Applied Optics Letters to the Editor

Luminance and Brightness P. K. Kaiser

Psychology Department, York University, Downsview, Ontario. Received 16 February 1971.

Brightness and luminance have been denned in a number of places.1 - 3 The reader is also referred to Wyszecki and Stiles2

who provide references for a number of additional sources pre­senting discussions on photometric units.

The photometric unit of luminance has been defined by the International Commission of Illumination (CIE).4 Photometric units in general were formulated because the human visual system is differentially sensitive to various portions of an equal energy spectrum, in that they appear unequally bright. Thus, the photometric system of light measurement was devised as an at tempt to relate radiometry to human visual sensitivity. Luminance, L, the photometric analogy to radiance, is defined by the following equation1:

Km is a constant depending on the units in which L and Le

are measured. Ledλ is the radiance emitted in the wavelength interval dλ. Vλ is the photopic relative luminous efficiency function. The Vλ function is defined by the CIE and has its basis in the psychophysical data collected by the step by step brightness matching and minimum flicker methods. These methods yielded a function with irregularities. The function adopted by the CIE had these irregularities mathematically smoothed.

I have been unable to find a definition of brightness in terms of the operation by which it is measured. Brightness depends on the responses of individual observers and viewing conditions. I t has been defined as the "attr ibute of sensation by which an observer is aware of differences of luminance. - 3 This definition provides no metric with respect to luminance, and a one-to-one correspondence is not valid. I propose tha t the term be restricted to data collected by direct brightness judgments or magnitude estimation procedures.

A distinction has been made between the terms lightness and brightness.1,2 Brightness refers to self-luminous objects, while lightness refers to nonself-luminous ones. The term brightness, in this Letter, will be used to include both conditions.

I t is not difficult to interpret mistakenly the concept of lumi­nance as a unit of measurement that unequivocally relates to brightness. The sources of confusion between brightness and luminance may be in part historical, and in part due to current discussions in reference texts. The following are examples of these sources.

(1) At one time the term luminance was called photometric brightness.3 Confusion between the concepts of subjective or apparent brightness and photometric brightness prompted the CIE to adopt the term luminance for the latter concept.

(2) LeGrand1 reported tha t " . . . in particular two sources in juxtaposition (in space or time) appear equally bright if the products VλLe are equal and it is natural to say in this case the luminances are equal."

Letters to the Editor should be addressed to the Editor, APPLIED OPTICS, AFCRL, Bedford, Mass. 01730

(3) The Committee on Colorimetry of the Optical Society of America3 refers to the ordinate of the relative luminous efficiency curve as being "proportional to the efficiency of radiant power for the production of brightness." They also point out that individual variations of luminosity data are large and that the adoption by the CIE of standard luminosity data was an arbi­trary procedure.

(4) An additional source of confusion stems from the CIE, who has given the terms luminance and luminosity different meanings. Luminance is defined by Eq. (1), while luminosity is equivalent to brightness. Since luminance and luminosity have common stems and refer to similar but different concepts this could also be a source of confusion.5 Brenneman and Bartleson6 presented an excellent discussion on the meaning of the terms luminosity and brightness. They pointed out tha t the CIE recommends that "luminosity is to stand for what has been called 'brightness' . . . . " They concluded that "brightness makes semantic sense when used to refer to an attribute of visual perception . . . , whereas luminosity has almost always referred to a stimulus concept."

Might it not make sense to use the word luminosity as a generic term referring to all photometric measures, of which one specifically defined term would be luminance?

Since it can be somewhat difficult to distinguish clearly between the terms luminance and luminosity it is not surprising that the unwary could easily fall into the t rap of confusing luminance with brightness. If these two terms were equivalent, the additivity law (Abney's law) should hold for additivity of brightnesses as well as additivity of luminances. I t does not.7 - 1 0 The CIE, according to Dresler7 " . . . is not concerned with brightness, but with luminances." He concludes tha t brightnesses assessed with even slight color differences are usually nonadditive but, " . . . luminances are additive by definition ...."

There are experiments in addition to those testing Abney's law that differentiate between luminance and brightness. Dres­ler,7 Sanders and Wyszecki,11 and Kaiser12 had subjects match a white light for brightness with a chromatic one. If fields of equal luminance but differing in color were judged as equally bright, the ratios of luminance of the white to chromatic lights should of course equal 1.0. They were almost always greater than unity.

All photometric quantities sanctioned by the CIE refer to a so-called standard observer whose spectral sensitivity has been defined by the CIE. Therefore, this international commission has made it possible to " . . . link up photometry with radiometric quantities . . . " . 1 The definition of luminance as given by Eq. (1) does " . . . not postulate tha t the human eye should under all circumstances be capable of measuring luminances by assessing brightnesses."7 Brightness would be an appropriate term to use when observers are specifically asked to make brightness judgments or magnitude estimates of brightness. Thus, one would not use minimum flicker judgments to match stimuli of various wavelengths for brightness.

Under special circumstances, two fields of equal luminance can be said to be equally bright. These circumstances are when the two fields have identical spectral distributions (or at least are judged to be equal in color) and have identical surrounds.

2768 APPLIED OPTICS / Vol. 10, No. 12 / December 1971

If two fields of equal luminance have different colors and contain different surround characteristics, they can be considered equally bright only when so judged by an observer.

The concept of luminance is appropriate, when any detector (e.g., photoelectric photometer) with a spectral sensitivity identical to the CIE standard observer is used. Another possible reason for a continued confusion between these concepts may lie in a longstanding inability to relate luminance and brightness to specific types of physiological processes. The remainder of this Letter addresses the question: Under what psychophysical procedures is it proper to use the concept lumi­nance and why?

Since brightness and luminance data can differ, and both have a basis in psychophysical judgments, it seems reasonable to seek a physiological correlate based on these differences. Guth et al.13 suggested that brightness is mediated by the combined outputs of the opponent and nonopponent systems, whereas flicker judgments are mediated only by the nonopponent system. Their suggestion is, in my opinion, entirely correct. In fact, an even stronger statement can be made with respect to the nonopponent system. It mediates not only flicker judgments but all luminance judgments. The following discussion provides further evidence for the suggestion of Guth et al.13 as well as the stronger one regarding luminance.

Boynton and his colleagues9-14 hypothesize five types of neural activity in the visual system: four chromatic ones associated with the unique hues red, yellow, green, and blue and one achro­matic associated with white. They assume that the activity of each chromatic element, in addition to giving information about the hue of a colored field also yields brightness information. Similarly the white neural activity yields whiteness and bright­ness information about the colored field. Central to the model is an assumption that when the relative brightnesses of a bipartite field are adjusted to yield a minimally distinct border, the amount of white neural activity associated with each half will be equal. Kaiser et al.14 present several empirical justifications for this assumption.

Based on this model and especially the latter assumption, Boynton and Kaiser9 correctly predicted that Abney's additivity law would hold for bipartite fields if minimally distinct border was used as the criterion response. Based on this additivity experiment and because the minimum flicker criterion also yields additivity,15 Kaiser12 expected and found that the mini­mally distinct border and minimum flicker criteria yield similar results when measuring how much white light was required to match lights of varying wavelengths. This result has recently been verified by Wagner and Boynton.16

Since the flicker criterion yields data similar to luminance7

and since minimally distinct border and minimum flicker yield results as noted above, one would expect that heterochromatic matches made with the minimally distinct border criterion should also yield results similar to luminance. Kaiser found evidence for this prediction also.12

Finally, DeValois and Pease17 have recorded from the lateral geniculate nucleus of the monkey while this animal was viewing white and green fields that were equated for luminance. As the edge formed by the juxtaposition of these fields was moved back and forth across the receptive field from which they re­corded, the frequency of electrophysiological responses increased as the border reached the center of the receptive field and then decreased, producing a function of frequency vs position similar to a bell shaped curve. However, they found that, when achro­matic fields of unequal luminance were used, an additional peak of neural activity was found on each side of the border formed by the bipartite fields. These looked like enhancement effects or Mach bands. From their work and discussions one could conclude that the absence of enhancement effects with heterochromatic

fields of equal luminance would be manifested psychophysically as a minimally distinct border. Ratliff18 reports that " . . . the eye itself sacrifices accuracy about information of little consequence, such as absolute levels of illumination, in order to enhance features that are more significant, such as contours and edges." Thus, one can infer from DeValois et al.17 and from Kaiser12 that when pre­cisely juxtaposed bipartite fields are adjusted for minimally dis­tinct border then their luminances are equal, although their brightnesses may differ.

The evidence in favor of luminance being mediated by the whiteness or nonopponent system can be summarized:

(1) the minimum flicker judgment is made after color fusion occurs;

(2) minimally distinct border occurs when the white neural activity associated with each half of a bipartite field is equal14;

(3) enhancement effects or Mach bands are not evidenced electrophysiologically when fields are equated for luminance17;

(4) minimally distinct border and minimum flicker yield data very similar to direct luminance measures12;

(5) luminance is additive by definition7 and the criteria of minimally distinct border9 and minimum flicker15 both can be used to demonstrate this additivity;

(6) the spectral sensitivity of the nonopponent excitatory cells recorded from the macaque LGN is similar to the CIE photopic luminous efficiency curve.19

The evidence in favor of brightness being mediated by the chromatic or opponent plus the whiteness or nonopponent system is:

(1) chromatic fields are brighter than equally luminous white fields7,11,12;

(2) additivity of brightness is not found, primarily due to the inhibition among the chromatic channels.10

Support from the National Research Council of Canada (APA 295) is acknowledged.

References 1. Y. LeGrand, Light, Colour and Vision (Chapman & Hall,

London, 1968). 2. G. Wyszecki and W. S. Stiles, Color Science (Wiley, New

York, 1967). 3. Committee on Colorimetry, Optical Society of America, The

Science of Color (Optical Society of America, Washington, D. C , 1963).

4. CIE, International Lighting Vocabulary (2nd ed.) I Publica­tion CIE 1.1 (CIE Bureau Central, Paris, 1957); cited by G. Wyszecki and W. S. Stiles, in Color Science (Wiley, New York, 1967).

5. Additional confusion can arise from the adjective luminous (e.g., luminous intensity, luminous flux, etc.). This ad­jective is not related to the noun luminosity but is related to the noun luminance via the luminous efficiency function

6. E. J. Brenneman and C. J. Bartleson, J. Opt. Soc. Am. 56, 983 (1966).

7. A. Dresler, Trans. Ilium. Eng. Soc. (London) 18, 141 (1953). 8. M. Tessier and M. Blottiau, Rev. Opt. 30, 309 (1951). 9. R. M. Boynton and P. K. Kaiser, Science 161, 366 (1968).

10. S. L. Guth, Vis. Res. 7, 319 (1967). 11. C. L. Sanders and G. Wyszecki, CIE Proceedings of 15th

session, Vienna, 1963 (CIE Bureau Central, Paris, 1964). 12. P. K. Kaiser, J. Opt. Soc. Am. 60, 1572A (1970). 13. S. L. Guth, N. J. Donley, and R. T. Marrocco, Vis. Res. 5,

537 (1969). 14. P. K. Kaiser, R. M. Boynton, and P. A. Herzberg, Vis. Res.

in press (1971). 15. H. E. Ives, Phil. Mag. Ser. 6, 24, 845 (1912).

December 1971 / Vol. 10, No. 12 / APPLIED OPTICS 2769

16. G. Wagner and R. M. Boynton, University of Rochester, per­sonal communication.

17. R. L. DeValois and P. Pease, Science 171, 694 (1971). 18. F. Ratliff, Mach Bands (Holden-Day, San Francisco, 1965). 19. R. L. DeValois, I. Abramov, and G. H. Jacobs, J. Opt. Soc.

Am. 56, 966 (1966).

2770 APPLIED OPTICS / Vol. 10, No. 12 / December 1971