PRIME COLOR, INCORPORATED
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Certified
Color-Matching Functions
A major problem arose around
1965 when we at Westinghouse Lamp Divisions began to develop what became
known as TRIPHOSPHOR LAMPS.
The CIE Standard Observers
could not properly evaluate either brightness or color-rendering of the new
lamps.
So we had to leave them
behind, and the new lamps spread worldwide, because the lamplight was both
brighter and rendered colors better than conventional lamplights of the
time.
The Problem: Strong Metamerism. New lamplight versus old.
Strong metamerism stresses
color-matching functions.
Because, when matching lights
are so different spectrally,
the
color-matching functions meant
to substitute for the spectral sensitivities of the human visual system
must be very precisely
identical to those
visual-system sensitivities.
CIE 2-degree
color-matching functions
These DEFINE the Standard Observer. The 10-degree observer
(larger visual field) is similar.
Their function is to compute the red, green and blue power content of each incoming light.
R
G
B
Approximation
to Rainbow Coloration
Spectral
Power Distribution of Average Daylight
Reminder:
Redefine strong metamerism
The admirable work done 70
years ago rested on assumptions, necessary then, which turn out to be
incorrect.
Today’s far more accurate
measurement capabilities, together with better means of extracting the
needed functions from the color-matching data, have led to color-matching
functions that are better representative of the normal human
visual system.
n
Certified color-matching
functions
The New Approach
Step 1:
Start with the strongly metameric lights of the sort that
cause all the trouble.
Step 2:
Extract the new CMFs directly from the troublesome
examples, thus ensuring that the new
CMFs work under the worst of conditions.
Step 3:
Start with a simple shape, and nibble at it, changing
its shape incrementally, reducing the errors bit by bit until they are
essentially gone.
Example of the new CMFs
The
new functions: They are not pretty…
But they WORK
a*b* color diagram
New (dotted) color diagram
The new color diagram has some unfamiliar
features:
1. The
green region of the traditional diagram
is swollen…as seen by the large MacAdam
ellipses. In the new
diagram, the swelling has subsided.
2.
In the new diagram, but not in the old, the points representing
adjacent spectral colors on its periphery are clustered in the spectral
region of the peaks of the sensitivity curves. This is to be expected.
Traditional color diagram
New (dotted) color diagram
a*b* color diagram
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