Reply Letter to March 1993 SID Information Display article "Brightness, Luminance, and Confusion"

More confusion??

July/August 1993 - Information Display

Referring to Charles P. Halsted's "Brightness, luminance, and confusion,'- appearing in the March 1993 issue, there are some unfortunate confusions in his otherwise commendable clarification of the language of displays. The main problem is in the definition of projected area. Projection in this sense does not involve lenses or optical systems. Rather, the term comes from geometry, and is illustrated in Fig. 1. The projection of a line (or surface) onto another line or surface is simply the length (or area) defined by dropping perpendiculars from one surface to the other. Hence. the projection of line AB on line CD has the length (AB cos Theta). (This is where the cosine term in luminance comes from.) lamhertian surface has a luminous intensity in any direction which is proportional to the cosine of the normal to the surface. The projected area seen from that direction is also proportional to that same cosine. so the luminance B which is the luminous intensity in some direction divided by the area projected in the same direction, is B = (I0 cos Theta)/(A0 cos Theta) = I0/A0, which is independent of the angle (see Fig. 2) . The luminance of a lambertian surface is independent of the viewing angle.

A few other minor matters:

There is some confusion in the literature about the definition of light. Some authors use light to mean only visible radiation, while others use the term to mean any electromagnetic radiation between radio frequencies and x rays (both terms which are also often carelessly defined). It might be clearer to use the term visible light. In discussing brightness, Halsted writes that "The stimulus is non-linear and complex." I suggest that better language is that the response is non-linear and complex; the stimulus, luminous flux, is a simple, measurable quantity. It is also noteworthy that while the eye is capable of operating over an enormous range of luminance's, this is not true at any one instant: the eye needs time to adapt to a particular average level of luminance.

The thrust of Mr. Halsted's article is that we should use the right words; this will help keep the concepts behind them clear. I heartily applaud this idea. but we must be careful not to introduce new errors as we strive to eradicate the old ones.

Alan Sobel Consultant Evanston, lllinois

(Dr. Sobel is a consultant. The Editor of the Journal of the SID. and a Contributing Editor to Information Display.)

The author replies:

I greatly appreciate Dr. Sobel's comments on my article. Dr. Sobel's description of the geometrical concept ''projected area'' is correct, but he is not describing the same thing I did. We have a situation here in which one expression is used to denote two different entities. In pho- tometry, what I called 'projected area'' is also called "measured area'' or ''measurement field.'' Using one of these terms would have avoided this confusion.

Dr. Sobel mentions the two definitions of light, as I did in the article. For historical and practical reasons, physicists often include ultraviolet and infrared wavelengths in their definition of light. and this is logical in the context of atomic physics. It is not logical or helpful in the context of displays and applied vision. Luminance and illuminance measurements only involve radiant power visible to humans. The CIE chromaticity diagram does not include UV and IR radiation. If a display is not emitting wavelengths humans can see, nobody will be interested in it. I do not believe that there should be two kinds of light: that which we measure with photometers and that which includes UV and IR. The word "stimulus" was inadvertently substituted for "response" during the revision process. I thank Dr. Sobel for bringing this to the attention of my readers.

Charles P. Halsted Naval Air Warfare Center Warminster, PA

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