Color rendering index: Difference between revisions

Revision as of 13:32, 13 March 2007

Revision as of 23:19, 3 April 2007

Line 13:

which gives the color rendering index with respect to each sample.

The factor 4.6 was so chosen that the Ra of a standard warm-white Thalium lamp would be about 50. It also appeared that Ri could be negative (∆Ei ≥ 22), and this was indeed

calculated for some lamp test colourscolors

The general color rendering index <math>R_a</math> is then the average of these eight separate indices.

:<math>R_a=\frac{1}{8}\sum_{i=1}^8 R_i</math>

In 1965, in order to be able to objectively compare the colourcolor rendering

properties of light sources, the CIE introduced a standardised measuring

method. This method calculates the colourcolor change of 14 test colourscolors under

the light source being tested relative to the colourscolors measured under a

reference illuminant. The first 8 test colourscolors are relatively non-saturated

colourscolors and are evenly distributed over the complete range of hues.

These 8 test colourscolors are employed to calculate the general colourcolor

rendering index Ra. The last 6 colourscolors (numbered 9 to 14) are employed to

supply extra information about the colourcolor rendering properties of the light

sources.

Line 34:

importance of Ri decreases as its value relative to 100 increases. This is

even more true for the Ra, which is the average of 8 individual Ri values,

and which gives only a global impression of the colourcolor rendering properties

of a light source. Indeed, in practice it can occur that a light source with

Ra = 85 is not always better than a light source whose Ra = 80.

A second disadvantage of the Ra value is the fact that it gives no

information as to the direction of the colourcolor shift. A colourcolor can be more

saturated or less saturated without a change in the numerical value of ∆Ei,

while in general a saturated colourcolor is experienced as being more attractive.

An attempt at rectifying this has been made by the introduction of the

ColourColor Discrimination Index (CDI). Here the surface of the octagon is

formed by the eight test colourscolors in the u,v diagram as a measure of the

colourcolor rendering quality. A smaller surface means less saturated, pale

colourscolors. A larger surface means greater saturation, stronger contrasts,

more lively, and so on. The objection to this method is that the principle of

true-to-nature colourcolor rendering is abandoned. It also appears that equal

surfaces do not always correspond to equal visual assessments. The CIE

is rather hesitant about this method. The same goes for the so-called

ColourColor Preference Index (CPI) in which even greater emphasis is placed

on the flattering rendition of well-known objects (butter, grass, skin colourcolor,

etc.).

Line 60:

== References ==

* CIE Publication 13.3. Method of measuring & specifying colour rendering properties of light sources. CIE,1995.

==External link==