are three types of cone or color sensing cells within the eye which are typically called long wavelength medium wavelength and short wavelength sensors l m and s cones as shown in fig1 the frequency response of the l and m cones is broad whereas the s cones have a narrow frequency response and although there is considerable overlap in the sensitivities of the l and m cones there is relatively little such overlap in the frequency response of the s cones it is well known that there are no s cones in the most sensitive region of the retina which is called the fovea whereas most regions of the retina consist of a mixture of rod and cone sensing cells the fovea only contains cone type cells and only those which are of the l and m types s type cones are only detected in the periphery of the fovea region and their spatial distribution is much lower than either the l or m cones it is estimated that there are less than one sixteenth as many s cones as either l or m cones in the retina since short wavelength sensing cells s cones do not occur in the central region of the fovea it is well known that it is visually impossible to attain focus on an object emitting only shortwave length e g blue only the elements of the image are defocused by virtue of the achromatic characteristics of the lens of the eye and also by the absence of appropriate sensor cells within the prime focus region of the retina the combination of defocused short wavelength blue light and lack of s cones in the area of.