A lens that works near the focal plane of the objective lens and can effectively reduce the size of the detector is called a field lens.
To be exact, the field lens should work in the image plane of the objective lens. On the one hand, the object plane of the field mirror coincides with the main plane. According to the characteristics of the main plane, its magnification is 1, so it has no contribution to the magnification of the system; On the other hand, the field lens should project the aperture surface of the objective lens on the entrance pupil of the eyepiece. For a single lens, the aperture surface is the exit pupil, which can ensure that the outgoing light of the objective lens can pass through the eyepiece to the greatest extent. If used in a scanning system, the detector replaces the entry pupil of the eyepiece.
F-theta field mirror is the most widely used plane focusing lens in laser marking machine, which is generally combined with galvanometer system. The more important ones are as follows:
Scanning range: This is the most concerned problem of users. The larger the area that the lens can scan, of course, the more popular it is with users. However, if we blindly increase the scanning area, it will bring many problems, such as thicker light spots, increased distortion and so on.
Focal length (related to the working distance, but not equal to the working distance): the scanning range is directly proportional to the focal length of the field mirror - the increase of the scanning range will inevitably lead to the increase of the working distance. The increase of working distance will inevitably lead to the loss of laser energy.
The diameter of the focused spot is directly proportional to the focal length. This means that when the scanning area reaches a certain degree, the light spot diameter obtained is large, that is, it is not fine enough, and the laser power density decreases very fast (the power density is inversely proportional to the second power of the spot diameter), which is not conducive to processing.
Since the f-theta field mirror is used, y '= f* θ The relationship to work, but practical θ And TG θ There is still a difference in the value of. Moreover, with the increase of focal length f, the degree of distortion will be greater and greater. If there is no correction program in the control system, the marked pattern will be seriously deformed.
Working wavelength: it mainly depends on the wavelength of the laser. The lens of the field mirror is coated at a given laser wavelength. If the field mirror is not used within a given wavelength range, the field mirror may be burned out by the laser or the required laser transmittance is very low.