Untitled - Journal of Lighting Engineering

J SCHANDA
as usual. With the introduction of the gas
discharge lamps that rendered the colours of
the environment differently as the
incandescent lamp or daylight, it became
necessary to introduce a metric to describe
the colour rendering properties of the
different types of light sources 1 . The first
CIE colour rendering index was based on
the dissimilarity of the spectrum of the test
and a reference light source 2 . But the
purpose of a colour rendering index is to
find a correlate of the visual impression the
observer has when viewing the illuminated
scene. Thus researchers looked for
alternative methods. One of these was
based on the colour difference of test
samples illuminated with a test and a
reference source 3,u4 . CIE decided to
introduce a recommendation to evaluate
colour rendering of light sources by the test
sample method 5,u6 . CIE published an
updated, revised edition of this publication
in 1974 7 , and republished it later with
minor editorial changes 8 .
Soon it was observed that one can tailor
the spectrum of a lamp in such a form that
objects look nicer under the modified
spectrum as under daylight or incandescent
light. Judd coined the term flattery index to
describe this as early as in 1967 9 . Later the
term was modified to colour preference, to
soften the negative over tune of “flattery”.
A source with a high colour flattery index
might falsify the observer and show e.g a
not too fresh meet to be quite appetizing.
There are applications where the
inspection under a light source is intended
to discriminate between small colour
differences, i.e. high colour discrimination
ability is required. To make the selection of
light sources for such purposes easier, the
concept of colour discrimination ability,
and the colour discrimination index were
constructed 10, 11 .
Neither the colour preference, nor the
colour discrimination index got yet general
acceptance, although much research went
into both directions recently.
2. The three aspects of light source
colour rendering
As shortly mentioned in the Introduction
light source colour rendering can be
subdivided to give answers on the
following three questions:
• how well a source reproduces
colours compared to a reference
illuminant, this we call now colour
fidelity;
• whether the source provides a more
pleasing atmosphere of the scene, or
of some objects, compared to their
colour appearance under a reference
illuminant, this we call colour
preference;
• how easy it is to distinguish between
small colour differences under the
given light source, this we call
colour discrimination.
An early overview of all three questions
was delivered in a paper presented at the
Plovdiv colour conference in 1980 12 . For all
three aspects indices have been developed,
thus they can be measured by the colour
fidelity, colour preference and colour
discrimination indices. To be exact the still
valid definition of colour rendering 13
“effect of an illuminant on the colour
appearance of objects by conscious or
subconscious comparison with their colour
48 Ingineria Iluminatului 2012; 14, 2: 47-54