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

New Trends in Physics Teaching IV
White light sources
The chromaticity diagram was specified in terms of an equal energy spectrum, so for such a
spectrum of white light, x = 1/3, y = 1/3 and z = 1/3. The white light found in nature does not
normally have these values. Hence, CIE defined three standard illuminants, A, B and C.
When a given density of electromagnetic radiation is in thermal equilibrium with matter, as,
for example, in a furnace, it has a characteristic spectrum called ‘black body’ radiation. If we
peek through a small hole in a furnace wall, we see this characteristic spectrum, red coloured at
low temperatures, white when hot - and the furnace temperature at which this colour exists
is called the colour temperature. Many bodies, such as a filament lamp, or the sun, give approximate
black body spectra, so CIE found it useful to employ simulations of black bodies at three
colour temperatures for their standards.
Standard A was a gas-filled incandescent lamp at a temperature of 2856 K, giving x = 0.4476,
y = 0.4074, z = 0.1450. Standard B approximated sunlight at a temperature of 4870 K, giving
x = 0.3486, y = 0.35 16, z = 0.3000, and Standard C approximated daylight, which, because of
the blue sky, is at a higher temperature 6770 K, giving x = 0.3 100, y = 0.3 162 and z = 0.3738.
(The three standard illuminants were actually obtained by passing the light from an incandescent
lamp through filters.) These three points are plotted on the chromaticity diagram in figure 9,
together with the curve of black body radiation.
0.8
0.6
Y
500
0.4
0.2
0 0 2 04 06 08
X
Figure 9. The CIE diagram for black body radiation at different temperatures in K, showing also the CIE standard illuminants
A, B and C.
202