simulations [15]. The importance of the ideal di!use character of the sphere wall was a$rmed, and errors due to non-ideal coatings have to be taken into account in high accuracy measurements. A high total re#ectance of the wall coating reduces the e!ect of non-Lambertian characteristics. Incorrect total re#ectance values from integrating spheres are most probably related to the errors outlined above. They all have in common that they are caused mainly by the integrating sphere itself or the choice of reference standard. We cannot exclude the possibility of other sources of error, such as the illumination system or the detector. However, there are few publications reporting of such errors, probably because they tend to be more casual and rare. 2.3. The instruments 2.3.1. Single-beam integrating sphere with Si-photoelement detector The employed single-beam integrating sphere equipped with a Si-photoelement detector has been produced by LMT Lichtmesstechnik Berlin, and it follows the standard DIN 5036 part 3 described in Section 2.1. The sphere diameter is 50 cm. Barium sulphate (BaSO ) covers the inside of the sphere, giving it the wanted optical properties; a total re#ectance of approximately 80% independent of the wavelength of the incident light, and a strong di!use re#ectance. A halogen lamp is used as light source (Osram 64410, 6 V, 10 W). The detector is a light-sensitive Si-photoelement positioned behind a di!using screen, which follows the sphere wall. The accuracy of the instrument is $1%. The uncertainty appears to be mainly caused by a 1% uncertainty in the calibration values of the reference standards. This should not a!ect relative di!erences between measurements made with the same standard. The precision is therefore thought to be much better than the accuracy of the measurements. The single-beam instrument can be used as a substitution sphere if a white cap is applied in the reference standard port. Experience from measuring di!erent aluminium surfaces showed that approximately the same results were obtained when the sphere was used in the substitution mode as in the comparison mode, which it is built for. This is because the Al samples and the reference standard in general have similar total re#ectance. After considering our demands for accuracy in the measurements, we chose to use the instrument as a substitution sphere. 2.3.2. Beckman double-beam integrating sphere with spectrophotometer The Beckman 198851 double-beam integrating sphere has a slightly di!erent geometry than the former sphere presented, but the same parameters can be measured using this sphere [12}14,16]. It contains seven ports, which are shown in Fig. 2. The Beckman 5240 spectrophotometer measures intensity in the wavelength range 300}2500 nm. The large wavelength range necessitates the use of two detectors, one PbS and one PM (6 and 7 in Fig. 2). They are both shielded with barium-sulphatecoated collars to avoid incident light directly from the sample.
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2.3.3. Colour measurement sphere The two measurement systems described above have the same small angle of incidence and di!use detection. Another measurement geometry was used in addition to them for comparison; a commercial colour measurement sphere called ACS Chroma Sensor CS-3. It has di!use illumination and detects re#ected light with spectral resolution at 83. From the amount of light detected in this direction, it calculates an overall `total re#ectancea. Such data have been compared to results from the double-beam sphere. The colour measurement sphere is not suitable for total re#ectance measurements in general, and a detailed description of it is therefore not included here.
3. Experimental results and discussion 3.1. Ewect of sample orientation relative surface directionality The topography of rolled aluminium materials generally consists of ridges following the rolling direction, making the surface very directional. Fig. 3 shows a scanning electron microscopy (SEM) micrograph from a laboratory-rolled material containing such a strong surface directionality. Single-beam total re#ectance measurements of rolled aluminium surfaces revealed that the results depend on the sample orientation relative to the sphere geometry. To illustrate this tendency, the measured total re#ectance of a series of etched and
Fig. 3. SEM micrograph of a laboratory-rolled aluminium material, illustrating the directionality of rolled surfaces.
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Fig. 4. Total re#ectance of etched and laboratory-rolled aluminium samples as measured using the single-beam sphere (white light). The measurements were made with either the rolling direction (RD) or the transverse direction (TD) of the samples parallel to the plane of incidence. The large deviation between the corresponding results is due to erroneous radiation losses in the TD orientation.
laboratory-rolled aluminium materials (AA1050) is plotted in Fig. 4. The starting material was etched thoroughly after the previous industrial hot rolling, in order to remove surface features generated in this process. Even though the etching changed the topography substantially, it did not fully remove the directionality of the surface. The etched material was then laboratory rolled in four passes. After each pass some material was taken out for optical measurements and characterisation. The total re#ectance was measured in the single-beam sphere relative to a di!use reference standard with total re#ectance of 83.0 $1%. The samples were measured in two orientations, with the rolling direction parallel to the plane of incidence (here called RD for `rolling directiona) and with the rolling direction normal to this plane (here called TD for `transverse directiona). The results are averages of at least 3 measurements from each material. It can be seen from Fig. 4 that the total re#ectance is considerably reduced after the "rst rolling pass. It then increases to some extent in the third and the fourth pass. The RD results are signi"cantly higher than the TD measurements for all samples, especially the rolled ones, and the explanation to this deviation was found to be the same as described by Roos and Ribbing [12]. A strong directionality in the aluminium surfaces causes the light to be re#ected into a long disk-shaped pattern perpendicular to the rolling direction. When the sample has the TD orientation, a signi"cant amount of this disk falls onto the entrance port, giving losses through the port and thereby a reduced detected signal. The deviation is smaller for the etched sample than for the three rolled samples because the etching process reduced the surface directionality.
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In the RD orientation the re#ected disk falls into the sample-detector plane of the sphere. Roos and Ribbing described how this created erroneously high re#ectance values due to unsatisfactory screening in their double-beam sphere [12]. The detector in our single-beam sphere appears to be well shielded against direct radiation from the sample, and such problems are therefore avoided for all sample orientations. This was con"rmed by a simple set of angle resolved measurements. They showed that the total re#ectance was constant for all sample orientations ranging approximately from !453 to #453 around the RD orientation, whereas it decreased as the orientation approached TD. Due to radiation losses through the entrance port for the TD orientation, we consider RD measurements to give the most correct re#ectance values. In Section 2.1 we referred to the ISO standard 6719 and how it requires the resulting re#ectance values to be an average of measurements from three di!erent sample orientations. It follows from the paper of Roos and Ribbing [12] and from the results presented above that this way of averaging may lead to less correct re#ectance values than if measurements are made at only one particular orientation. This is important to be aware of whenever making measurements in accordance with the ISO standard or analysing such results. 3.2. Choice of reference standard Total re#ectance is always measured relative to a reference standard. There are di!erent types of standards with variations in re#ectance properties, specularity, and accuracy. The choice of standard may be of in#uence to the measurement results, and determines to a large extent the accuracy of the measurements. In this investigation we have employed two di!erent reference standards in the single-beam sphere. One is di!use with a total re#ectance of 83.0$1%. The other is specular and re#ects 96.1$1% of incident light. These calibration values have been obtained employing the same type of white-light source as used in our instrument. A series of nine commercial, anodised aluminium re#ector materials were measured with both standards, and the results are plotted in Fig. 5. The plot also includes the deviation between the results for each sample, which varies from 1.0 to 1.8%. A dotted line indicates the mean di!erence for the nine samples, which is 1.2$0.2%. Sample A has a very high purity ('99.9% Al). The amount of impurities and alloying elements increases towards sample H and I, which contain more than 1% alloying elements. The "gure reveals that the specular standard leads to higher re#ectance values than the di!use standard. The discrepancies are so small, however, that they are within the uncertainty limit of the measurements. As long as the uncertainty in the calibration values of the reference standards is as large as 1%, this could easily cause di!erences of the magnitude that we observe. It could also explain why the specular standard gives higher total re#ectance values than the di!use. If the di!erences were due to errors outlined in Section 2.2, there is a large probability that the di!use reference standard would have given the highest re#ectance values. Both losses through the entrance port as well as lack of su$cient screening would lead to higher values when using a di!use standard. On the contrary, if the di!erences are caused by
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Fig. 5. Comparison of the total re#ectance of nine commercial aluminium re#ector materials (A}I) as measured with two di!erent reference standards, one di!use and one specular. The di!erence between corresponding measurements is plotted in the lower graph. The specular standard results in higher total re#ectance values than the di!use standard.
uncertainty in calibration, the deviation could go in either direction depending on the two reference standards. If calibration uncertainties were the only cause of discrepancies between measurements made employing the di!use and the specular standard, we would expect the deviations to be of approximately constant size. Errors related to the sphere itself would most probably vary in size depending on the exact angular distribution of re#ected light from the sample. Even though the total re#ectance of most of the nine samples in Fig. 5 is rather constant, the angular distribution of re#ected light varies considerably. Some variation can also be seen in the deviations between results obtained with di!erent reference standards. They are, however, rather small compared to the mean di!erence, a tendency suggesting that the main contribution to the discrepancies is systematic, most probably related to the calibration of the reference standards. Most of the aluminium samples that we analyse are considered to be far from di!use. When they re#ect light, the majority of them produce a strong specular component. Clarke and Compton [11] showed by an example that measurements of mirror samples employing a di!use reference standard may lead to severe errors. The problem could, however, be avoided by choosing a specular standard when measuring mirror samples. With a specular standard, the sample and the standard produce the same type of errors, which thereby cancel out. Based on this argumentation, the specular standard should preferably be used for our aluminium samples in order to reduce the e!ect of possible artefacts in the sphere. Whenever single-beam measurements have been made only using the di!use standard, a correction of 1.1 has been added to the data, in order to make them comparable to measurements made with the
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specular standard. This "gure is the mean di!erence in the measurements of a large number of aluminium materials. 3.3. Comparison of results from single- and double-beam integrating spheres In order to study how the total re#ectance varies as function of wavelength, a double-beam-integrating sphere equipped with a spectrophotometer was employed. The samples from the laboratory rolling series together with some industrially rolled and etched samples were measured. Both the total and the di!use re#ectance were determined with spectral resolution. A di!use reference standard was employed in the measurements, and rough corrections of the results were made afterwards according to the method developed by RoK nnow and Roos [16]. Parameters necessary for the calculation of correction factors were selected based on experience from similar measurements. Whereas the single-beam results were averages of several measurements from each material, the double-beam results are obtained from only one measurement. A simple comparison was made between the spectrally resolved measurements from the double-beam sphere and the white-light measurements from the single-beam sphere The eye and the detector of the single-beam sphere have a sensitivity maximum around j"550 nm. Double-beam sphere results for this wavelength were therefore compared to data from the single-beam sphere. Table 1 contains measurement data for the etched and laboratory-rolled samples and for two industrially cold rolled materials that have been etched gradually in phosphoric acid (85 wt% H PO ). The purity of the three alloys varied from 99.7% for the AA1070 alloy to 99.2% for the AA1200 alloy. All materials except one exhibit highest re#ectance values in the single-beam measurements, indicating that this is a systematic tendency. Nevertheless, Table 1 Total re#ectance of laboratory-rolled samples and two industrially rolled materials before and after etching, as measured with the two integrating spheres Material
Total re#.
Di!erence
Single-beam sphere White light (%)
Double-beam sphere j"550 nm (%)
(%)
AA1050 hot rolled and etched AA1050 lab-rolled 1 pass AA1050 lab-rolled 4 passes
90.0 78.4 82.5
83.4 78.7 81.7
6.6 !0.3 0.8
AA1070 as received AA1070 etched 30 s (0.97 lm) AA1070 etched 300 s (6.80 lm)
81.3 90.3 89.8
77.4 89.3 87.1
3.9 1.0 2.7
AA1200 as received AA1200 etched 30 s (1.04 lm) AA1200 etched 300 s (7.24 lm)
78.0 89.9 90.0
77.6 88.6 82.7
0.4 1.3 7.3
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the di!erences in the obtained results show large variations from one sample to another. Many of the di!erences are too large to be within the uncertainty limit of the measurements, and other explanations must be sought. Most probably comparison of measurements made using white light with results for one single wavelength shows some minor discrepancies due to the di!erence in wavelengths. In addition the angular distribution of re#ected light varied from sample to sample, and it is possible that the estimates of correction factors for the double-beam measurements have been based on incorrect assumptions. It is also necessary to consider errors related to the choice of reference standards as well as other types of sphere artefacts. Intuitively, however, it is hard to understand how any of these suggestions can explain a deviation as large as 7%. This illustrates that measurements from two di!erent spheres should not be compared without considering the di!erences between the instruments. However, a full analysis of the sources of deviation has not been performed here, since our main interest has been the spectral dependence resolved in the double-beam sphere. 3.4. Spectral dependence Fig. 6 shows the total re#ectance of the etched and laboratory-rolled samples as measured using the double-beam sphere. Whereas the etched sample has a rather constant total re#ectance for all wavelengths in the visible area (350}750 nm), the re#ectance of the rolled materials decreases somewhat towards shorter wavelengths.
Fig. 6. Total re#ectance measurements of the etched and laboratory-rolled materials made in the doublebeam sphere equipped with a spectrophotometer. The rolled samples show a red shift in the total re#ectance data. The sharp minimum at the lower side of 325 nm is due to detector artefacts. At approximately 825 nm there is another minimum, which is mainly due to the optical properties of pure aluminium [17]. In addition the data are slightly discontinuous in this area due to the change of detectors.
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Fig. 7. Specular, di!use, and total re#ectance of sample laboratory rolled 1 pass. The total and the specular re#ectance decrease towards shorter wavelengths, whereas the di!use re#ectance is rather constant.
Total re#ectance data from other rolled aluminium surfaces show the same tendency: Rolling does not only decrease the overall total re#ectance for visible wavelengths, it also creates a weak red shift. This characteristic cannot be revealed in white-light measurements. Fig. 7 shows the total, the di!use, and the specular re#ectance of the sample that was rolled 1 pass in the laboratory. Whereas the di!use re#ectance is rather constant throughout the visible area, both the total re#ectance and the specular component exhibit a red shift. This tendency applies to other rolled materials as well. Intuitively, we would expect the scattering of light to be stronger for the shorter wavelengths in the spectrum, producing a reduction in the specular component and an increase in the di!use component. This, however, should not lead to a red shift in the total re#ectance, like we observe. Since light of shorter wavelengths may be scattered to a larger extent than light of longer wavelengths, it is hard to know whether the spectral dependence of the total re#ectance of rolled materials originates in the specular component, in the di!use component, or in both. The results from the double-beam sphere demonstrate that additional information can be found in measurements made with spectral resolution. However, similar information may also be obtained from other types of instruments. Fig. 8 shows results from measurements of the etched and laboratory-rolled materials in the commercial colour measurement sphere presented in Section 2.3. The results clearly demonstrate the weak red shift of rolled surfaces. Accordingly, information on spectral dependence may be obtained employing other commercial instruments that are more common than integrating spheres specialised for re#ectance measurements.
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Fig. 8. Total re#ectance of etched and laboratory-rolled samples as measured with a colour measurement sphere. The data show the same red shift for rolled samples as seen in the double-beam integrating sphere measurements.
4. Conclusions We have given an introduction to the use of integrating spheres for total re#ectance measurements, and reviewed literature concerning the method. Aluminium samples of di!erent surface "nish have been measured using two integrating sphere systems. One is a single-beam sphere with incident white light and a Si-photoelement detector. The other is a double-beam sphere, which measures the re#ectance properties with spectral resolution. Results from structured materials with surface directionality illustrate how integrating sphere measurements may depend on the sample orientation. The topography of rolled aluminium was found to cause radiation losses through the entrance port in the single-beam sphere for a certain range of sample orientations. This can only be avoided by taking certain precautions when the sample is positioned in the sphere. In many Ulbricht's spheres the most correct total re#ectance values of rolled aluminium materials are obtained if the sample is oriented with RD close to the plane of incidence. Careful selection of one sample orientation generally reduces the e!ect of arti"cial losses compared to averaging results from di!erent sample orientations, as prescribed in the ISO 6719 standard. Rolled aluminium is in general a very glossy material with a large specular component in the re#ected light. In the single-beam sphere measurements of aluminium, it is better to use a specular metallic reference standard than the more common white/di!use one, since the specular standard has similar light scattering characteristics to those of aluminium sample surfaces. They should therefore experience
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approximately the same losses of radiation from the sphere or arti"cial gains in the detector. However, we have no experimental foundation for the choice, since di!erences between the results obtained using the two reference standards are within uncertainty limits of the measurements ($1%). The relatively constant di!erence between such results indicates that uncertainties in calibration account for most of the deviation. Integrating sphere measurements with spectral resolution show a red shift in the total re#ectance of rolled aluminium materials. This characteristic cannot be revealed using white light. The spectral variations may, however, appear in measurements from more common optical instruments. This was demonstrated employing a commercial colour measurement sphere, and the obtained results corresponded well with measurements from the double-beam instrument. The reduced and red-shifted total re#ectance of rolled aluminium surfaces cannot by any means be attributed to artefacts in the instruments.
Acknowledgements We are grateful to J. E. Lein and A. Augdal for making the measurements with the single-beam sphere and the colour measurement sphere, and to A. Roos and coworkers at the University of Uppsala for the double-beam sphere measurements. We also acknowledge J. Ma rdalen and E. Wold for helpful discussions. References [1] Lindseth I, Bardal A. Relations between the optical re#ectance and the near-surface microstructure of aluminium alloys. Proceedings of the Nineth Intrnational Conference on Modern Materials Techn * CIMTEC, symp. II 1998. p. 49}56. [2] Lindseth I, Bardal A, Wold E, Hunderi O. Optical re#ectance of industrial aluminium surfaces. Part 1: Assessment of possible absorption mechanisms, to be published. [3] Lindseth I, Bardal A, Ma rdalen J, Pettersen G, WilzeH n L, H+ier R. Optical re#ectance of industrial aluminium surfaces. Part 2: Evolution of total re#ectance and surface morphology upon rolling and etching, to be published. [4] Carr KF. Integrating sphere theory and applications. Part I: integrating sphere theory and design. Surf Coat Int 1997;80(8):380}5. [5] Carr KF. Integrating sphere theory and applications. Part II: integrating sphere applications. Surf Coat Int 1997;80(10):485}91. [6] DIN Deutsches Institut fuK r Normung E V: Deutsche Normen, DIN 5036, teil 3, 1979. [7] International Organization for Standardization: ISO 6719}1986. [8] Jacquez JA, Kuppenheim HF. Theory of integrating sphere. J Opt Soc Am 1955;45(6):460}70. [9] Goebel DG. Generalized integrating-sphere theory. Appl Opt 1967;6(1):125}8. [10] Finkel MW. Integrating sphere theory. Opt Commun 1970;2(1):25}8. [11] Clarke FJJ, Anne Compton J. Correction methods for integrating-sphere measurement of hemispherical re#ectance. Col Res Appl 1986;11(4):253}62. [12] Roos A, Ribbing CG, Bergkvist M. Anomalies in integrating sphere measurements on structured samples. Appl Opt 1988;27(18):3828}32. [13] Roos A, Ribbing CG. Interpretation of integrating sphere signal output for non-Lambertian samples. Appl Opt 1988;27(18):3833}7.
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[14] RoK nnow D, Roos A. Stray-light corrections in integrating-sphere measurements in low-scattering samples. Appl Opt 1994;33(25):6092}7. [15] Hanssen LM. E!ects of non-Lambertian surfaces on integrating sphere measurements. Appl Opt 1996;35(19):3597}606. [16] RoK nnow D, Roos A. Correction factors for re#ectance and transmittance measurements of scattering samples in focusing Coblentz spheres and integrating spheres. Rev Sci Instr 1995;66(3):2411}22. [17] Smith DY, Shiles E, Inokuti M. The Optical Properties of Metallic Aluminium. In Palik ED, editor. Handbook of Optical Constants of Solids. San Diego: Academic Press, 1998.
Re#ectance measurements of aluminium surfaces using integrating spheres I. Lindseth *, A. Bardal, R. Spooren Department of Physics, Norwegian University of Science and Technology, 7491 Trondheim, Norway SINTEF Materials Technology, 7465 Trondheim, Norway Received 16 November 1999; accepted 11 January 2000
Abstract In the investigation of the considerable absorption of visible light in industrially rolled aluminium surfaces, a thorough knowledge of the total re#ectance measurement method is required. In this paper a general introduction to the integrating sphere method is given, with emphasis on the current understanding of instrumental artefacts and ways of correcting them. Selected aluminium surfaces were measured employing two spheres; a single-beam instrument equipped with a white-light source and a Si-photoelement detector, and a double-beam sphere, which measures re#ectance properties with spectral resolution. It was found necessary to take precautions concerning the orientation of rolled samples relative instrument geometry, to avoid arti"cial losses from the sphere. The use of a specular reference standard is assumed to minimise the e!ect of several sphere artefacts, since it produces similar angular distribution of re#ected light as the rather glossy aluminium samples. Measurements with spectral resolution show that the total re#ectance of aluminium is somewhat red shifted after rolling, a tendency that cannot be revealed in ordinary white-light measurements. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Aluminium; Integrating spheres; Optical re#ectance; Rolling
1. Introduction Aluminium does in general have a very high optical re#ectivity. A `cleana surface of pure aluminium re#ects more than 90% of incident light for most visible wavelengths.
* Corresponding author.Tel.: #47-73-59-81-31; fax: #47-73-59-77-10. E-mail address: [email protected] (I. Lindseth). 0143-8166/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 0 ) 0 0 0 1 0 - 5
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This makes aluminium suitable to use in many products requiring high total (hemispherical) re#ectance for visible light. By total re#ectance we mean the sum of specularly re#ected and di!usely scattered light. One group of products demanding high total re#ectance is light re#ectors, which are often produced from industrially rolled aluminium materials. Measurements show, however, that the total re#ectance of aluminium decreases in the rolling process [1]. Some rolled materials exhibit absorption of more than 15%, even after thorough degreasing to remove lubricants from the surface. In the investigation of this phenomenon a thorough understanding of the total re#ectance measurement method is essential. It is common in the lighting industry to use commercial integrating spheres for determining the total re#ectance of aluminium materials. The use of such spheres is considered to be straightforward, and the method is well standardised. Our experience is, however, that in order to avoid incorrect measurements from aluminium materials, certain precautions may be necessary to take, and a basic knowledge of the measurement method is essential. This paper gives an overview of the integrating sphere method, particularly with respect to total re#ectance measurements of aluminium surfaces. A general introduction to the method is given. Based on literature, the current understanding of sources of error and ways of correcting them are outlined. Experimental results from total re#ectance measurements of aluminium materials are presented and discussed in light of this. The measurements have been made employing two di!erent integrating spheres; one measures total re#ectance of white light, whereas the other measures with spectral resolution. Results from the two spheres are compared and discussed with respect to the details of the instruments and their limitations and artefacts. Measurements with spectral resolution revealed tendencies in the re#ectance properties of rolled surfaces that cannot be revealed in ordinary white-light measurements. All the presented results are measurements from aluminium materials of di!erent surface "nish, and restricted to the wavelength range of visible light. Even though this investigation is limited with respect to choice of integrating spheres, samples, and wavelengths, the observations and conclusions most probably apply to other measurement geometries and samples, as well as to a wider spectral range. The outline and observations presented in this paper emphasise that the measured reduction in total re#ectance of aluminium upon rolling by no means can be related to the measurement method and companion artefacts. The phenomenon will therefore be discussed in relation to material structure and topography elsewhere [2,3].
2. Methods 2.1. Introduction to the integrating sphere method Integrating spheres, also known as Ulbricht's spheres, have the ability of collecting and spatially integrating optical radiation, and this makes them useful for a wide range of applications [4,5]. The luminous #ux originating from lamps and lasers can be measured. An integrating sphere can also serve as large area source of uniform,
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di!use radiation. Their most important application, however, is in re#ectance and transmittance measurements of di!erent materials. An integrating sphere is a closed sphere with a few port openings, such as for the incoming light, the sample, and the detector. The inside of the sphere has a coating, which ideally should re#ect 100% of the incident light, and be uniform and ideally di!use. (A Lambertian or ideal di!user is one whose radiance is independent of the angle of view under any condition of illumination.) The total port area should in general be as low as possible, and as a rule of thumb constitute less than 5% of the total sphere surface. When doing measurements, a radiation balance should be established inside the sphere after as few internal re#ections as possible. The amount of light hitting the detector then corresponds to the average light intensity inside the sphere. However, light re#ected from the sample should not pass directly to the detector. This is because the re#ectance of the wall coating in real spheres is less than 100%, and radiation is therefore attenuated whenever undergoing multiple re#ections from the walls. Radiation direct from the sample is not attenuated in this way and becomes overweighed with respect to the rest of the re#ected light. This error can be avoided by mounting ba%es in the sphere wall to block the detector's view of incident #ux coming directly from the sample. The ba%es should be coated with the same material as the sphere wall. There are di!erent sphere geometries available. In a substitution sphere there is only one port for the sample and the reference standard, and the two surfaces have to be measured subsequently. When changing from the sample to the standard, the average re#ectance of the sphere is altered. This makes it hard to determine the sample re#ectance with high accuracy. To avoid this problem comparison spheres have been designed. They have two ports for sample and reference standard, so that the average sphere re#ectance is kept constant. There are two types of comparison spheres. In single-beam instruments the position of the sample and the reference standard is alternated, so that each of them is placed in the incoming beam subsequently. In double-beam instruments there are two equivalent optical paths, one for the sample and one for the reference standard. These two paths are then usually chopped alternately and synchronised with the detector system. Schematic illustrations of a single-beam sphere and a double-beam sphere are shown in Figs. 1 and 2, respectively. Many integrating spheres are equipped with some sort of standardised white-light source, which may be similar to daylight. They may also be equipped with detectors that are modulated to replicate the eye's light sensitivity (photopic vision). In this way the choice of light source and detector re#ects our interest in a certain type of optical re#ectance, strongly connected to the visual appearance of the surface, and to the functional properties that are essential for a speci"c application. Total re#ectance measurements are often made in accordance with some sort of standardisation. The lighting industry often refers to the German standard DIN 5036 part 3 [6], which treats di!erent methods for measuring hemispherical parameters like di!use and total re#ectance and transmittance. The DIN standard gives a rather detailed speci"cation regarding the design of integrating spheres, the choice of light source and detector, and how to make re#ectance measurements. DIN 5036 integrating spheres contain "ve ports in addition to a light source and a detector. The
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Fig. 1. Schematic cross-sections of a single-beam sphere following the standard DIN 5036, part 3. A horizontal cross-section is shown in (a), and a vertical cross-section in (b). Ports 1 and 2 are sample and reference standard ports. The light enters the sphere through port 3. Port 5 is the specular exit port, which should be open when measuring di!use re#ectance, and closed when measuring total re#ectance. Port 4 is used only when measuring transmission properties. The position of port 2 may depart from what is shown in the "gure.
Fig. 2. Schematic illustration of the Beckman double-beam sphere geometry. Ports 1 and 2 are reference and sample beam entrance ports. Port 3 is for the sample, and 4 is for the reference standard. Port 5 is the specular exit port, whereas 6 and 7 are the PbS and the PM detector, respectively. When measuring di!use re#ectance, port 5 has to be covered with a beam dump. For total re#ectance measurements, it must be closed with a BaSO reference plate.
geometry of the sphere is shown in Fig. 1. The light source is a standardised "lament lamp (type A) giving a relatively white light, and the light's angle of incidence on the sample is 83. The detector is usually a spectrophotometer or a Si-photoelement. In the latter case the detector sensitivity is adjusted to spectrally simulate the light sensitivity of the eye. The detector is positioned behind a di!using screen in the sphere wall, shown in the vertical cross section of the sphere in Fig. 1b. The coating of the inside of
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the sphere should ideally re#ect 100% of the incident light, but a total re#ectance as low as 80% is satisfactory according to the standard. The sphere inside should be perfectly di!use and wavelength non-selective. For further details we refer to the standard itself [6]. ISO 6719 [7] standardises the use of integrating spheres for measurements of re#ectance characteristics of aluminium surfaces. The main contents are the same as for the DIN-standard, but the ISO standard is not as detailed and covers a wider range of integrating sphere geometries. When it comes to calibration standards, ISO 6719 speci"es three reference samples. A certain black surface should be used for the stray-light measurements or dark current compensation. In addition to a white di!use calibration standard, a specular surface should be used as working-standard. According to the ISO standard, each sample should be measured in three di!erent orientations. The exact orientation is given by the angle between the machining direction of the sample and the optical plane of the instrument, and the three orientations are 0, 45 and 903. Three re#ectance measurements in each orientation should be made and an average value obtained. We argue later that this way of averaging measurements from di!erent orientations not necessarily gives the most correct re#ectance values for directional samples. 2.2. Integrating sphere theory, known artefacts, and ways of correction The "rst theories of integrating spheres considered the case of a perfect sphere assuming the apertures to be too small to have any signi"cant e!ect on the results. The reference standard, the sample and the sphere inside were considered to be perfectly di!use. These assumptions are in general not ful"lled in real integrating spheres, and several authors have estimated the e!ect of sphere imperfections. Jacquez and Kuppenheim made calculations both for a perfect sphere, and including e!ects like #at sample and standard, and a specular instead of a di!use sample surface [8]. The authors considered both substitution and comparison types of spheres. One of their conclusions was that errors related to the substitution method are larger than for the comparison method. Goebel developed a general equation for the e$ciency of an integrating sphere with a non-uniform coating [9]. Finkel made a simpli"ed approach to the integrating sphere theory, and thereby found expressions for the e!ect of angular dependence in the detector, port losses, and sample #atness [10]. Clarke and Compton have given a full review of possible errors in re#ectance measurements caused by the integrating sphere itself, and how to correct them [11]. They divide the errors in several groups, and those of greatest relevance to our measurement systems and samples are summarised below. The general integrating error is a term collecting all e!ects departing the sphere from ideal conditions, e.g. non-uniform coatings, losses through ports, internal screening, ageing e!ects, and deposition of dirt. If radiation passes directly from the sample to the detector without any re#ections from the sphere wall, we get a diwuse reyectance-screening error. In most modern spheres this is avoided by internal screening. A similar source of error exists only for rather glossy samples. Since they create a strong specular (or regular) component of re#ected light, the detector should preferably be screened from view of
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where the patch of this radiation falls. Without such screening, some radiation from the specularly component may pass directly onto the detector without being attenuated by multiple re#ections. The specular part is then overweighed with respect to the di!use component, giving the regular-reyectance screening error. Most spheres lack adequate screening for glossy samples. When measuring total re#ectance, the specular component of light usually falls onto a white cap covering the exit port. It is important that this cap has a re#ectance equal to the average sphere wall re#ectance, but this is di$cult to achieve. Re#ectance deviations of the cap may cause a regular reyectance-coating error. Clarke and Compton state that any of these or the rest of the groups of errors could be the largest experienced in a particular instrument, and in some cases the errors may compound additively. However, they have shown how each of them to a greater or lesser extent can be determined and compensated for. When it comes to errors related to the specularly re#ected component (regularre#ectance coating-uniformity error and regular-re#ectance screening error), they can be determined by measuring a mirror with known re#ectance relative an ordinary di!use reference standard. Clarke and Compton show examples of errors of 7}10% for Al and Cr mirrors measured relative a matt standard in an ordinary sphere. According to them, the majority of the error is due to lack of screening between the specularly re#ected-component patch and the detector port. Such e!ects make it necessary to derive corrections for measurements of glossy samples, as long as a matt reference standard is used. The larger the specular component of re#ected light from the sample, the stronger the correction. However, valid results can be obtained for mirror samples by replacing the di!use reference standard with a specular one. The errors are then of the same proportion for both sample and standard, and thereby cancel out. In 1988 Roos, Ribbing and Bergkvist reported how experiments of structured samples showed a relationship between directional properties of the re#ectance and port imperfections in the integrating sphere [12]. A steel sample polished in only one direction was rotated stepwise in the sample port, and measurements were made at each orientation. The re#ectance varied signi"cantly as function of orientation. This could only be explained by considering the strong directionality in the sample surface, causing the light to be scattered in a long, disk-shaped pattern perpendicular to the polishing grooves. For a certain range of orientations this disk-shaped pattern fell onto the entrance port, causing a signi"cantly reduced re#ectance. Another artefact occurred for a small range of orientations as light directly from the sample fell on the inside of the detector collar, causing an erroneously high re#ectance value. Such anomalies should be taken into consideration whenever measuring samples with strong directionality in the surface (e.g. rolled samples). Roos and Ribbing published also in 1988 a paper on interpretation of integrating sphere output for samples that are not ideally di!use [13]. They show through calculations and experiments that when estimating correction factors to integrating sphere results, the common approach of dividing the total re#ectance into a specular and a di!use component is su$cient only in certain cases. Generally the di!use component must be further divided into a fully Lambertian and a more specularly
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di!use component. In 1994 RoK nnow and Roos extended the model to take stray light into consideration [14]. Hanssen has studied the e!ects of non-Lambertian surface coatings on integrating sphere measurements, using Monte Carlo simulations [15]. The importance of the ideal di!use character of the sphere wall was a$rmed, and errors due to non-ideal coatings have to be taken into account in high accuracy measurements. A high total re#ectance of the wall coating reduces the e!ect of non-Lambertian characteristics. Incorrect total re#ectance values from integrating spheres are most probably related to the errors outlined above. They all have in common that they are caused mainly by the integrating sphere itself or the choice of reference standard. We cannot exclude the possibility of other sources of error, such as the illumination system or the detector. However, there are few publications reporting of such errors, probably because they tend to be more casual and rare. 2.3. The instruments 2.3.1. Single-beam integrating sphere with Si-photoelement detector The employed single-beam integrating sphere equipped with a Si-photoelement detector has been produced by LMT Lichtmesstechnik Berlin, and it follows the standard DIN 5036 part 3 described in Section 2.1. The sphere diameter is 50 cm. Barium sulphate (BaSO ) covers the inside of the sphere, giving it the wanted optical properties; a total re#ectance of approximately 80% independent of the wavelength of the incident light, and a strong di!use re#ectance. A halogen lamp is used as light source (Osram 64410, 6 V, 10 W). The detector is a light-sensitive Si-photoelement positioned behind a di!using screen, which follows the sphere wall. The accuracy of the instrument is $1%. The uncertainty appears to be mainly caused by a 1% uncertainty in the calibration values of the reference standards. This should not a!ect relative di!erences between measurements made with the same standard. The precision is therefore thought to be much better than the accuracy of the measurements. The single-beam instrument can be used as a substitution sphere if a white cap is applied in the reference standard port. Experience from measuring di!erent aluminium surfaces showed that approximately the same results were obtained when the sphere was used in the substitution mode as in the comparison mode, which it is built for. This is because the Al samples and the reference standard in general have similar total re#ectance. After considering our demands for accuracy in the measurements, we chose to use the instrument as a substitution sphere. 2.3.2. Beckman double-beam integrating sphere with spectrophotometer The Beckman 198851 double-beam integrating sphere has a slightly di!erent geometry than the former sphere presented, but the same parameters can be measured using this sphere [12}14,16]. It contains seven ports, which are shown in Fig. 2. The Beckman 5240 spectrophotometer measures intensity in the wavelength range 300}2500 nm. The large wavelength range necessitates the use of two detectors, one PbS and one PM (6 and 7 in Fig. 2). They are both shielded with barium-sulphatecoated collars to avoid incident light directly from the sample.
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2.3.3. Colour measurement sphere The two measurement systems described above have the same small angle of incidence and di!use detection. Another measurement geometry was used in addition to them for comparison; a commercial colour measurement sphere called ACS Chroma Sensor CS-3. It has di!use illumination and detects re#ected light with spectral resolution at 83. From the amount of light detected in this direction, it calculates an overall `total re#ectancea. Such data have been compared to results from the double-beam sphere. The colour measurement sphere is not suitable for total re#ectance measurements in general, and a detailed description of it is therefore not included here.
3. Experimental results and discussion 3.1. Ewect of sample orientation relative surface directionality The topography of rolled aluminium materials generally consists of ridges following the rolling direction, making the surface very directional. Fig. 3 shows a scanning electron microscopy (SEM) micrograph from a laboratory-rolled material containing such a strong surface directionality. Single-beam total re#ectance measurements of rolled aluminium surfaces revealed that the results depend on the sample orientation relative to the sphere geometry. To illustrate this tendency, the measured total re#ectance of a series of etched and
Fig. 3. SEM micrograph of a laboratory-rolled aluminium material, illustrating the directionality of rolled surfaces.
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Fig. 4. Total re#ectance of etched and laboratory-rolled aluminium samples as measured using the single-beam sphere (white light). The measurements were made with either the rolling direction (RD) or the transverse direction (TD) of the samples parallel to the plane of incidence. The large deviation between the corresponding results is due to erroneous radiation losses in the TD orientation.
laboratory-rolled aluminium materials (AA1050) is plotted in Fig. 4. The starting material was etched thoroughly after the previous industrial hot rolling, in order to remove surface features generated in this process. Even though the etching changed the topography substantially, it did not fully remove the directionality of the surface. The etched material was then laboratory rolled in four passes. After each pass some material was taken out for optical measurements and characterisation. The total re#ectance was measured in the single-beam sphere relative to a di!use reference standard with total re#ectance of 83.0 $1%. The samples were measured in two orientations, with the rolling direction parallel to the plane of incidence (here called RD for `rolling directiona) and with the rolling direction normal to this plane (here called TD for `transverse directiona). The results are averages of at least 3 measurements from each material. It can be seen from Fig. 4 that the total re#ectance is considerably reduced after the "rst rolling pass. It then increases to some extent in the third and the fourth pass. The RD results are signi"cantly higher than the TD measurements for all samples, especially the rolled ones, and the explanation to this deviation was found to be the same as described by Roos and Ribbing [12]. A strong directionality in the aluminium surfaces causes the light to be re#ected into a long disk-shaped pattern perpendicular to the rolling direction. When the sample has the TD orientation, a signi"cant amount of this disk falls onto the entrance port, giving losses through the port and thereby a reduced detected signal. The deviation is smaller for the etched sample than for the three rolled samples because the etching process reduced the surface directionality.
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In the RD orientation the re#ected disk falls into the sample-detector plane of the sphere. Roos and Ribbing described how this created erroneously high re#ectance values due to unsatisfactory screening in their double-beam sphere [12]. The detector in our single-beam sphere appears to be well shielded against direct radiation from the sample, and such problems are therefore avoided for all sample orientations. This was con"rmed by a simple set of angle resolved measurements. They showed that the total re#ectance was constant for all sample orientations ranging approximately from !453 to #453 around the RD orientation, whereas it decreased as the orientation approached TD. Due to radiation losses through the entrance port for the TD orientation, we consider RD measurements to give the most correct re#ectance values. In Section 2.1 we referred to the ISO standard 6719 and how it requires the resulting re#ectance values to be an average of measurements from three di!erent sample orientations. It follows from the paper of Roos and Ribbing [12] and from the results presented above that this way of averaging may lead to less correct re#ectance values than if measurements are made at only one particular orientation. This is important to be aware of whenever making measurements in accordance with the ISO standard or analysing such results. 3.2. Choice of reference standard Total re#ectance is always measured relative to a reference standard. There are di!erent types of standards with variations in re#ectance properties, specularity, and accuracy. The choice of standard may be of in#uence to the measurement results, and determines to a large extent the accuracy of the measurements. In this investigation we have employed two di!erent reference standards in the single-beam sphere. One is di!use with a total re#ectance of 83.0$1%. The other is specular and re#ects 96.1$1% of incident light. These calibration values have been obtained employing the same type of white-light source as used in our instrument. A series of nine commercial, anodised aluminium re#ector materials were measured with both standards, and the results are plotted in Fig. 5. The plot also includes the deviation between the results for each sample, which varies from 1.0 to 1.8%. A dotted line indicates the mean di!erence for the nine samples, which is 1.2$0.2%. Sample A has a very high purity ('99.9% Al). The amount of impurities and alloying elements increases towards sample H and I, which contain more than 1% alloying elements. The "gure reveals that the specular standard leads to higher re#ectance values than the di!use standard. The discrepancies are so small, however, that they are within the uncertainty limit of the measurements. As long as the uncertainty in the calibration values of the reference standards is as large as 1%, this could easily cause di!erences of the magnitude that we observe. It could also explain why the specular standard gives higher total re#ectance values than the di!use. If the di!erences were due to errors outlined in Section 2.2, there is a large probability that the di!use reference standard would have given the highest re#ectance values. Both losses through the entrance port as well as lack of su$cient screening would lead to higher values when using a di!use standard. On the contrary, if the di!erences are caused by
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Fig. 5. Comparison of the total re#ectance of nine commercial aluminium re#ector materials (A}I) as measured with two di!erent reference standards, one di!use and one specular. The di!erence between corresponding measurements is plotted in the lower graph. The specular standard results in higher total re#ectance values than the di!use standard.
uncertainty in calibration, the deviation could go in either direction depending on the two reference standards. If calibration uncertainties were the only cause of discrepancies between measurements made employing the di!use and the specular standard, we would expect the deviations to be of approximately constant size. Errors related to the sphere itself would most probably vary in size depending on the exact angular distribution of re#ected light from the sample. Even though the total re#ectance of most of the nine samples in Fig. 5 is rather constant, the angular distribution of re#ected light varies considerably. Some variation can also be seen in the deviations between results obtained with di!erent reference standards. They are, however, rather small compared to the mean di!erence, a tendency suggesting that the main contribution to the discrepancies is systematic, most probably related to the calibration of the reference standards. Most of the aluminium samples that we analyse are considered to be far from di!use. When they re#ect light, the majority of them produce a strong specular component. Clarke and Compton [11] showed by an example that measurements of mirror samples employing a di!use reference standard may lead to severe errors. The problem could, however, be avoided by choosing a specular standard when measuring mirror samples. With a specular standard, the sample and the standard produce the same type of errors, which thereby cancel out. Based on this argumentation, the specular standard should preferably be used for our aluminium samples in order to reduce the e!ect of possible artefacts in the sphere. Whenever single-beam measurements have been made only using the di!use standard, a correction of 1.1 has been added to the data, in order to make them comparable to measurements made with the
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specular standard. This "gure is the mean di!erence in the measurements of a large number of aluminium materials. 3.3. Comparison of results from single- and double-beam integrating spheres In order to study how the total re#ectance varies as function of wavelength, a double-beam-integrating sphere equipped with a spectrophotometer was employed. The samples from the laboratory rolling series together with some industrially rolled and etched samples were measured. Both the total and the di!use re#ectance were determined with spectral resolution. A di!use reference standard was employed in the measurements, and rough corrections of the results were made afterwards according to the method developed by RoK nnow and Roos [16]. Parameters necessary for the calculation of correction factors were selected based on experience from similar measurements. Whereas the single-beam results were averages of several measurements from each material, the double-beam results are obtained from only one measurement. A simple comparison was made between the spectrally resolved measurements from the double-beam sphere and the white-light measurements from the single-beam sphere The eye and the detector of the single-beam sphere have a sensitivity maximum around j"550 nm. Double-beam sphere results for this wavelength were therefore compared to data from the single-beam sphere. Table 1 contains measurement data for the etched and laboratory-rolled samples and for two industrially cold rolled materials that have been etched gradually in phosphoric acid (85 wt% H PO ). The purity of the three alloys varied from 99.7% for the AA1070 alloy to 99.2% for the AA1200 alloy. All materials except one exhibit highest re#ectance values in the single-beam measurements, indicating that this is a systematic tendency. Nevertheless, Table 1 Total re#ectance of laboratory-rolled samples and two industrially rolled materials before and after etching, as measured with the two integrating spheres Material
Total re#.
Di!erence
Single-beam sphere White light (%)
Double-beam sphere j"550 nm (%)
(%)
AA1050 hot rolled and etched AA1050 lab-rolled 1 pass AA1050 lab-rolled 4 passes
90.0 78.4 82.5
83.4 78.7 81.7
6.6 !0.3 0.8
AA1070 as received AA1070 etched 30 s (0.97 lm) AA1070 etched 300 s (6.80 lm)
81.3 90.3 89.8
77.4 89.3 87.1
3.9 1.0 2.7
AA1200 as received AA1200 etched 30 s (1.04 lm) AA1200 etched 300 s (7.24 lm)
78.0 89.9 90.0
77.6 88.6 82.7
0.4 1.3 7.3
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the di!erences in the obtained results show large variations from one sample to another. Many of the di!erences are too large to be within the uncertainty limit of the measurements, and other explanations must be sought. Most probably comparison of measurements made using white light with results for one single wavelength shows some minor discrepancies due to the di!erence in wavelengths. In addition the angular distribution of re#ected light varied from sample to sample, and it is possible that the estimates of correction factors for the double-beam measurements have been based on incorrect assumptions. It is also necessary to consider errors related to the choice of reference standards as well as other types of sphere artefacts. Intuitively, however, it is hard to understand how any of these suggestions can explain a deviation as large as 7%. This illustrates that measurements from two di!erent spheres should not be compared without considering the di!erences between the instruments. However, a full analysis of the sources of deviation has not been performed here, since our main interest has been the spectral dependence resolved in the double-beam sphere. 3.4. Spectral dependence Fig. 6 shows the total re#ectance of the etched and laboratory-rolled samples as measured using the double-beam sphere. Whereas the etched sample has a rather constant total re#ectance for all wavelengths in the visible area (350}750 nm), the re#ectance of the rolled materials decreases somewhat towards shorter wavelengths.
Fig. 6. Total re#ectance measurements of the etched and laboratory-rolled materials made in the doublebeam sphere equipped with a spectrophotometer. The rolled samples show a red shift in the total re#ectance data. The sharp minimum at the lower side of 325 nm is due to detector artefacts. At approximately 825 nm there is another minimum, which is mainly due to the optical properties of pure aluminium [17]. In addition the data are slightly discontinuous in this area due to the change of detectors.
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Fig. 7. Specular, di!use, and total re#ectance of sample laboratory rolled 1 pass. The total and the specular re#ectance decrease towards shorter wavelengths, whereas the di!use re#ectance is rather constant.
Total re#ectance data from other rolled aluminium surfaces show the same tendency: Rolling does not only decrease the overall total re#ectance for visible wavelengths, it also creates a weak red shift. This characteristic cannot be revealed in white-light measurements. Fig. 7 shows the total, the di!use, and the specular re#ectance of the sample that was rolled 1 pass in the laboratory. Whereas the di!use re#ectance is rather constant throughout the visible area, both the total re#ectance and the specular component exhibit a red shift. This tendency applies to other rolled materials as well. Intuitively, we would expect the scattering of light to be stronger for the shorter wavelengths in the spectrum, producing a reduction in the specular component and an increase in the di!use component. This, however, should not lead to a red shift in the total re#ectance, like we observe. Since light of shorter wavelengths may be scattered to a larger extent than light of longer wavelengths, it is hard to know whether the spectral dependence of the total re#ectance of rolled materials originates in the specular component, in the di!use component, or in both. The results from the double-beam sphere demonstrate that additional information can be found in measurements made with spectral resolution. However, similar information may also be obtained from other types of instruments. Fig. 8 shows results from measurements of the etched and laboratory-rolled materials in the commercial colour measurement sphere presented in Section 2.3. The results clearly demonstrate the weak red shift of rolled surfaces. Accordingly, information on spectral dependence may be obtained employing other commercial instruments that are more common than integrating spheres specialised for re#ectance measurements.
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Fig. 8. Total re#ectance of etched and laboratory-rolled samples as measured with a colour measurement sphere. The data show the same red shift for rolled samples as seen in the double-beam integrating sphere measurements.
4. Conclusions We have given an introduction to the use of integrating spheres for total re#ectance measurements, and reviewed literature concerning the method. Aluminium samples of di!erent surface "nish have been measured using two integrating sphere systems. One is a single-beam sphere with incident white light and a Si-photoelement detector. The other is a double-beam sphere, which measures the re#ectance properties with spectral resolution. Results from structured materials with surface directionality illustrate how integrating sphere measurements may depend on the sample orientation. The topography of rolled aluminium was found to cause radiation losses through the entrance port in the single-beam sphere for a certain range of sample orientations. This can only be avoided by taking certain precautions when the sample is positioned in the sphere. In many Ulbricht's spheres the most correct total re#ectance values of rolled aluminium materials are obtained if the sample is oriented with RD close to the plane of incidence. Careful selection of one sample orientation generally reduces the e!ect of arti"cial losses compared to averaging results from di!erent sample orientations, as prescribed in the ISO 6719 standard. Rolled aluminium is in general a very glossy material with a large specular component in the re#ected light. In the single-beam sphere measurements of aluminium, it is better to use a specular metallic reference standard than the more common white/di!use one, since the specular standard has similar light scattering characteristics to those of aluminium sample surfaces. They should therefore experience
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approximately the same losses of radiation from the sphere or arti"cial gains in the detector. However, we have no experimental foundation for the choice, since di!erences between the results obtained using the two reference standards are within uncertainty limits of the measurements ($1%). The relatively constant di!erence between such results indicates that uncertainties in calibration account for most of the deviation. Integrating sphere measurements with spectral resolution show a red shift in the total re#ectance of rolled aluminium materials. This characteristic cannot be revealed using white light. The spectral variations may, however, appear in measurements from more common optical instruments. This was demonstrated employing a commercial colour measurement sphere, and the obtained results corresponded well with measurements from the double-beam instrument. The reduced and red-shifted total re#ectance of rolled aluminium surfaces cannot by any means be attributed to artefacts in the instruments.
Acknowledgements We are grateful to J. E. Lein and A. Augdal for making the measurements with the single-beam sphere and the colour measurement sphere, and to A. Roos and coworkers at the University of Uppsala for the double-beam sphere measurements. We also acknowledge J. Ma rdalen and E. Wold for helpful discussions. References [1] Lindseth I, Bardal A. Relations between the optical re#ectance and the near-surface microstructure of aluminium alloys. Proceedings of the Nineth Intrnational Conference on Modern Materials Techn * CIMTEC, symp. II 1998. p. 49}56. [2] Lindseth I, Bardal A, Wold E, Hunderi O. Optical re#ectance of industrial aluminium surfaces. Part 1: Assessment of possible absorption mechanisms, to be published. [3] Lindseth I, Bardal A, Ma rdalen J, Pettersen G, WilzeH n L, H+ier R. Optical re#ectance of industrial aluminium surfaces. Part 2: Evolution of total re#ectance and surface morphology upon rolling and etching, to be published. [4] Carr KF. Integrating sphere theory and applications. Part I: integrating sphere theory and design. Surf Coat Int 1997;80(8):380}5. [5] Carr KF. Integrating sphere theory and applications. Part II: integrating sphere applications. Surf Coat Int 1997;80(10):485}91. [6] DIN Deutsches Institut fuK r Normung E V: Deutsche Normen, DIN 5036, teil 3, 1979. [7] International Organization for Standardization: ISO 6719}1986. [8] Jacquez JA, Kuppenheim HF. Theory of integrating sphere. J Opt Soc Am 1955;45(6):460}70. [9] Goebel DG. Generalized integrating-sphere theory. Appl Opt 1967;6(1):125}8. [10] Finkel MW. Integrating sphere theory. Opt Commun 1970;2(1):25}8. [11] Clarke FJJ, Anne Compton J. Correction methods for integrating-sphere measurement of hemispherical re#ectance. Col Res Appl 1986;11(4):253}62. [12] Roos A, Ribbing CG, Bergkvist M. Anomalies in integrating sphere measurements on structured samples. Appl Opt 1988;27(18):3828}32. [13] Roos A, Ribbing CG. Interpretation of integrating sphere signal output for non-Lambertian samples. Appl Opt 1988;27(18):3833}7.
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[14] RoK nnow D, Roos A. Stray-light corrections in integrating-sphere measurements in low-scattering samples. Appl Opt 1994;33(25):6092}7. [15] Hanssen LM. E!ects of non-Lambertian surfaces on integrating sphere measurements. Appl Opt 1996;35(19):3597}606. [16] RoK nnow D, Roos A. Correction factors for re#ectance and transmittance measurements of scattering samples in focusing Coblentz spheres and integrating spheres. Rev Sci Instr 1995;66(3):2411}22. [17] Smith DY, Shiles E, Inokuti M. The Optical Properties of Metallic Aluminium. In Palik ED, editor. Handbook of Optical Constants of Solids. San Diego: Academic Press, 1998.