A detailed analysis of the principle of spectral confocal
As we all know, sunlight in nature is a kind of white light. White light is not pure light, but is composed of many monochromatic lights. The wavelengths of light visible to the human eye range from 400nm (blue light) to 700nm (red light). The phenomenon of angular deflection may occur when light propagates in different media.
However, under the actual white light, there will be many single-line light refractions in different media. Optical materials (lenses) have different refractive indices for different monochromatic lights, that is, different angles of refraction. The shorter the wavelength, the greater the refractive index, and the longer the wavelength, the smaller the refractive index (this is also the reason for the so-called chromatic aberration of different telescopes). The same thin lens has different focal lengths for different monochromatic light, each monochromatic light.
According to the wavelength of the color light from short to long, their image points separate the lenses from near to far and are arranged on the optical axis (the wavelengths of different monochromatic lights are different), so that the so-called chromatic lens error occurs in imaging. Chromatic lens errors cause color spots or halos in the image. In photographic equipment, special treatment should be carried out to minimize the imaging problems caused by chromatic aberration lens errors. Commonly used elimination methods are double gluing system and double separation system.
The chromatic aberration of a single lens produces different focal lengths for different wavelengths of colored light. For an achromatic doublet, the wavelengths of visible light have approximately the same focal length. Diffractive optics with the property of canceling dispersion can be used to correct chromatic aberration and the spectrum is confocal. The measurement method uses precisely the properties of this physical phenomenon.