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Color Temperature -- The ICI (CIE) Chart

Lots of strange comments, here, on perceptions of the different color 
temperatures of lamps makes me want to say "WHOA!" :-)

First, go to:


and examine the good chart provided at the top of the page.

This is effectively a slice through the entire human vision coordinate 
space. [Our ability to see color drops as the light gets too bright or two 
dark, so the 3D shape is about like a sweet potato, with points at the 
black (dark) and white (too bright) limits. That brightness axis is 
perpendicular to the chart, as shown. See the gray scale, 2 pictures down 
the page.]

A dark line is plotted through the color space, running from red through 
bluish-white. Color Temperatures fall on this line, or they are not white. 
Yes, some tubes are far from white. Nevertheless, the color temperatures 
are defined as being on or imperceptably close to the line. Every color 
temperature can be given a set of x and y coordinates defining its exact 
location on the chart.

A tube that looks magenta or greenish is well off that white curve and 
will probably have a very low CRI, as a result. CRI is how much the lamp 
makes colors appear to be illuminated by a true white source. That has 
little to do with its color temperature. IMHO, manufacturers shouldn't try 
to use color temperature on lamps below a CRI of 75 or so. To do so can be 
nearly meaningless.

Study that curved black line and note where color temperatures lie along 
it. The difference between 5000K and 6500K isn't a whole lot. The old B&W 
TV phosphors used to look very blue when we saw them through a window from 
outside. That's because the color temperature of those phosphors was about 
8600K. Old incadescent lamps (before they tinkered with them so much) were 
about 3200K and looked very yellow compared to the "white" of the TV.

I hope this helps some to see what color temperature is all about.


PS. The chart was generated by the International Committee on Illumination 
about 60-70 years ago. [The French initials are sometimes still used 
despite our better judgement. ;-)] The purpose was to provide standards 
and definitions for decorating store windows, displaying meat, etc. They 
did the human testing to define Lumens, Lux, and a host of other 
"psychophysical" or photometric units, as opposed to the radiometric units 
used by physicists and engineers (Watts, Einsteins, etc.).

Wright Huntley -- 209 521-0557 -- 731 Loletta Ave, Modesto CA 95351

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