Recently, injection molded plastic optical products are widely used in many fields, because injection molding process has advantages of low cost and high productivity. However, there remains residual birefringence and residual stresses originated from flow history and differential cooling. The present study focused on developing a technique to measure the birefringence in transparent injection-molded optical plastic parts using two methods as follows: (i) the two colored laser method, (ii) the R-G-B separation method of white light. The main idea of both methods came from the fact that more information can be obtained from the distribution of retardation caused by different wavelengths. The comparison between two methods is demonstrated for the same sample of which retardation is up to 850 nm.