A lens that works near the focal plane of the objective lens to effectively reduce the size of the detector is called a field lens.
To be precise, the field lens should work on the image plane of the objective.On the one hand, the object plane of the field mirror is coincident with the principal plane, and its magnification rate is 1 according to the characteristics of the principal plane, so there is no contribution to the magnification of the system.On the other hand, the field lens should project the aperture plane of the objective lens onto the entrance pupil of the eyepiece. For single lens, the aperture plane is the exit pupil, which can ensure that the exit light of the objective lens can pass through the eyepiece to the maximum extent.If used in a scanning system, the probe replaces the entrance pupil of the eyepiece.
According to the wavelength: 1064nm field mirror (also called YAG field mirror), 10.6um field mirror (also called CO2 field mirror), 532nm field mirror (also called green field mirror), 355nm field mirror (also called ultraviolet field mirror)According to the design concept, f-theta lens and telecentric lens have high cost. And the industrial system mainly adapts to the f-theta lens.
The main function of field mirror is:
(1) improve the ability of the edge beam to incident to the detector;
(2) in the same primary optical system, the additional field lens will reduce the detector area.If the same detector area is used, the field of view can be expanded to increase the flux of incidence;
(3) the image plane can be positioned to place the modulator to solve the problem of no place for the modulator;
(4) the non-uniform illumination on the photosensitive surface of the detector can be homogenized;
(5) when using flat field mirror, the flat field image plane can be obtained;
(6) in the aspect of aberration correction, the field curvature and distortion of the system can be compensated.
