[A new high-power biconcave contact lens for observation and coagulation tasks of the posterior segment].
At the present time, positive, wide-angle contact (and preset) lenses, having a large field of view, are widely used for inspecting and irradiating the fundus. They provide a real, inverted image of the fundus, but illuminate only the slit projected. The wide-field they provide (125 degrees ) is considered to be an advantage, but their image inversion and the lack of illumination of the whole field is disadvantageous. Contact lenses of the Goldmann type have a comparably restricted field of view (standard Goldmann: 46 degrees, CGR 3: 53 degrees). They illuminate the whole field inspected, but it is necessary to use internal mirrors when looking at the periphery. The new contact lens has a lateral magnification of 0.44 and a field of view of 64 degrees. Owing to the prismatics of its biconcave structure, the field of view may be shifted by tilting the lens and/or by induced eye movements, beyond the ora serrata. The new contact lens CGR4 has an upright field of view of 64 degrees and tilting enables one to reach the equator with very little optical image degradation. The new lens has a negative power of - 139 dioptres. Safety during laser irradiation tasks is increased as compared to positive lenses because the irradiance at the crystalline part is significantly reduced. The lens is made of laser-resistant glass with a refractive index of 1,516 BK 7 (Schott).