Reply Letter to March 1993 SID Information Display article
"Brightness, Luminance, and 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
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
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.
(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
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.
Return to the prior Document
Charles P. Halsted
Naval Air Warfare Center