Mino M, Okano Y
Appl Opt. 1971 Oct 1;10(10):2219-25. doi: 10.1364/AO.10.002219.
To improve the imaging properties of a defocused optical system, the use of shaded apertures is studied theoretically and experimentally. The study is based on the optical transfer function (OTF). The two shaded apertures studied are the type in which the amplitude transmittance decreases gradually from the center of the pupil toward its rim, T(A), and the type in which the amplitude transmittance decreases from its rim toward the center, T(B). For comparison, the effects achieved with a clear aperture, T(C), are included. The results of the calculations show that near focus the OTF for T(A) has higher values in the low frequency region than has either T(B) or T(c). When the system is defocused, the shaded aperture of the type T(A) yields an improved defocused image that is faithful to the outline of the object. The quality of the defocused image obtained with T(B) is worsened. When the OTF is used as a means for judging the quality of the defocused image, the two necessary conditions on the functions appear to be that the OTF (1) must be a monotonically decreasing function and (2) must be nonnegative. These conditions are confirmed by experiment. Since the transmittance variation of the shaded apertures is achieved by absorption, the effects due to the resultant decreases in light level are also considered.
为了改善离焦光学系统的成像特性，对使用遮蔽光阑进行了理论和实验研究。该研究基于光学传递函数（OTF）。所研究的两种遮蔽光阑，一种是振幅透过率从光瞳中心向边缘逐渐减小的类型，即T(A)；另一种是振幅透过率从边缘向中心减小的类型，即T(B)。为作比较，还纳入了使用清晰光阑T(C)所取得的效果。计算结果表明，在近焦时，T(A)的OTF在低频区域的值高于T(B)或T(C)。当系统离焦时，T(A)型遮蔽光阑能产生更清晰的离焦图像，该图像能忠实反映物体轮廓。用T(B)得到的离焦图像质量变差。当用OTF作为判断离焦图像质量的手段时，对函数的两个必要条件似乎是：OTF（1）必须是单调递减函数，（2）必须是非负的。这些条件得到了实验的证实。由于遮蔽光阑的透过率变化是通过吸收实现的，因此还考虑了由此导致的光强降低所产生的影响。
