Achromatic cemented lens, also known as doublet lens, is made of two concave and convex lenses with different refractive index and different dispersion rate, which can reduce or eliminate color difference to a greater extent. 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.