Contrast

When you determine the right contrast for your LPR camera, consider the difference in gray value (when images are converted to 8-bit grayscale) between the license plate’s characters and the license plate’s background color:

Good contrast

Acceptable contrast; recognition is still possible

Pixels in an 8-bit grayscale image can have color values ranging from 0 to 255, where grayscale value 0 is absolute black and 255 is absolute white. When you convert your input image to an 8-bit grayscale image, the minimum pixel value difference between a pixel in the text and a pixel in the background should be at least 15.

Note that noise in the image, the use of compression, the light conditions, and similar can make it difficult to determine the colors of a license plate’s characters and background.