An improved test for quantifying luster of petroleum products in particle form, especially coke, as an index of quality. A representative sample of the product is obtained and prepared. A digital image of the sample is formed using a scanner. The image is then processed digitally to produce a representative luster or optical density measurement for the sample. The preceding process may be repeated several times for each sample and the resultant luster or optical density measurements for each iteration are totaled and averaged. Once the luster measurement or optical density for the sample is obtained, it is compared to established parameters to assign a CTE value to the sample, assuming there is sufficient historical data correlating the two measurements. Various refining operating parameters including feedstocks, temperatures and pressures, may be altered to obtain a desired product.