Practical application of double cemented lenses

The composition of double cemented lens: it is made of two concave and convex lenses with different refractive index and different dispersion rate, which can greatly reduce or eliminate color difference.

For better performance, usually the side with the larger radius of curvature (the flat surface) should be turned away from the collimated beam. Color difference comes from the difference in the dispersion and refraction coefficient of different wavelengths of light in the transmission material, so that different wavelengths of light converge at different focal points, double-bonded achromatic lens through the adhesive design of different material lenses, so that the dispersion of the two materials compensation each other, thereby reducing the comprehensive color difference. Achromatic design also helps to improve the image aberration of light sources in a wide wavelength range (composite light sources), improve spherical aberration at a suitable radius of curvature, and improve coma at the same time when the material is selected arbitrarily. Achromatic lenses provide sharper images than optical systems that use a single lens and can form smaller points of light, further improving the optical performance of the lens. Achromatic lenses are suitable for a variety of applications such as fluorescence microscopy, image relaying, detection or spectroscopy.

Double lens and thin lens

The gluing achromatic lens can correct the chromatic aberration, but these corrections can be calculated using a thin lens. The most common and easiest to explain is the chromatic aberration correction double gluing lens of the objective lens of the telescope. Even if it is not a telescope, this form of objective lens is also used elsewhere, such as collimating lens, we usually reverse design according to the actual use, which makes it exactly the same as the telescope objective lens !

For these systems, the focal length is usually extremely long relative to the physical thickness of the lens, making it close to a thin lens (in practice). That is, in terms of calculation, the thickness of the lens is so small compared to the entire focal length that any calculations affected by the thickness are so small that we can ignore them. The bigger effect is the radius of curvature, which can be simplified and modified to the optical focal length (the reciprocal of the focal length), which is the thin lens equation. Therefore, for many applications, double-glued lenses can be designed as thin lenses. We still have three degrees of freedom for lens design.

Since the system can be approximated as a thin lens, we don't have to worry about additional calculations for the time being. But we can use the parameters at hand to get enough information to understand how aberrations work. This is especially important for double-bonded lenses.