Abstract: The main illumination effects are brilliance, fire, and scintillation.In this paper, we took photographs of some diamond samples in the light source with different incident angles in order to evaluate the fire of round brilliant cut diamond.The light source was composed of LED light arranged in six circle, with incident angles at 14.5°, 18.5°, 21.5°, 25.5°, 28.5° and 32.5°.The photographs by software programmed were BMP bitmap files.The software could capture the RGB colour model of each spot in the image, and converse the RGB colour space into HSI (Hue, Saturation, Intensity) colour space.Until the value of saturation is more than 25%, and the value of intensity is more than 20%, the pixel point can be regarded as fire, which add up intensity of all pixel points.The fire value is equal to the sum of intensity divided by the number of the pixel points related to the diameter of the diamond.The result showed that the accuracy and stability of system were enhanced by averaging the values of tests.
There are 48 diamond samples with table 56%±1%, star facet 53%±5%, lower girdle facet 76%±6%, girdle 3%±0.5%, the crown angle range from 27° to 40° and the pavilion angle range from 38° to 43°.The fire value of 48 diamond samples are 2.0-12.0.Observing the images of diamond by naked eye, the fire value was quite higher when the colour area in the image is more, the test results were consistent with the feeling of people.The fire value is irregular between the fire and crown angle or pavilion angle of round brilliant cut diamond.The ideal cut diamond (suggested by Tolkowsky M) really can present good fire.
In conclusion, the method to evaluate the fire in round brilliant cut diamond is exact.The diamonds with pavilion angle at 40° or 41° and the crown angle at 35° or 39° can display better fire.The relationships among the fire, crown angle, and pavilion angle are nonlinear, it is necessary to evaluate the fire in round brilliant cut diamond.