Programs available for generating ishihara plates

I've been attempting to make my own ishihara plate however when I rasterize to grayscale its clear my attempt did not work.

I'm wondering if anyone has any suggestions on programs available to "ishihara-ize" an image. My goals was to make up a checkerboard (of different colour combos i.e red and green) and when grayscale the image no longer held the square boundaries of the checkers on the board.

I appreciate any help thank you in advance!