Stray light performance of UV multichannel spectral measuring instruments

Spectrophotometry, Luminescence and Coulour; Science and Compliance C. Burgess and D.G. Jones (eds.) 1995 Elsevier Science B.V.

Stray light performance of UV multichannel spectral measuring instruments Eugene G Arthurs, Iain Drummond, and Alex Kremer Oriel Instruments, 250 Long Beach Blvd., Stratford, CT 06497, USA.

Abstract We show the stray light performance of seven different spectrographs with multichannel detectors when measuring low levels of ultraviolet radiation in a high visible and near infrared background. The poor performance precludes the use of simple spectrographs for accurate multichannel based ultraviolet spectroradiometry.

Keywords Ultraviolet, Spectroscopy, Spectroradiometry, Spectrograph, Charge coupled detector, Photodiode array, Stray light, Scattering. INTRODUCTION Stray out-of-band radiation has always complicated the accurate measurement of low levels of ultraviolet (UV) radiation in the presence of high levels of visible and near infrared radiation. Kaye [1] has pointed out how stray radiation can limit the performance of spectrophotometers where the source and detector are included in the instrument. Stray radiation also complicates solar UV spectroradiometry and the calibration of UV-A/LW-B spectroradiometers with FEL irradiance standards. Multichannel spectroradiometers based on silicon photodiode array (PDA) or charge coupled device (CCD) detectors suffer from this problem more than traditional scanning instruments with photomultiplier detectors. This is because of the extended spectral responsivity of silicon detectors (compared with traditional photomultipliers), the large exposed active detector area and back reflections from the detector and the poor performance of some black surface finishes in the near infrared. We report measured results for PDA and CCD detectors used on several different spectrographs including conventional and crossed Czerny-Turner instruments and on a prism spectrograph. We identify the radiation responsible for the background in the UV by using filters to limit the wavelength range of the incident radiation.

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400 MOTIVATION It is possible to optimize any spectrograph design for a single application. The application defines a spectral range of interest and the radiation that enters the spectrograph can be both limited to this range and modified by filters to equilibrate the signal from the detector elements. Spectrographs require compromise among spectral bandwidth, spectral resolution and responsivity. Crossed dispersion or MultiTrack TM [2] approaches allow wideband coverage with good resolution at lower total responsivity. Changing the grating angle while using a grating with high groove density allows wideband coverage, good resolution and the opportunity to use the full sensitive area of the detector with a simple low cost spectrograph. Our interest is in the design of a low cost versatile spectrograph for use with Si multichannel detectors. Applications include studies of the ozone sensitive solar UV edge, spectroscopy of flames, studies of the spectral distribution of UV sources and UV Raman. The detectors have evolved rapidly to have high dynamic range; we find t h a t the spectrographs used with the detectors do not allow use of the dynamic range in many applications, because of stray light limitations. While filters and predispersers can be used to reduce stray light, instruments without the need for these accessories are simplest. Additionally, bandpass filters are also difficult to find for the UV below 300 nm. Here we study the stray light levels recorded using spectrographs in the UV, under "worst case conditions" allowing the entire broadband radiation from a quartz tungsten halogen lamp to enter the spectrograph. Although artificial, these conditions apply to many low cost spectrographs used to measure solar UV.

HOW IS STRAY LIGHT DEFINED?

"Stray light" is a term routinely used in the specifications of monochromators and spectrographs. We do not consider radiation leaks into the instrument here. All data we report here is after light leaks have been eliminated. Unfortunately the definitions and measurement conditions are not usually fully specified and the detailed studies [3-5] concentrate on grating scatter. Most data refer to the signal levels in the wings of and normalized to the peak of a single spectral line such as that from a HeNe laser stripped of the plasma tube background. The efficiency of the grating at the measurement wavelength is important. Scatter also increases with grating groove density [5]. The residual background well out on the wings is due to scatter from optical components inside the instrument, and from the unabsorbed radiation at the input wavelength that reaches the detector. Verrill [3] pointed out that the scatter from gratings, after Fraunhofer diffraction from the grating edges has been eliminated, has two components, "grass", and diffuse scatter. Grass is scatter distributed mainly in the plane of the incident and

401 diffracted beams; general diffuse scatter is similar to that from any optical component with distribution into the full 2~ solid angle. The measured stray signal in a monochromator then depends on the width of the slit for the "grass" and on the area of the slit for the diffuse when the detector "sees" the complete slit. Here "slit" refers to the illuminated exit opening, that is identical to the physical opening when the imaged entrance slit is larger t h a n the exit aperture. Kaye [1] points out that for monochromatic illumination the signal depends on slit width while the stray radiation varies quadratically with slit width. We also find, not surprisingly, that the stray light levels depend strongly on how the light is coupled into the spectrometer. So, even in the traditional definition of stray light for monochromatic illumination, several qualifying instrument parameters should be reported. In a monochromator it is only the stray light that exits a small slit along a path carrying it to the detector that is of concern. With multichannel detectors, the entire detector area is always exposed to stray radiation, the grass closer to the plane of the optical axis and the diffuse scatter over the entire detector. Further, in contrast to most monochromators, the large exposed detector reflects radiation back into the instrument. Silicon reflectance increases with decreasing wavelength. This reflected radiation poses an additional design problem adding to the classic re-entrant spectra problem. Stray light with broadband sources and detectors is often [1,2,6] quantified by using a cut-off filter and recording the signal where there should be none. This method is the basis of ASTM E-387 [7] for spectrophotometers, but a key difference is that it is the performance of all the elements of the spectrophotometer, source, filters, spectrometer and detector that are tested. This is not the case with spectrometers. The results of this test on a spectrometer depend on the source used and the detector. At Oriel Instruments we publish data for our monochromators using deuterium light sources, photomultipliers with S-5 (180-650 nm) response and a 320 nm blocking filter [2]. Internal tests also use tungsten halogen light sources. The former technique is a fair measure of UV stray light performance for this source and photomultiplier. It is a poor indicator of how good the spectrometer is with a Si detector and tungsten halogen source or when looking at sunlight. The typical UV Si detector is very sensitive to scattered NIR, and has low UV responsivity. The spectral power distribution of a typical quartz tungsten halogen lamp used for calibration peaks at ca. 930 nm. When expressed in photons/nm/s the peak is at 1155 nm. (Figure 1) Combining these factors with the high NIR reflectance of some black finishes results in poor S/N in the UV.

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Figure 1 Quantum efficiencies for the PDA and CCD (Top) and normalized spectral distribution of the light source in quanta/s/nm, and the products of the spectral distribution with the quantum efficiencies of the detectors (bottom). QUANTUM EFFICIENCIES OF THE PDA AND CCD

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SPECTROGRAPHS For our tests we used five commercial spectrographs and two breadboard units. Figures 2-8 show the units. They range in size from a miniature 35 mm focal length system with fiber optic input to a 257 mm focal length Czerny-Turner (CT) type i n s t r u m e n t designed to eliminate re-entrant spectra problems. Spectrograph 7, a prism spectrograph, is based on a traditional Russian spectrograph design (ISP-22) for use with photographic plates. The prism removes the need for order sorting, and as a computer is used to process the data from the multichannel detector, calculation of the wavelength assignment is only marginally more difficult for the prism instrument though the resolution changes dramatically over the spectral range. Durability and simplicity also make the use of a prism attractive as the basis of a broadband solar spectroradiometer for field deployment (8). MULTICHANNEL DETECTORS We used an Oriel Si photodiode array (PDA) and Oriel Si charge coupled device detector (CCD) (Figure 9) to record the signals in the output plane of six of the instruments. Both silicon detectors were UV sensitive (Figure 1). For these tests we used the CCD like a PDA, as a series of vertical detector elements, 27 mm wide by 6900 mm high. We summed ("binned") the signal over the "vertical column" of pixels though some of the spectrographs had imaging capability. Use in this mode requires no shutter, the ideal "no moving parts" implementation. We set the exposure time on the instruments at or close to saturation signal levels at the long wavelength end of the spectrum. For all six instruments we used the integrate capability of our InstaSpec TM software to quickly accumulate m a n y exposures. Typical exposure times were 0.025 s, the minimum, for the CCD and 0.1 s for the PDA. The PDA exhibited slight spectral modulation of responsivity below 400 nm due to the thin surface film on the Si surface. The modulation was significantly less t h a n reported by Mount et al [9] and that which we observed earlier with Reticon arrays in a similar arrangement. There was no indication of any modulation with the CCD nor did we record any significant back diffraction as reported by Bormett and Asher [10] even with laser sources. Spectrograph 3 had a small custom CCD as part of the instrument. We found that very long exposure cycles were needed to provide usable signal with this device.

404 F i g u r e s 2 to Spectrograph Spectrograph Spectrograph

4 1, a fast compact unit with a concave grating 2, a fast imaging spectrograph with a corrected concave grating. 3, a miniature spectrograph on a computer board with fiber optic input and custom CCD.

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7 4, a classic CT designed for zero re-entrant spectra. 5, a compact crossed CT. 6, an imaging Czerny Turner design.

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Figure 8 Spectrograph 7, a breadboard prism instrument based on established prism spectrograph design

Figure 9 The detecting elements. The PDA surface is oxide overcoated silicon; the CCD surface is mainly oxide coated silicon with lithographic microstructure, all overcoated with a thin layer of UV absorbing, visible and NIR transmitting fluorescent material.

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MEASUREMENT SET-UP Figure 10 shows the m e a s u r e m e n t set-up. We used an F/4 quasi-diffuse input. The spectral distribution of the highly regulated lamp is shown in Figure 1. We recorded the lamp spectrum and then used a long pass (LP) filter to remove UV below ~360 nm. The 3 mm filter had measured transmittance of less t h a n 10 .6 for all wavelengths from 200-320 nm.

Figure 10 The m e a s u r e m e n t set up

RESULTS We show the signal levels recorded with, and without, the filter for each spectrograph in Figures 11 to 17. What we show is "electron counts corrected for the dark electrical background inherent in these detectors. If the spectrographs were ideal, the signal below the filter cut-on would rapidly go to zero. Where possible we indicate the ratio of the signal 40 nm above and below the nominal 360 nm filter cut-on. The actual UV signal levels (for equivalent exposure times) and resolution are other measures of the instruments not presented here.

408 F i g u r e s 11 a n d 12 Data from Spectrograph 1, with the PDA. Top: With no filter and with a 360 nm LP filter and Bottom: With a series of long pass filters. Data from Spectrograph 2. Top: With the PDA, and 360 nm LP filter. Bottom: With the CCD and 360 nm LP filter RESULTS WITH PDA

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412 Fig 17 shows the result using the prism spectrograph. The stray light performance was disappointing and as the lower graph in this figure shows, the detector tilt required by the optical design resulted in "spectral ghosts" due to multiple reflections within the detector structure. We further studied the spectral content of the stray signal at 320 nm using a series of long pass filters sequentially to exclude radiation below 420, 500, 630, 780 and 850 nm, and by using bandpass filters.

DISCUSSION Recorded signal levels depend on the spectral irradiance at the entrance slit, slit dimensions, speed and optical imaging characteristics of the spectrograph, grating spectral efficiency, detector dimensions and spectral responsivity. The signals shown and those recorded over other spectral ranges matched expectations based on the detector responsivity, grating efficiency and spectral distribution of the source. The stray background also depends on the radiation admitted into the instrument, scatter of the radiation from the optical components, from the walls and from the internal structure of the spectrograph into the detector. It also depends on the significant backward radiation reflected from the detector and on the disposition of any re-entrant portion of the broadband spectrum. Figures 11 to 17 show that the measured ratio of stray background to signal varies significantly from instrument to instrument. Generally the stray light signal contribution corresponded to that expected from spectrally neutral scattering, i.e match the spectral distribution of the light source weighted by the detector spectral responsivity (Fig 1). The stray radiation had a relatively smooth distribution in all cases. There were no dramatic spectral features indicative of retro-reflected radiation meeting sharp re-entrant criteria, with the possible exception of the unusual feature on the curve in Figure 13 for spectrograph # 3. We assume this came from an illuminated structural element in the field of view of the detector but we were unable to confirm this conclusively. Bumps and steps in the other stray light data could be accounted for in most cases by the physical structure inside the spectrograph. The data with the long pass filters shown in Figure 11 shows that most of the stray radiation is in the NIR, approximately corresponding to the product of the lamp and detector curves. We noted higher contribution from the NIR in one spectrograph that used a "black" finish that was highly reflective in the NIR on some internal mounts. The PDA had consistently better performance than the CCD. We believe this is due to the smaller vertical dimension of the PDA used, 2.5 mm compared with 6.9 mm for the CCD and the difference in the spectral responses. The height of the

413 spectral image in the detector plane depends on the input sht height and on the imaging properties of the spectrograph and typically increases from IR to UV. The CCD captures proportionately more stray signal due to its larger height and the increase in stray/signal ratio with off axis vertical position. In the simple pixel binning used, stray signal from all pixels was added, whether or not the pixels at the top and bottom edges of the array recorded any of the desired spectral image. The prism spectrograph had especially disappointing stray light performance traced to the prism itself. Further tests with improved surfaces are planned. CONCLUSIONS This brief initial report of stray light levels recorded with a variety of spectrographs and multichannel detectors shows severe limitations for these devices for recording UV spectra in the presence of high levels of visible and NIR radiation. Obviously bandpass filters will greatly improve the UV performance though many UV bandpass filters have high NIR transmittance. The input optical configuration is important; and the illuminated input slit height should be minimized so that the image only fills the PDA. Selective reading out of the rows of a CCD can reduce the problem and proper use of "imaging spectrographs" promises better performance for both PDAs and CCDs. REFERENCES

Kaye, W.I. in, Advances in Standards and Methodology in Spectrophotometry, 1987 Burgess,C., and Mielenz, K.D., (Editors) Elsevier

10.

Science Publishers. Oriel Catalogue, Volume II, 1993. Verrill, J.F., Optica Acta 25, 7, 1978, pp 531-547. Woods, T.N., Wrigley III, R.T., Rottman, G.J. and Haring R.E., Appl. Opt, 33, 19,1994, pp 4273-4285. Geikas, G.I., SPIE ,675,1986, Stray Radiation V., pp 140-151. Pierson, A., and Goldstein, J., 1989, Lasers and Optronics, Sept.1989, pp 67-74 Standard Method of Estimating SRE, ASTM E-387-72. Michalsky, J.J., Harrison, L., Beik, M., Berkheiser III, W, and Schlemmer, J., in Proceedings of the Third Atmospheric Radiation Measurement Program., Science Team Meeting, Mar 1-4, 1993, Norman OK Mount, G.H., Sanders, R.W. and Brault, J.W., Appl. Opt., 31,7, 1992, pp 851-858. Bormett, R.W. and Asher, S.A., Appl. Spec., 48,1, 1994, pp 1-6.