The filter is made of plastic or glass with special dyes. The transmittance of the glass sheet is originally similar to that of air, and all colored light can pass through, so it is transparent, but after dyeing the dye, the molecular structure changes and the refractive index also changes, which changes the passage of certain colored light. For example, a beam of white light passes through a blue filter and emits a beam of blue light, while green light and red light are very few, and most of them are absorbed by the filter.

　　The filter is used to attenuate (absorb) certain light wavebands in the light wave or to pass through the light wave in a small range of precise selection, while reflecting (or absorbing) other undesirable light waves. Hope to pass the band. By changing the structure of the filter and the optical parameters of the film, various spectral characteristics can be obtained, so that the filter can control, adjust and change the transmission, reflection, polarization or phase optical lens state of the light wave.

　　 Optical filters also have crystals, blue glass, and AR coating on the outer surface of the crystals is used to improve light penetration, and blue glass is used to filter infrared. In addition to special filters for black and white and color photography, there are also some filters that can be used universally, such as ultraviolet lenses, polarizers, neutral gray lenses, soft lenses, fog lenses, etc.

　　 Lens is an optical element made of transparent materials (such as glass, crystal, etc.). The lens is a refracting lens, and its refractive surface is a transparent body with two spherical surfaces (part of the spherical surface), or one spherical surface (part of the spherical surface) and one plane. The images it forms have real and virtual images. Lenses can generally be divided into two categories: convex lenses and concave lenses. The central part is thicker than the edge part called convex lens, there are three kinds of biconvex, plano-convex, and concave-convex; the central part is thinner than the edge part is called concave lens optical lens, there are three kinds of bi-concave, plano-concave, convex-concave.

　　lens classification:

　　 Convex lens has the function of converging light, so it is also called converging lens; positive lens (can be used for long vision With reading glasses). This type of lens can be divided into:

　　a, biconcave lens; is a two-sided lens;

　　b, plano-concave lens; It is a lens with a concave surface and a flat surface;

　　c, a convex-concave lens; a lens with a convex surface and a concave surface. Thin lens-a lens whose central part has a large thickness compared to the radius of curvature of its two sides. In the early days, the camera only had a convex lens, so it was called a single lens. With the development of science and technology, modern lenses have several convex and concave lenses of different forms and functions to form a convergent lens, called a compound lens. The concave lens in the compound lens is used as a focusing lens to correct various aberrations.

　　 Optical glass has high transparency, purity, colorlessness, uniform texture, and good refractive power, so it is the main raw material for lens production. Due to the difference in chemical composition and refractive index, optical glass has:

　　1, flint glass-lead oxide is added to the glass composition to increase the refractive index (1.8804);

　　2, crown glass---made by adding sodium oxide and calcium oxide to the glass composition to reduce its refractive index (the refractive index of barium crown glass is 1.7055);

　　3. Lanthanum crown glass-a variety discovered in recent years, it has the excellent characteristics of high refractive index and low dispersion rate, which provides conditions for the creation of large-aperture advanced lenses.

　　The principle of lens imaging:

　　 A glass or plastic component used in lamps that can change the light. Aspheric mirror Direction or control the light distribution situation.

　　 lens is the most basic optical element that composes the microscope optical system. The objective lens, eyepiece, condenser and other components are composed of single and multiple lenses. According to the difference of its shape, it can be divided into two categories: convex lens (positive lens) and concave lens (negative lens).

　　 When a beam of light parallel to the main optical axis passes through the convex lens and intersects at a point, this point is called the focal point, and the plane passing through the focal point and perpendicular to the optical axis is called the focal plane. There are two focal points. The focal point in the object space is called the object focal point, and the focal plane there is called the object focal plane; conversely, the focal point in the image space is called the image focal point, and the focal plane there is called After the light from the focal plane of the image side passes through the concave lens, it becomes an erect virtual image, while the convex lens becomes an inverted real image. The real image can be displayed on the screen, but the virtual image cannot.