ThisletterintroducescolorconstancyandRetinextheoryforimageenhancement.Itclas-sifiesRetinexalgorithmsintofourcategoriesandprovidestheirprinciplesandimplementationsingeneral.TheexperimentalresultsofFrankle-McCann,MSR(Multi-ScaleRetinex)andPNSD(Pro-jectedNormalizedSteepestDescent)Retinexalgorithmsarepresentedandcompared.Moreover,varianceandaveragegradientareproposedtoevaluatetheperformanceofthedifferentalgorithms.