To study the fire of a diamond, the GIA group uses an infinitely distant point source, just the same configuration as that shown in Fig. 2. In such a model, a light beam passed through a prism consists of only colored rays at an infinite distance from the prism.

Therefore:

In the GIA model, the probability of observing a green ray is the same as that for any other color. In real diamonds, green rays are observed much more rarely than blue and yellow rays;

GIA metric for dispersed color light return DCLR is almost independent of dispersion. For example, if we consider a material, the dispersion of which is two times weaker than that of diamond, the DCLR will be almost the same as that for diamond! Moreover, if we suppose that there is absolutely no dispersion in the material of the considered stone, the DCLR not only fails to become zero, but even remains close to the value calculated by GIA (just dispersion, not refractive index). Let us consider green color, which is located in the center of visual color range and red and blue colors lie on opposite sides of the green color, at equal distances on visual spectrum. The deviation of blue and red spots from green spot on registration sphere depends on the dispersion of the material. In any case, these deviations are small enough and almost equal one another. ^Note

When calculating DCLR, the GIA team performs summation. As a result, the red and blue rays compensate each other, and their overall mean DCLR is almost equal to that of the central green ray. Then let us change (decrease) dispersion value of material fixing refractive index for green color. The location of all such green spots on the sphere used by the GIA group for registration will not change, and blue and red spots will be closer to green. When calculating DCLR blue and red rays again compensate each other and we will get the same DCLR value as before!

This statement can be illustrated by the following formula:

Here for each arbitrary light beam:
a - direction of green ray
2d - angle between red and violet rays.

It is clear from the common sense that if dispersion equal zero fire will not appear. Thus, the following questions arise: What is the merit of the GIA's approach from the viewpoint of modeling the fire and what does the DCLR metric actually describe?

Important notes : 1. Actually blue ray can be located further from green than red ray, especially when rays refract from diamond at the angle close to critical. It does not influence on DCLR, however it influences on picture really observed. This is one reason why we observe blue color more often that red, and why most frequently observable colors in diamond viewed at close distance - blue and yellow. Here authors consider angles near critical and smaller it on 2-3 degrees (for example range from 16 up to 21 degrees.) At the large angles the refraction losses are sharply increased, and also the beam area changes, that results in sharp loss of intensity and decreasing of possibility to observe any color. 2. The second reason why more often we can see blue and yellow colors when look at diamond from close distance or use photo camera ("Macro" mode) is color mixture properties. Taking range of spectrum from blue to green and adding green color a little we will get cyan color as a mixture. The range of spectrum from red to green with addition of the same little amount of green will be mixed into orange. Under real not very bright illumination conditions a human vision will see cyan as blue color, and orange as yellow or brown color, not as red.

We have just shown that the DCLR takes into account fire that does not actually exist. It is also important to show that the DCLR does no take into account fire that does actually exist. The problem is that the GIA's illumination source produces only one type of rays, namely, those rays normally incident on the table of the diamond. In reality, a ray may be incident on the diamond at any angle. As far as we know, observers usually grade a diamond, holding it in the face-up position. In this case, the fire they observe originates from light incident not normally to the table plane (typical range of incidence angles is 10-15 degrees, with respect to the table normal). Anyone who has a diamond, may observe the fire phenomenon with a source that obliquely illuminates the table of the diamond. Thus, the DCLR coefficient does not take into account a considerable portion of the actual fire, maybe the most of it. This is also due to the fact that the illumination model is inadequate.

As it can be seen, the results obtained by the GIA group contradict practice. The inadequacy of the illumination model is the main reason of this contradiction. A real light source emits rays in all directions.

Therefore, not parallel but diverging rays enter the prism (see Fig.3 ). If the dispersion angle of the material of the prism is less than the divergence angle of the incident light, there are some white rays at any distance from the prism. Moreover, in the most of real cases, the white rays prevail over the colored.

Therefore, in practice green rays are observed rarely, and when the dispersion decreases, colored rays disappear, leaving only white-light beam to observe.