Introduction to the measurement parameters and principles of the optical parameters of the lens

The optical measurement parameters of the lens (lens group), the most frequently appearing in many optical measuring instruments, and the most basic include: radius of curvature (ROC), effective focal length (EFL), front and rear focal length (FFL/BFL), center deviation, angle Compare and other parameters.

What are the instruments for optical testing? Nowadays, the optical testing instruments on the market are really a bit dazzling, such as digital eccentricity meter, automatic focal length meter, photoelectric goniometer, lens comprehensive tester, laser interferometer, etc. There are many different models for each instrument according to different manufacturers, test ranges, test product types, etc., but the test principles still have some similarities. Here, we choose one of the automatic lens comprehensive testers. The test principle is explained.

The lens comprehensive measuring instrument, as its name implies, is the comprehensive measurement of the lens and lens group, and can also measure the above-mentioned parameters. It is widely used for precision measurement by lens manufacturers and lens manufacturers. It is mainly used in the control of important processing indicators and shipment inspection in lens processing, and can accurately monitor the feeding inspection and assembly accuracy of the functional indicators of the single lens assembled by the lens.

1. Measurement of radius of curvature

The radius of curvature is one of the most important indicators that reflect the information of a single spherical surface, and it is also a direct parameter that needs to be controlled in spherical processing. The processing control accuracy of the radius of curvature directly affects the lens focal length and other functional indicators.

The traditional radius of curvature measurement is controlled by a template, which requires a precise template to be made in advance, and increases the risk of damaging the processed lens. Now many manufacturers have gradually replaced the traditional measurement method with non-contact and non-destructive measurement methods, using fully automated Measurement, operation is simple, convenient, and measurement efficiency is high.

Principle of radius of curvature measurement:

The radius of curvature is measured in a reflective state. A high-quality CCD camera separately focuses and images the lens surface and the center of curvature. The focus position is accurately calculated by the test software on the cross-hair image and grating data collected by the CCD. The image on the right shows the imaging position of the surface image and the center image of the concave-convex lens. The radius of curvature of the lens to be measured is equal to the distance between the two positions.

2. Measurement of focal length

Focal length is one of the most important lens design indicators. Although it cannot be directly controlled during processing, it reflects the comprehensive information of each curved surface including curvature, material, thickness, etc. The accuracy of the focal length directly affects the actual function of the lens. The focal length can be divided into effective focal length (EFL), back focal length (BFL), and front focal length (FEL) according to different calculated positions. The principle of its measurement is slightly different.

Principle of focal length measurement:

The effective focal length refers to the distance from the principal optical plane to the corresponding focal point. The measurement method is transmission mode. The measurement wavelength adopts cold light source, double vertical cross-hairs, collimator, lens to be measured, achromatic objective lens and photoelectric autocollimator objective lens. A double vertical line image is formed on the CCD. The measurement software controls the electric translation stage to accurately focus the image, and automatically calculates the focal length of the special measurement lens based on the collected image.

The front and back focal length is the distance from the apex of the optical surface of the lens to the corresponding focal point. The measurement method is similar to the measurement of the radius of curvature. The detection components of the system respectively focus and image the lens surface (reflection method) and focal plane (transmission method). The focus position is tested The software accurately calculates the cross-hair image and grating rule data collected by the CCD, and the front and back focal length is equal to the distance between the two positions.

When measuring the focal length, the operator needs to select the “focal length measurement” software selection, and after selecting the lens and reticle type according to the range of the focal length of the lens to be tested, adjust the position of the photoelectric autocollimator, and find a clear pair on the screen. For the vertical crosshair image, you can get the focal length value, and the true result will be displayed in the display box in order.

3. Measurement of center deviation

Decentering (or center deflection) plays a vital role in lens assembly and application functions. The decentering of a lens (or lens group) refers to the position or direction deviation of the optical axis from the reference axis when the optical axis does not coincide with the reference axis. The method of measuring the eccentricity of the lens (lens group) is usually to rotate the sample. By adding a certain lens rotation device, the comprehensive measuring instrument can also be used as a high-precision eccentricity measuring instrument.

Fourth, the contrast measurement of angular accuracy

The measurement principle also uses the collimator principle. The crosshairs illuminated on the focal plane of the collimating lens are projected to infinity, and then reflected by the mirror, and then imaged on the photosensitive surface of the high-precision CCD industrial camera by the collimating lens. . The slight change between the optical axis of the autocollimator and the angle of the reflector will cause a deviation of the position of the cross image. The system converts this deviation into the angle value that needs to be measured.