New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
Wavelength /nrn
IO
Figure 5. Spectral tristimulus values. The amounts of the red (rh), green (gh) and blue (bh) primaries needed by a normal observer
to match each of the colours of the equal energy spectrum (CIE 1931).
So far, we have seen that the quantities for the red, blue and green primaries can be obtained
experimentally to match a test la,mp. Now we have the spectral tristimulus values, we can calculate
the quantities of the matching primaries from the spectrum produced by the test lamp. We
split the spectrum into small wavelength divisions. The spectral energy at each division Eh is
multiplied by the corresponding tristimulus value at that wavelength; this product is summed,
and normalized as shown in figure 6. We should get the same value, using this calculation, as
would an observer experimentally matching the three lamps to get (Y), (g) and (b).
THE CIE CHROMATICITY DIAGRAM
These three variables for different sources may be represented on a three dimensional plot, but
this is obviously inconvenient. To ensure that the results have practical interest, CIE made three
adjustments. The first involved the luminance of the source. CIE defined new co-ordinates
(X, Y and 2) where
X = 2.7869 (Y) + 0.38159 (g) + 18.801 (b)
Y= (Y) + (s) + (b 1
Z= 0.12307 (s) + 93.066 (b)
We can now easily calculate X, Y and 2 from our measured values of (Y), (g) and (b).
It wil be seen that, since the luminance of the source wil be the sum of the luminances of the
three matching primaries, Y is the luminance of the source in the psycho-physical unit lumens.
198