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Sensation luminance: A new name to distinguish CIE luminance from luminance dependent on an individual's spectral sensitivity
0042-6989/88$3.00+ 0.00 Pergamon Press plc
Visiont&s. Vol. 28, No. 3, pp. 455-456, 1988 Printed in Great Britain
RESEARCH NOTE
SENSATION LUMINANCE: A NEW NAME TO DISTINGUISH CIE LUMINANCE FROM LUMINANCE DEPENDENT ON AN INDIVIDUAL’S SPECTRAL SENSITIVITY PETER K. KAISER Department of Psychology, York University, 4700 Keele St, North York, Ontario, Canada M3J lP3 (Received 15 June 1987; in revised &w-m2 1 September f 987) Abstract-The term sensation luminance is proposed when light measures are made which depend on individual observer’s spectral sensitivity. The unit of measure for sensation luminance is ives/m’. When sensation luminance is used to calculate retinal illuminancc the term sensation trolands is proposed. Color vision
Luminance
Photometry
Sensation luminance
INTRODUCTION
Sensation trolands
Trolands
dependent on the CIE V(A) and (2) luminance dependent on an individual observer’s spectral When vision researchers equate chromatic lights sensitivity. It is preferable to distinguish clearly based on their individual observer’s spectral between these two light measures. sensitivities they use specific, psychophysical Such a distinction is made in audition and procedures such as heterochromatic flicker phoacoustics. Sound pressure level measurements tometry, minimally distinct border, etc. It has refer to sound levels, in decibels, relative to become common place in the vision research to an inte~ationally defined reference level. This say that stimuli, so equated, are eq~luminous. reference level represents the internationally This may or may not be the case depending agreed average observer’s threshold for hearing. on whether the stimuli are equal according to Sensation level, on the other hand, refers to equation (1). similar measures except that the reference level is the threshold of the individual listener to L=K, Le,,W)d,J (1) whom the measures apply (Moore, 1977). In the s following I recommend a similar procedure for where L = luminance, cd/m’; K,,, = maximum the measurement of light, luminous efficiency; Le,1 = spectral radiance; V(A) = CIE photopic luminous efficiency function. It has also become common place to identify SENSATION LUMINANCE neurons that are spectrally non-opponent as When an individual observer’s spectral sensiluminance units. Following from this anativity is the basis of light measurement, this tomical designation it is common place to speak of a luminance channel. The rationale for call- should be called sensation luminance or sluminance for short. I propose as the associated ing spectrally non-opponent neurons luminance units is that these are the neurons considered to scale of measurement, ivef per square meter, implying for point sources a unit of sensation be activated during flicker photometry (Kaiser, 1971; Lee et al., 1987). The term luminance, in intensity, the ive. It is appropriate to honor Ives because he did the pioneering research on flicker vision research has acquired a generic connophotometry (Ives, 1912a-e, 1914, 1917) and tation leaving open the possibility for confusion and misinterpretation. flicker photometry is probably the most freIn this paper I would like to focus on lumi- quent psy~hophysi~l procedure, used by visual stimuli. nance as a light measurement term with two scientists, to equate heterochromatic different but related meanings: (1) luminance Sensation luminance can be calculated knowing 455
456
Research Note
the spectral radiance of the stimulus and the spectral sensitivity of the observer, e.g. L,,,, ~(2) di
SL=K, s
(2)
SL = sensation luminance, ives/m*; radiance; K,,, = maximum luminous efficiency; u(1) = spectral sensitivity of the observer. It is important to note that in equation (1) the luminous efficiency function is designated by V(n). In equation (2) the spectral sensitivity is designated by u(n), the critical difference being capitalization. Whereas luminance is measured in cd/m2, sensation luminance is measured in ives/m*. Luminance is conveniently measured using a well calibrated photoelectric photometer. Sensation luminance, however, depends on an appropriate psychophysical procedure. A practical method for measuring sensation luminance is as follows. Define a reference stimulus and measure its luminance, i.e. its CIE luminance, using a well calibrated photoelectric photometer. The choice of the reference stimulus is not critical, but a monochromatic one is recommended since it is the easiest to reproduce reliably. A convenient choice is 570 nm because it is the least saturated wavelength. Once the reference stimulus is determined and measured, test stimuli are matched to it by an appropriate psychophysical procedure such as flicker photometry or minimally distinct border. The numeric value of sensation luminance would be the numeric luminance value of the reference, with the measurement unit in ives/m*, not cd/m*. For example, consider a 570 nm reference stimulus of 10 cd/m*. A series of test spectral stimuli are matched to this reference by flicker photometry. Each of the latter stimuli would now have a sensation luminance of 10 ives/m*. Stimuli equal in sensation luminance probably will not be equal in luminance. I have not limited the psychophysical procedures that are appropriate for measuring sensation luminance. They are intentionally left open. However, in order for sensation luminance to qualify as a “luminance” type measure, the psychophysical procedure must be additive. Consequently, brightness matching would be an inappropriate procedure, but flicker photometry and minimally distinct border would be appropriate (Kaiser, 1984).
SENSATION
TROLANDS
Is-td/
It is proposed that the product ot‘ sens~ition luminance (ivesjm’) times pupil area (mm’) i ield the number of sensation trolands (s-tdj.
where
L,,j, = spectral
SUMMARY
The nomenclature sensation luminance is proposed when light measurements are made which depend on an individual’s spectral sensitivity rather than on the CIE photopic standard observer’s spectral sensitivity [V(E,)]. A sensation luminance of, for example, 10 ives/m’ is that amount of test field light required to match a reference field of 10 cd/m*. The matching must be done with a psychophysical method that will insure additivity of sensation luminances.
REFERENCES Ives H. E. (1912a) Studies of the photometry
of lights of different colours. I. Spectral luminosity curves obtained by the equality of brightness photometer and the t?ieker photometer under similar conditions. Phil. Mug. 24, 149-188. Ives H. E. (1912b) Studies of the photometry of lights of different colours. II. Spectral luminosity curves by the method of critical frequency. Phil. Msg. 24, 352.-370 Ives H. E. (1912~) Studies of the photometry of lights of different colours. III. Distortions in spectral luminosity curves produced by variations in the character of the comparison standard and of the surroundings of the photometric field. Phil. Mag. 24, 74475 1. Ives H. E. (1912d) Studies of the photometry of light of different colours. IV. The addition of luminosities of different color. Phil. Mag. 24, 845-853. Ives H. E. (1912e) Studies of the photometry of light of different colours. V. The spectral luminosity of the average eye. Phil. Mag. 24, 853-863. Ives H. E. (1914) The theory of the flicker photometer. Phil. Mag. 28, 708-728.
Ives H. E. (1917) A polarization flicker photometer and some data of theoretical bearing obtained with it. Phil. Mag. 33, 360-380.
Kaiser P. K. (1971) Luminance and brightness. Appl. Opt. 10, 2768-2770. Kaiser P. K. (1984) Photometric measurements. Optical Radiation Measurements, Vol. 5, Visual Measurements (Edited by Bartleson C. J. and Grum F.), pp. 563-614. Academic Press, New York. Lee B. B., Martin P. R. and Valberg A. (1987) The physiological basis of heterochromatic photometry. Imrest. Ophrhal. visual Sci., Suppl. 2i%,240. Moore 3. C. J. (1977) Introduction to the Psychology of Hearing, pp. 304-305. MacMillan, New York.
Visiont&s. Vol. 28, No. 3, pp. 455-456, 1988 Printed in Great Britain
RESEARCH NOTE
SENSATION LUMINANCE: A NEW NAME TO DISTINGUISH CIE LUMINANCE FROM LUMINANCE DEPENDENT ON AN INDIVIDUAL’S SPECTRAL SENSITIVITY PETER K. KAISER Department of Psychology, York University, 4700 Keele St, North York, Ontario, Canada M3J lP3 (Received 15 June 1987; in revised &w-m2 1 September f 987) Abstract-The term sensation luminance is proposed when light measures are made which depend on individual observer’s spectral sensitivity. The unit of measure for sensation luminance is ives/m’. When sensation luminance is used to calculate retinal illuminancc the term sensation trolands is proposed. Color vision
Luminance
Photometry
Sensation luminance
INTRODUCTION
Sensation trolands
Trolands
dependent on the CIE V(A) and (2) luminance dependent on an individual observer’s spectral When vision researchers equate chromatic lights sensitivity. It is preferable to distinguish clearly based on their individual observer’s spectral between these two light measures. sensitivities they use specific, psychophysical Such a distinction is made in audition and procedures such as heterochromatic flicker phoacoustics. Sound pressure level measurements tometry, minimally distinct border, etc. It has refer to sound levels, in decibels, relative to become common place in the vision research to an inte~ationally defined reference level. This say that stimuli, so equated, are eq~luminous. reference level represents the internationally This may or may not be the case depending agreed average observer’s threshold for hearing. on whether the stimuli are equal according to Sensation level, on the other hand, refers to equation (1). similar measures except that the reference level is the threshold of the individual listener to L=K, Le,,W)d,J (1) whom the measures apply (Moore, 1977). In the s following I recommend a similar procedure for where L = luminance, cd/m’; K,,, = maximum the measurement of light, luminous efficiency; Le,1 = spectral radiance; V(A) = CIE photopic luminous efficiency function. It has also become common place to identify SENSATION LUMINANCE neurons that are spectrally non-opponent as When an individual observer’s spectral sensiluminance units. Following from this anativity is the basis of light measurement, this tomical designation it is common place to speak of a luminance channel. The rationale for call- should be called sensation luminance or sluminance for short. I propose as the associated ing spectrally non-opponent neurons luminance units is that these are the neurons considered to scale of measurement, ivef per square meter, implying for point sources a unit of sensation be activated during flicker photometry (Kaiser, 1971; Lee et al., 1987). The term luminance, in intensity, the ive. It is appropriate to honor Ives because he did the pioneering research on flicker vision research has acquired a generic connophotometry (Ives, 1912a-e, 1914, 1917) and tation leaving open the possibility for confusion and misinterpretation. flicker photometry is probably the most freIn this paper I would like to focus on lumi- quent psy~hophysi~l procedure, used by visual stimuli. nance as a light measurement term with two scientists, to equate heterochromatic different but related meanings: (1) luminance Sensation luminance can be calculated knowing 455
456
Research Note
the spectral radiance of the stimulus and the spectral sensitivity of the observer, e.g. L,,,, ~(2) di
SL=K, s
(2)
SL = sensation luminance, ives/m*; radiance; K,,, = maximum luminous efficiency; u(1) = spectral sensitivity of the observer. It is important to note that in equation (1) the luminous efficiency function is designated by V(n). In equation (2) the spectral sensitivity is designated by u(n), the critical difference being capitalization. Whereas luminance is measured in cd/m2, sensation luminance is measured in ives/m*. Luminance is conveniently measured using a well calibrated photoelectric photometer. Sensation luminance, however, depends on an appropriate psychophysical procedure. A practical method for measuring sensation luminance is as follows. Define a reference stimulus and measure its luminance, i.e. its CIE luminance, using a well calibrated photoelectric photometer. The choice of the reference stimulus is not critical, but a monochromatic one is recommended since it is the easiest to reproduce reliably. A convenient choice is 570 nm because it is the least saturated wavelength. Once the reference stimulus is determined and measured, test stimuli are matched to it by an appropriate psychophysical procedure such as flicker photometry or minimally distinct border. The numeric value of sensation luminance would be the numeric luminance value of the reference, with the measurement unit in ives/m*, not cd/m*. For example, consider a 570 nm reference stimulus of 10 cd/m*. A series of test spectral stimuli are matched to this reference by flicker photometry. Each of the latter stimuli would now have a sensation luminance of 10 ives/m*. Stimuli equal in sensation luminance probably will not be equal in luminance. I have not limited the psychophysical procedures that are appropriate for measuring sensation luminance. They are intentionally left open. However, in order for sensation luminance to qualify as a “luminance” type measure, the psychophysical procedure must be additive. Consequently, brightness matching would be an inappropriate procedure, but flicker photometry and minimally distinct border would be appropriate (Kaiser, 1984).
SENSATION
TROLANDS
Is-td/
It is proposed that the product ot‘ sens~ition luminance (ivesjm’) times pupil area (mm’) i ield the number of sensation trolands (s-tdj.
where
L,,j, = spectral
SUMMARY
The nomenclature sensation luminance is proposed when light measurements are made which depend on an individual’s spectral sensitivity rather than on the CIE photopic standard observer’s spectral sensitivity [V(E,)]. A sensation luminance of, for example, 10 ives/m’ is that amount of test field light required to match a reference field of 10 cd/m*. The matching must be done with a psychophysical method that will insure additivity of sensation luminances.
REFERENCES Ives H. E. (1912a) Studies of the photometry
of lights of different colours. I. Spectral luminosity curves obtained by the equality of brightness photometer and the t?ieker photometer under similar conditions. Phil. Mug. 24, 149-188. Ives H. E. (1912b) Studies of the photometry of lights of different colours. II. Spectral luminosity curves by the method of critical frequency. Phil. Msg. 24, 352.-370 Ives H. E. (1912~) Studies of the photometry of lights of different colours. III. Distortions in spectral luminosity curves produced by variations in the character of the comparison standard and of the surroundings of the photometric field. Phil. Mag. 24, 74475 1. Ives H. E. (1912d) Studies of the photometry of light of different colours. IV. The addition of luminosities of different color. Phil. Mag. 24, 845-853. Ives H. E. (1912e) Studies of the photometry of light of different colours. V. The spectral luminosity of the average eye. Phil. Mag. 24, 853-863. Ives H. E. (1914) The theory of the flicker photometer. Phil. Mag. 28, 708-728.
Ives H. E. (1917) A polarization flicker photometer and some data of theoretical bearing obtained with it. Phil. Mag. 33, 360-380.
Kaiser P. K. (1971) Luminance and brightness. Appl. Opt. 10, 2768-2770. Kaiser P. K. (1984) Photometric measurements. Optical Radiation Measurements, Vol. 5, Visual Measurements (Edited by Bartleson C. J. and Grum F.), pp. 563-614. Academic Press, New York. Lee B. B., Martin P. R. and Valberg A. (1987) The physiological basis of heterochromatic photometry. Imrest. Ophrhal. visual Sci., Suppl. 2i%,240. Moore 3. C. J. (1977) Introduction to the Psychology of Hearing, pp. 304-305. MacMillan, New York.