An optical prism is a transparent optical component with a specific geometric structure, and its core function is to regulate the propagation path, polarization state, and spectral composition of light through plane refraction or reflection. Unlike ordinary lenses, which rely on surface curvature to achieve convergence, the optical action of prisms is based on a plane-to-plane interaction at a fixed angle, which gives prisms strong advantages in controlling the optical path direction, spectral separation, and other fields.
When we pull back to 1666, the famous physicist Newton first confirmed the apochromaticity of white light through triangular prism dispersion experiments, laying the foundation for optical principles; Back in the mid-20th century, with the breakthrough of high-precision diamond grinding and chemical polishing technology, prisms officially moved towards standardized industrial products, and the accuracy has been greatly improved, further supporting the large-scale application of telescopes, spectrometers and other equipment.
The current prism has entered a white-hot stage, and the scene segmentation characteristics are significant: in the field of consumer electronics, the autofocus prism for mobile phone cameras and AR glasses; in industrial testing, spectroscopy and laser rangefinders; in scientific research scenarios, astronomical telescopes, quantum communication, and other fields.
Materials and Fabrication of Prisms
Material selection
Prism materials need to be comprehensively considered according to the band, stability, and cost used, and there are obvious differences between different materials and applicable scenarios. At present, among the mainstream optical glasses, K9 glass is the first choice, with strong refraction, stability and a wide wavelength range, suitable for different scenarios; The transmittance band of quartz glass is 180-3500nm, and the coefficient of thermal expansion is only 1/10 of that of K9 glass, but the high-temperature resistance is outstanding, and it is mostly used in high-temperature scenarios such as high-precision spectrometers and laser equipment.
Do we have any other options besides these?
Of course, in terms of special substrates, sapphire crystals have a light transmission range of 200-5000nm, a hardness of 9 on the Mohs scale, and have strong high temperature resistance (melting point 2050°C), which can be used in harsh environments such as aerospace and high-temperature testing; The transmitted wavelength of calcium fluoride extends to 150-12000nm, which can cover the deep ultraviolet to far infrared regions, but the texture is brittle and easy to absorb moisture, and it is mainly used in scientific research scenarios such as far infrared spectroscopy analysis and vacuum optical experiments.
The selection of materials should follow the principles of “band priority, environmental adaptability, and cost balance” (for example, calcium fluoride is used for infrared scenes and K9 glass is used for visible light scenes). Then, the substrate is determined according to environmental conditions such as temperature, humidity, and corrosivity. Finally, the cost is controlled while meeting the performance requirements (the price of special substrates is usually 10-100 times that of ordinary glass).
Manufacturing process and precision control of prisms
The manufacturing of prisms is based on shape control and surface quality, and the whole process can be roughly divided into three stages: roughing, finishing, and post-processing.
- In the roughing stage, workers first use diamond cutting equipment to cut the glass blank into a blank close to the target shape, and then determine the geometric structure through rough grinding of the diamond grinding wheel, such as right angle or equilateral shape, and the dimensional error is usually controlled at 0.1 to 0.5 mm, leaving the foundation for subsequent finishing.
- In the finishing stage, fine grinding is used to remove surface microcracks and scratches, so that the surface roughness is reduced to a smaller range. At the same time, it is necessary to use chemical polishing methods (commonly silica sols) and strictly follow the surface requirements to ensure there is no obvious distortion when light penetrates.
- The final treatment requires edge chamfering to prevent edge chipping or operational damage, and coating according to customer requirements: transmission prisms usually use anti-reflection films to achieve a visible light transmittance of more than 95%; reflective prisms use high-reflectivity films with a reflectivity of over 98%.
The production accuracy of prisms is critical to subsequent performance, so the clamping method and inspection standards need to be strictly controlled during the manufacturing process. Usually, using hot wax fixation (barrier method) is easy to operate and suitable for medium and low precision products, but thermal expansion and contraction may bring small errors; Van der Waals force fixation (contact method) eliminates the need for adhesive and avoids additional deviations, making it a common method for high-precision prisms.
In terms of inspection, the industry usually requires surface flatness not less than λ/10 (λ=632.8 nm), angular tolerance controlled within 3 arc minutes, and parallelism error less than 5 arc seconds. Interferometers can accurately measure surface topography with laser interference stripes with resolutions up to λ/20, while goniometers use high-precision optical sensors to measure prism angles with measurement errors as low as 0.1 arc minutes.
Optical principles and engineering characteristics of prisms
The core principle of the prism comes from the refraction and reflection laws of light, among which Snell’s law and total internal reflection are the basis of design calculation. Snell’s law states that the relationship between angle of incidence and angle of refraction follows n₁sinθ₁ = n₂sinθ₂. Taking the right-angle prism of BK7 glass (n=1.5168) as an example, when the angle of incidence is 45°, the refractive angle is about 27.7°, the deflection angle is about 45.4°, and the angular error needs to be controlled within ±0.1° during manufacturing.
Total internal reflection occurs when light is emitted from a high refractive index medium to a low refractive index medium and the angle of incidence exceeds the critical angle, and is often used in engineering for the 90° reflection of a right-angle prism to achieve a high-reflectivity structure without coating. However, it should be noted that if oil or water vapor is attached to the surface, it will reduce the critical angle and lead to a decrease in reflectivity, so hydrophobic coatings are often used for protection in outdoor or high-humidity environments.
In image processing applications, the number of reflections of a prism directly determines imaging rotation. The light bounced off odd times causes the image to be upside down or rotated, while even reflections keep the image upright. Microscope systems often use secondary reflection to achieve upright imaging, and the prismatic structure in binoculars is based on the same principle. In engineering design, the correspondence between the number of reflections and the image direction can be confirmed through optical path simulation, so as to avoid the influence of rotation error on the imaging quality.
Another important factor is pyramid tolerance, which is a small angular deviation between the parallel planes of a prism. Although extremely small, it can cause significant optical path shifts or wavelength errors in high-precision systems. Precision devices such as spectrometers or laser rangefinders typically require tolerances of less than 0.5 arc minutes, while common optical devices such as toy telescopes or AR glasses can be relaxed to ±5 arc minutes. Deviations caused by compensating tolerances can be offset by installing a compensation prism with a reverse angle or corrected by fine-tuning the mounting angle during system assembly.
Prismatic selection and application points
The selection of prisms should first determine the type according to the application scenario. For spectral separation, such as wavelength detection or material analysis, a dispersive prism should be chosen. If the optical path needs to be deflected or framed turned, a biased prism can be used; If the system involves image stabilization or angle correction, a rotating prism is optional; If the target is optical axis translation or beam shaping, a displacement prism is recommended; Scenarios involving polarization or beam splitting, such as 3D imaging and interferometric detection, require polarizing prisms or specialty functional prisms.
In the selection and evaluation, three types of parameters should be integrated: optical, mechanical and environmental. The optical parameters need to match the operating band and transmittance requirements, such as K9 glass for visible light scenes and calcium fluoride for infrared scenes. The coating type needs to be divided into transparency enhancement or high reflection according to the function. Mechanical dimensions should match the installation space and accuracy level, such as mobile phone prisms are typically less than 5 mm, while astronomical equipment may exceed 100 mm. Environmental conditions are also critical, with industrial-grade applications meeting the operating temperature range of -40°C to 85°C, and coatings and substrates with hydrophobic or corrosion-resistant properties in humid or corrosive environments.
Common selection mistakes include blind pursuit of high precision, neglect of coating and band matching, and failure to consider image rotation. Although high-precision prisms have excellent performance, they will cause unnecessary cost burden for ordinary scenarios; mismatched coatings can severely reduce the light transmittance of the system; Rotation errors can lead to image inversion. Therefore, scientific evaluation of actual needs and selection in combination with optical path analysis is key to ensuring the balance between prism system performance and cost.
Application status
As a passive optical component, prisms have the advantages of no power supply, strong environmental adaptability, and controllable manufacturing costs, and have become the core basic components in optics. Today, its applications have expanded from traditional imaging and ranging to cutting-edge fields such as AR/VR optical waveguides, lidar, and quantum communications. In the future, with the development of new materials and precision coating technology, prisms will play a wider role in higher wavelengths, more complex optical paths, and intelligent optical systems.
