Characteristics of Visual Perception: Brightness


Equal amounts of radiant flux (a purely objective or physical measure) of different wavelengths do not produce visual sensations of equal brightness (a purely subjective measure). Careful experiments have been made with a very large number of observers to determine the relative effectiveness of monochromatic radiant flux in evoking the brightness sensation.
 

If the energy output of each of these three lights is equal, yellow
will appear to be the brightest light, green will appear somewhat
less bright and red will appear to be the least bright!

 
 
 

 

 

The Photometric System

Light flux, for the purposes of illumination engineering, is measured in lumens.  A lumen of light, no matter what its wavelength (color), appears equally bright to the human eye.  As we have seen, the human eye has a stronger response to some wavelengths of light than to others.  The strongest response for the light-adapted eye (i.e.,when scene luminance > .001 Lambert) comes at a wavelength of 555 nm.  A light-adapted eye is said to be operating in the photopic region.  A dark-adapted eye is operating in the scotopic region (i.e.,scene luminance </= 10^-8 Lambert).  In between is the mesopic region.  Again as we have seen, the peak response of the eye shifts from 555 nm to 510 nm as scene luminance is decreased from the photopic region to the scotopic region.

The standard lumen is approximately 1/680 of a watt of radiant energy at 555 nm.  Standard values for other wavelengths are based on the photopic response curve and are given with two-place accuracy by the table below. The values are correct no matter what region you're operating in - they're based only on the photopic region. If you're operating in a different region, there are corrections to apply to obtain the eye's relative response, but this doesn't change the standard values given below.

Wavelength, nm
Lumens/watt
Watts/lumens
400
 0.27
3.704
450
26
0.038
500
220
0.005
550
680
0.001
600 
430
0.002
650
73
0.014
700
2.8
0.357


See,  for example, the discussion of the Heterochromatic Brightness Matching Method which makes use of the following protocol:
 
The test wavelengths are placed in one hemifield and the reference in the other. The observer's task is to adjust the chromatic test field until it is equally bright to the reference field. The energy (radiance) of the equally bright chromatic test field is then measured with a radiometer.

This procedure is repeated for each of the test field wavelengths. The reciprocal of these radiances is plotted as a function of wavelength which produces a graph variously called a spectral sensitivity function, luminous efficiency function, relative luminance function.


This page was prepared and is maintained by R. Victor Jones
Comments to: jones@deas.harvard.edu.

Last updated November 31, 1999