To investigate the effect of preprocessing techniques including contrast enhancement and illumination correction on retinal image quality, a comparative study was carried out.

We studied and implemented a few illumination correction and contrast enhancement techniques on color retinal images to find out the best technique for optimum image enhancement. To compare and choose the best illumination correction technique we analyzed the corrected red and green components of color retinal images statistically and visually. The two contrast enhancement techniques were analyzed using a vessel segmentation algorithm by calculating the sensitivity and specificity. The statistical evaluation of the illumination correction techniques were carried out by calculating the coefficients of variation (CVs).

The dividing method using the median filter to estimate background illumination showed the lowest CVs in the red component. The quotient and homomorphic filtering methods after the dividing method presented good results based on their low CVs. The contrast limited adaptive histogram equalization (CLAHE) increased the sensitivity of the vessel segmentation algorithm up to 5% in the same amount of accuracy. The CLAHE technique has a higher sensitivity than the polynomial transformation operator as a contrast enhancement technique for vessel segmentation.

Three techniques including the dividing method using the median filter to estimate background, quotient based and homomorphic filtering were found as the effective illumination correction techniques based on a statistical evaluation. Applying the local contrast enhancement technique, such as CLAHE, for fundus images presented good potentials in enhancing the vasculature segmentation.