Abstract: In this paper,the instrument for observing the illumination effects in diamond,invented by Professor Yuan Xinqiang,is used to capture the actual image pattern of 48round brilliant cut cubic zirconia(CZ)under the incident light with angle of 7.12degree to 12.58 degree.The cutting parameter of samples is diverse from each other.Table width ranges from 57%to 63%and crown angle ranges from 25degree to 42degree,with pavilion angle changing from 36degree to 42degree.The actual image pattern is a real colour bitmap file,and the pixel is comprise of red,green and blue.The quantification procedure transforms RGB colour space to YUV colour space.The Ycomponent determines the brightness of the colour and refers to luminance or luma,which ranges from 0to 255in digital formats.Accumulating Yvalue of all pixels of the actual image pattern and the summation are regarded as the brilliance of the sample.Then the brightness value is normalized and the contour map is done with crown angle as the X-axis and pavilion angle as Y-axis and brightness values as Z-axis.Brightness values of several samples are relatively high and they can be easily found in the map.The crown angle ranges from 27degree to 39degree,and pavilion angle ranges from 38.5degree to 41.5degree.However,not all the samples in the scope own the high brightness value.The change of brightness values over cutting parameters is discontinuous uneven.Sometimes,the brightness values vary considerably,even their cutting parameters of the samples are very similar.While sometimes the brightness values are similar,but their cutting parameters are quite different.The computer 3D modeling system created by Shibin is used to simulate the Viewable Brightness Return(VBR)and virtual image pattern of the round brilliant cut CZ.Contrasting the similarities and differences between virtual image patterns and actual image patterns,we can find that virtual image patterns and actual image patterns of 36samples are generally consistent,as symmetrical characteristic of these samples is good and cutting parameter is exact.However,examining the image patterns carefully,there are still some differences in details.Obviously,the virtual image patterns are perfect,and the actual image patterns have some irregular bright and dark regions.The phenomenon is possible because of the complicated path of the light,which goes through the CZ samples.The light may deviate from the ideal light path while the cutting parameter,the symmetrical characteristic or the internal characteristics vary.The virtual image patterns and actual image patterns of the other 12samples are different and some even show opposite patterns.Their symmetrical characteristics are bad,such as bigger than 12%ovality,culet significantly deviating from the center,uneven thickness of girdle.In a conclusion,symmetrical characteristic has a great influence on the actual illumination effects of the samples,which should not be neglected.Calculating the brightness values of the virtual image patterns,comparing and analyzing the brightness values and the simulative VBR,the authors find variation tendencies of the VBR and the brightness values are similar.It is proved that the method used to deal with the image pattern is accurate and reasonable.The results show that the symmetrical characteristics or subtle differences of some parameters may lead to big changes of brilliance to a certain range.The reverse change of crown angle and pavilion angle can produce a similar good brilliance.The method of measuring the appearance to evaluate the illumination effects of round brilliant diamond is viable.