A filter mask for use in a flow cytometer includes light blocking features and light passing apertures. The flow cytometer operates to evaluate one or more characteristics of a sample by illuminating the sample and a carrier fluid and collecting light rays that are radiated from the sample and the carrier fluid. The light rays are passed through the filter mask. The light blocking features of the filter mask are arranged to selectively block radiated light at certain radiation angles, while permit light rays having other radiation angles to pass therethrough. A sensor analyzer receives the light rays that pass through to evaluate at least one characteristic of the sample. The light rays can also be separated into two beams, which can be independently filtered using different filter masks. The results can then be compared to provide even more information regarding characteristics of the sample.