The degenerate illumination model for the optics camera system is introduced

and a fast method for gray-level correction is presented. It needn't additional parameters and mathematic model

only performs on the image. It corrects degenerate illumination based on the strategy of original image subtraction from background image. Generally say

the method includes three steps: first

background subset extraction. The image is divided into blocks

then according to the image characteristic

one point is extracted as the background point for each block; second

the whole background building. The whole background image can be got by interpolation from the background subset extracted in previous step; then the last step is correction using subtract the background image from original image This method can not only keep the object of original image

but also be efficacious

fast and simple. It has proved good result from the crack detection of surface of highway.