Untitled - Journal of Lighting Engineering

appearance under a reference illuminant”
describes the above colour fidelity. Fidelity,
preference and discrimination have not
been defined in the CIE International
Lighting Vocabulary yet.
3. Colour fidelity
The first colour fidelity/rendering index 6
was based on only chromaticity differences,
but as a three dimensional colour difference
metric became available, it was extended to
three dimensional description 7 . Despite the
fact that the colour rendering index (CRI) is
in practically unchanged form current,
during the past almost forty years many
attempts were made to update the method,
Colour rendering – a personal view
or propose alternative techniques (for a
review see 14, 15 ).
Some authors questioned whether the
use of reference illuminants of equal
correlated colour temperature (CCT) as that
of the test source is a good technique, or
one could use a single reference
illuminant 16 . We have discussed the
question of using a single reference
illuminant as early as in 1981 17, and we
have shown that if the influence of different
light sources on the colour appearance on
objects is tested in a simulated scene, the
rank order of light sources changes
drastically compared to that of the
prediction of the current CRI test method 18, 19 .
Figure 1 CIE Ra versus visual rank order of different light sources based on a computer simulation.
Figure 1 shows the results of the
computer simulation experiment: Pictures,
where the spectral reflectance was known
pixel by pixel, were rendered using
different light source spectra and
chromatically transformed to D65 using
CIECAM02 colour appearance model, and
were compared to the rendered picture
using illumination with D65 illuminant.
The CIE CRI Ra values are shown for the
visual evaluation rank order. For D65 the
best position (rank order 0) is obvious. But
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