Today to share with you: optical glass prism lens element surface requirements

I Surface quality/finish

The quality of the optical surface is used to measure the surface characteristics of the optical product and covers some defects such as scratches and pits. Most of these surface defects are purely surface defects and do not have a large impact on system performance, although they may cause a slight slip in the amount of light passing through the system, resulting in more subtle scattering of scattered light. However, some surfaces are more sensitive to these effects, such as: (1) surfaces on the image plane, where these flaws create focus, and (2) surfaces with high power levels, where these flaws increase energy absorption and destroy optical products. The most commonly used specifications for surface quality are the scratch and pitting specifications specified by MIL-PRF-13830B. The scratch name is determined by comparing the scratch on the surface with a series of standard scratches provided under controlled lighting conditions. Therefore, the scratch name does not describe its actual scratch, but compares it to a standard scratch according to the MIL specification. However, the pothole point name is directly related to a point or small pit on the surface. Pothole names are calculated by dividing the pothole diameter in micrometers by 10, usually a scratch pothole specification between 80 and 50 will be considered standard quality, between 60 and 40 will be an accurate quality, and between 20 and 10 will be considered a high precision quality.

I Surface flatness

p> Surface flatness is a type of specification for measuring surface accuracy, it is used to measure the deviation of a plane such as a mirror, window piece, prism, or flat lens. You can measure this deviation using an optical flat crystal, which is a high-quality, high-precision reference plane for comparing the flatness of a sample. When the plane of the tested optical product is placed against the optical flat crystal, there will be stripes, whose shape indicates the surface flatness of the tested optical product. If these fringes are equally spaced and are parallel straight lines, then the optical surface being detected is at least as flat as the reference optical flat crystal. If the stripe is curved, the number of stripes between two dotted lines (one tangent to the midpoint of the stripe and the other crossing the end of the same stripe) will indicate flatness errors. The flatness deviation is usually measured by the ripple value (λ), which is composed of multiple wavelength test sources. One fringe corresponds to the wavelength of ½. If the flatness is 1λ, it indicates the general quality level; A flatness of λ/4 indicates an accurate quality level. The flatness is λ/20, indicating a high precision quality level.

I Number of stops

Aperture number is a type of specification that measures the accuracy of a surface, and it is suitable for curved optical surfaces or surfaces with power. The aperture number test is similar to the flatness test in that the surface is compared to a reference surface with a higher standard radius of curvature. Using the same interference principle generated by these two surface gaps, the interference pattern of the fringes is used to describe the deviation between the test surface and the reference surface. The deviation from the reference will produce a series of rings called Newton rings. The more loops present, the greater the deviation. The number of dark or bright rings, but not the total number of both, is equal to twice the wavelength error.

I Degree of irregularity/IAS

Irregularity is a type of specification for measuring surface accuracy that describes the deviation between the surface shape and the reference surface shape (also called local aperture, profile accuracy, PV). Irregularity is measured in the same way as the number of apertures. Regularity refers to the spherical circular streak formed by comparing the test surface with the reference surface. When the number of apertures on the surface exceeds 5 stripes, it will be difficult to detect small irregular shapes smaller than 1 stripe. Therefore, it is common practice to specify the ratio of the number of apertures on the surface to the degree of irregularity so that it is approximately 5:1. For more details about optical flatness and the fringe pattern that describes test flatness, aperture number, and irregularity, see Optical Flatness.

I Surface finishing/surface roughness

Surface machining, also known as surface roughness, is used to measure some small irregularities on a surface. They are usually an adverse consequence of the polishing process. Rough surfaces tend to be more wear-resistant than smooth surfaces and may not be suitable for some applications, particularly those using lasers or in superheated environments, due to the possibility of subtle cracks or imperfections at the nucleation site. The production tolerance for surface machining is 50A RMS for average quality, 20A RMS for accurate quality, and 5A RMS for high quality.