| 摘要 |
The focal length of an objective lens system is substantially equal at two wavelengths: one within the visible spectrum and one within an eyesafe IR spectrum centered about 1.54.mu.m; the lens is essentially achromatic throughout the entire visible spectrum (0.45.mu.m-0.70.mu.m) and is wellcorrected for other monochromatic aberrations (spherical aberration, coma, astigmatism, field curvature, and distortion). By having an objective lens (12) that is common to both optical paths (visual 36, 42 and laser 36, 44) and has the property of having essentially the same focal length and essentially the same angular deviation for the two optical paths upon lens decentration, then a practical mechanism can be included in the opto-mechanical design of a laser rangefinder to adjust the x and y decentration of the objective lens to accomplish a corresponding x and y adjustment of the system boresight. One specific embodiment is a cemented triplet (14, 16, 18; 14', 16', 18') in which the front lens element (14, 14') is bi-convex and is formed of a crown glass having a relatively high index of refraction (n) and a moderately high inverse relative dispersion (V); the rear element (18, 18') is substantially plano convex and is formed of a crown glass having a relatively low index of refraction and a relatively high inverse relative dispersion; and the middle lens element (16, 16') is bi-concave and is formed of an anomalous dispersion flint glass having an index of refraction intermediate that of the front and rear elements and an inverse relative dispersion that is substantially below that of those other two elements, with partial relative dispersion being displaced from the "normal" line by at least twice and preferably at least six times the distance associated with the normal glasses forming the two convergent elements (14, 18; 14', 18').
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