The spatial contrast sensitivity model given in the previous chapter is restricted to the normal situation of foveal vision. At foveal vision, the eyes of an observer are directed to an object in such a way that the center of the object is imaged on the center of the retina where the contrast sensitivity of the eye is maximum. This process is called fovea! fixation. In daily practice, for instance in traffic, it is also important that the eye can observe objects that are outside the area on which the main attention is concentrated. In this chapter the model given in the previous chapter will be extended, so that it can also be used for extra-fovea! vision. Outside the fovea, the contrast sensitivity and the resolution of the eye is much less. To measure the local contrast sensitivity outside the fovea, a marker is usually placed in the object plane and the observer is asked to fixate his eye on this marker, while the actual object is placed at some distance from the marker. This distance is usually expressed in an angular measure called eccentricity and the contrast sensitivity is measured as a function of eccentricity. As the instruction to fixate the eye on the marker is not always easy to follow, extra-fovea! contrast sensitivity measurements usually show more spread than fovea! contrast sensitivity measurements. The fovea! area has a diameter of about 1°. Although the center of the object at fovea! vision is imaged on the fovea, a large part of the image will usually also cover the retinal area outside the fovea. At extra-fovea! vision, the center of the object is imaged outside the fovea, but a part of the image can still cover the fovea! area.
For the extension of the contrast sensitivity model to extra-fovea! vision, it is sufficient to adapt the constants used in the model as a function of eccentricity. It may be assumed that the variation of the constants with eccentricity is caused by the density variation of the cones and ganglion cells over the retina. Therefore, first some approximation formulae will be given for the density distribution of these types of cells over the retina, then the effect of these cell types on the contrast sensitivity will be analyzed, and finally the so extended contrast sensitivity model will be compared with various published measurements.
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