Chromostereopsis due to eye glasses in patients with different characteristics of natural chromostereopsis
In modern ophthalmological practice, optical correction often includes some prismatic components. In treating strabismus, it is usual to employ eye glasses with prisms compensating deviation of the squinting eye in order to save and develop binocular vision. The ordinary eye glasses with spherical lens can also exhibit a prismatic action if the distance between the lens centers in the rim does not equal to the proper value determined by the inter pupil distance and the position of the observed object. Since the refraction of the short-wave and long-wave light rays is different, the prismatic eye glasses demonstrate chromatic aberration leading to relative shift of the blue and red images on the retina. If the prisms are fixed symmetrically in the eye glasses (both bases to the nose or to the temples) this shift appears to be equivalent to a retinal disparity producing an illusion of difference in distance to objectively equidistant visual stimuli. This stereo effect of prismatic eye glasses depends on the prism power, interpupil distance and the refractive indexes for different wavelengths. In this study, it has been found that the prismatic effect is not the same in all patients due to existence of natural chromostereopsis that is determined by the individual properties of the optical and sensory apparatus of the subject's eyes as well as of his central visual mechanisms. The contribution of this effect should be taken into consideration in the case of prismatic correction.