【Optical lens】Application of dispersion lens in spectral confocal displacement sensor

In: Industry information On: Wednesday, March 18, 2020 Hit: 2449

In recent years, with the continuous development of precision manufacturing industry, higher requirements have been put forward for the accuracy of measurement. As the basis of precise measurement of geometric quantities, displacement measurement technology not only needs ultra-high measurement accuracy, but also needs a wide range of adaptability to the environment and materials, and gradually tends to real-time, non-destructive testing. Compared with the traditional contact measurement method, the confocal sensor has obvious advantages such as high speed, high precision and high adaptability.

Let's start with the principle of confocal sensors. Everyone knows that light is a wave. When a beam of white light passes through a prism, the color will separate. Dispersion is essentially due to the fact that materials have different refractive indices for different wavelengths of light, which will lead to different propagation paths for different wavelengths of light. The principle of spectral confocal displacement sensor is used in this way. White light is divided into different colors of light. After passing through the lens, different wavelengths of light are focused at different positions on the axis. Will other wavelengths of light reflect back? Yes, but because other wavelengths are defocused at this location, the distribution of reflected light at the source is much larger than the core diameter, so most light cannot enter the spectrometer. The wavelength value at the maximum light intensity is obtained by decoding the spectrometer, and the corresponding distance value of the target is measured. Due to the use of confocal technology, the method has good chromatographic characteristics, improves the resolution, and is not sensitive to the properties of the measured objects and stray light.

As the eye of the whole instrument, the dispersive objective lens is very important, and this lens is very special in design. Generally, the lens is to eliminate color difference, while the dispersive objective lens needs to make the axial color difference as large as possible, so that its range will be larger. In the design of dispersion lens, besides the axial color difference, the following factors should be considered:

(1) the resolution can be improved by increasing the matter-side numerical aperture;

(2) increasing the numerical hole of image square can improve the light source utilization;

(3) the precision can be improved by reducing the spherical difference of the system;

(4) the system structure should be easy to assemble and adjust.

The above factors are mutually restricted. As the numerical aperture increases, the spherical aberration of the system also increases. If the spherical aberration system is to be corrected, the structure will become complex. Typically, such lenses need to be designed to the diffraction limit, and since most spectral confocal displacement sensors are transmitted by optical fibers, they can be approximated as point light sources. Designing a lens for a fixed wavelength is usually simple, but it is more difficult to work well across a spectrum. Dispersion lenses usually use white light, so they need to work around 400 to 700nm.

The next difficulty is the uniformity of the color difference. As we all know, linear change is the most perfect state of the sensor, when one parameter changes and the other parameter is just multiplied by a fixed parameter to get the result. But most of the time this is not easy to happen, because distance and wavelength itself is not a simple linear relationship. Therefore, the design can only be as close to linearity as possible to reduce the error after fitting.

Moreover, the range should be as large as possible. In the case of fixed band, the larger the dispersion range, the more subtle wavelength changes can be distinguished, which is the most effective way to improve the resolution of the system.

Finally camera resolution, the function of the lens is equivalent to a point light source dispersion after focusing on the object under test, so the light spot size requirement is very high, the commonly used multimode optical fiber core is 50 microns, after focusing on the object space with the size of the relationship between numerical aperture is large, and is inversely proportional, the greater the numerical aperture, the higher the resolution, and the numerical aperture of the fiber ordinary usually is 0.2, so in theory a support 0.2 numerical aperture of the camera resolution

d = 0.61 * 0.55 = 0.61 lambda/NA mu = 1.65 m / 0.2 microns


oconsidering the aberration and the error caused by manufacturing, the image quality of the lens will be reduced to some extent. Of course, all things within the allowable range are acceptable. Because of the advantages of the spectral confocal displacement sensor such as small size, high measurement accuracy and wide application fields, the demand for the dispersion lens has been increasing year by year in recent years. Our company undertakes the customized business of all kinds of special lenses. We also have the development experience of dispersion lenses. If you have any demand, please contact our sales staff.


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