What are the differences in the applicable scenarios of optical prisms made of different materials?

As the core component in optical systems that realizes functions such as reflection, refraction, and dispersion of light, the material selection of optical prisms directly determines their optical performance, environmental adaptability, and application boundaries. At present, the mainstream optical prism materials are mainly divided into three categories: optical glass, crystal materials, and optical plastics. Each category is further subdivided into multiple specific types, such as K9 glass and ZF series flint glass in optical glass, quartz crystal and calcium fluoride crystal in crystal materials, PMMA and PC in optical plastics, etc. Due to differences in light transmittance, refractive index, dispersion coefficient, mechanical strength, temperature resistance, and other characteristics, different materials are suitable for vastly different scenarios. Starting from material classification, combined with specific performance parameters and practical application cases, this article systematically analyzes the differences in the applicable scenarios of optical prisms made of different materials, providing reference for optical system design and selection.

1、 Optical Glass Prism: Balancing Performance and Cost, Suitable for General Optical Scenarios

Optical glass is currently a widely used prism material. By adjusting the glass composition (such as silicon, boron, lead, fluorine, etc.), different optical properties can be achieved, with good transparency, stability, and cost-effectiveness. It is suitable for most general optical scenes. According to the differences in optical performance, mainstream optical glass prisms can be divided into three categories: colorless optical glass, colored optical glass, and special optical glass, each with its own emphasis on applicable scenarios.

（1） Colorless Optical Glass Prism: Core Selection for General Imaging and Optical Systems

Colorless optical glass (such as K9 glass and BK7 glass) is the "basic type" of optical prism, which has the characteristics of wide light transmission range (usually visible light transmittance ≥ 92% in the 400-700nm wavelength band), stable refractive index (K9 glass refractive index d_d=1.5168), moderate dispersion coefficient (K9 glass ╧ d_d=64.1), high mechanical strength (Mohs hardness of about 6-7), easy processing and polishing, relatively low cost, suitable for general scenarios with moderate optical performance requirements and no special environmental requirements.

Typical applicable scenarios:

Consumer grade optical equipment: such as DSLR cameras and mirrorless cameras' viewfinder prisms (pentaprisms or pentamirrors), utilizing the high transmittance and stable refractive index of K9 glass to ensure clear and distortion free viewfinder images; The turning prisms of telescopes (such as Paul prisms and ridge prisms) fold the optical path through BK7 glass, reducing the size of the equipment while ensuring imaging brightness and color reproduction.

Industrial testing equipment: such as optical projectors and microscopes, using the low dispersion characteristics of colorless optical glass to evenly divide light into two paths for imaging and illumination, ensuring detection accuracy (such as size measurement error ≤ 0.001mm); The right angled prism of the machine vision system changes the direction of the light path through reflection, adapting to different installation angles of camera lenses without adjusting the position of the light source.

Civilian optical products, such as turning prisms for periscopes and refractive prisms for laser level gauges, utilize the low cost and ease of processing of colorless optical glass to meet mass production requirements while ensuring basic optical performance (such as light deviation of laser level gauges ≤ 0.1mm/m).

Not applicable scenarios:

Application in the ultraviolet (<350nm) or infrared (>2000nm) wavelength range, as the transmittance of colorless optical glass sharply decreases in these wavelength ranges (such as K9 glass with transmittance<50% in the 250nm ultraviolet wavelength range);

In high temperature environments (>200 ℃), glass is prone to cracking due to differences in thermal expansion coefficients (K9 glass has a thermal expansion coefficient of approximately 7.1 × 10 ^ -6/℃), which affects its optical properties.

（2） Colored optical glass prism: specialized for filtering and spectral selection scenarios

Colored optical glass (such as absorption type colored glass and cutoff type colored glass) achieves absorption or transmission of specific wavelength light by adding metal oxides (such as iron, cobalt, nickel oxides) to the glass. It is suitable for scenes that require filtering, splitting, or spectral selection. The core advantage is that wavelength screening can be achieved without additional coating, and the cost is lower than that of coated prisms.

Typical applicable scenarios:

Spectral analysis equipment: such as the dispersive prism of a spectrophotometer, using colored flint glass (such as ZF6 glass, dispersion coefficient ν d_d=36.3, refractive index n_d=1.7550), utilizing its high dispersion characteristics to decompose composite light into monochromatic light of different wavelengths, suitable for spectral analysis in the visible to near-infrared wavelength range (400-1100nm), widely used for chemical substance concentration detection and material composition analysis.

Optical filtering system: For example, the infrared cut-off prism of security monitoring cameras uses blue absorbing glass (such as BG39 glass) to absorb infrared light above 700nm, avoiding infrared light interference with CCD/CMOS sensor imaging and ensuring true color of daytime images; At night, it is used in conjunction with infrared lights to achieve day night mode conversion by switching prisms.

Medical optical equipment: such as wavelength selective prisms for dental curing lights, using specific colored glass (such as glass that transmits 450-480nm blue light) to filter out other wavelengths of light, ensuring precise blue light irradiation of the curing resin, improving curing efficiency and effectiveness, while avoiding harmful light damage to gingival tissue.

Not applicable scenarios:

For imaging scenes that require extremely high transparency (such as high-precision microscopes), colored glass also absorbs light in the target wavelength band to a certain extent (usually with a transparency of ≤ 85%), which affects the brightness of the image;

In wideband application scenarios (such as when visible light and near-infrared light need to pass through), the wavelength selectivity of colored glass will limit the optical path design.

（3） Special Optical Glass Prism: Extreme Environment and Special Performance Requirements Scenarios

Special optical glass (such as high temperature resistant glass, radiation resistant glass, low expansion glass) is designed with special components (such as adding titanium dioxide, aluminum oxide) or manufacturing processes, and has characteristics such as high temperature resistance, radiation resistance, and low thermal expansion. It is suitable for extreme environments (such as high temperature, strong radiation) or scenes with extremely high optical stability requirements, and the cost is higher than ordinary colorless optical glass.

Typical applicable scenarios:

High temperature industrial scenarios: such as the observation prism of high-temperature furnaces in the metallurgical industry, which uses high-temperature resistant quartz glass (softening temperature of about 1730 ℃, long-term use temperature ≤ 1100 ℃) to maintain stable light transmittance in high-temperature environments (light transmittance in the 400-2500nm wavelength band ≥ 90%), achieving real-time observation of the molten state inside the furnace and avoiding high-temperature softening and deformation of ordinary glass.

Nuclear industry and radiation environment: For example, the optical prism of radiation monitoring equipment in nuclear power plants uses radiation resistant glass (such as glass containing cerium oxide), which can maintain stable optical performance (transmittance attenuation ≤ 5%) in gamma ray and X-ray radiation environments (radiation dose ≤ 10 ^ 5 Gy), ensuring the normal transmission of radiation dosimeters and avoiding discoloration of glass caused by radiation (such as ordinary glass that is prone to yellowing after radiation).

High precision optical instruments, such as the fixed mirror prism of astronomical telescopes, use low expansion glass (such as ULE glass, with a thermal expansion coefficient of about 0 ± 0.3 × 10 ^ -7/℃). Under the temperature difference between day and night (such as -20 ℃ to 30 ℃), the size of the prism changes very little, ensuring precise alignment of the optical path and avoiding imaging deviation caused by thermal expansion and contraction. It is suitable for the strict accuracy requirements of astronomical observations (angular deviation ≤ 0.1 arcseconds).

Not applicable scenarios:

Cost sensitive batch applications (such as consumer grade cameras), special optical glass is usually priced 3-10 times higher than ordinary K9 glass, with low cost-effectiveness;

In scenarios that require complex processing (such as irregular prisms), special optical glass has high hardness (such as quartz glass with a Mohs hardness of 7), making it difficult to process and prone to edge breakage and scratches.

2、 Crystal material prism: core adaptation for wideband and special optical functional scenarios

Crystal materials, such as quartz crystals, calcium fluoride crystals, and lithium niobate crystals, have unique crystal structures and can achieve properties that ordinary optical glasses cannot possess, such as wideband transparency (from deep ultraviolet to far-infrared), birefringence, electro-optic effect, etc. They are suitable for scenarios that require wideband coverage, polarization control, or special optical functions. However, they are usually expensive, difficult to process, and have lower mechanical strength than glass.

（1） Quartz Crystal Prism: Wide Band Transmittance and Polarization Control Scenarios

Quartz crystals (divided into natural quartz and synthetic quartz) have an extremely wide range of light transmission (deep ultraviolet 180nm to far-infrared 4000nm), a low refractive index temperature coefficient (about 5 × 10 ^ -7/℃), good mechanical strength (Mohs hardness 7), and natural birefringence properties, making them suitable for wideband optical systems and polarization control scenarios. They are the core choice for applications in the ultraviolet and infrared bands.

Typical applicable scenarios:

UV optical equipment: such as the splitting prism of UV lithography machines, using synthetic quartz crystals (with a transmittance of ≥ 90% in the 193nm deep UV band), utilizing their high UV transmittance and low dispersion characteristics to accurately segment deep UV lasers, ensuring the resolution of lithography patterns (≤ 7nm). Ordinary optical glass is almost opaque in this band and cannot be replaced; The light guiding prism of the ultraviolet sterilization equipment uniformly guides 254nm ultraviolet light to the sterilization area through quartz crystals, avoiding energy loss caused by glass absorption.

Infrared detection system: such as the turning prism of an infrared thermal imager, which uses quartz crystals (with a transmittance of ≥ 85% in the 3-5 μ m infrared band) to fold and turn the infrared path, ensuring accurate detection of target temperature by the thermal imager (temperature measurement error ≤± 1 ℃), suitable for industrial temperature measurement, security monitoring and other scenarios; The dispersive prism of an infrared spectrometer utilizes the wide band transparency of quartz crystals, covering the 2-12 μ m infrared band, to analyze the infrared absorption spectrum of materials for qualitative detection of substances.

Polarization optical equipment: such as polarization prisms in polarization microscopes (such as Wollaston prisms and Nicol prisms), which utilize the birefringence property of quartz crystals to decompose natural light into two orthogonal polarized light beams, observe the polarization state changes of transparent materials, and use them for microstructure analysis of biological samples (such as fibers and crystals); The polarizing prism of the 3D movie projector achieves polarization separation of light through quartz crystals, ensuring that the left and right eyes receive images with different polarization directions, forming a 3D visual effect.

Not applicable scenarios:

Cost sensitive visible light scenes (such as regular cameras), quartz crystal prices are 5-8 times higher than K9 glass, with no cost-effectiveness advantage;

In scenarios that require high dispersion (such as visible light dispersion in spectrometers), quartz crystals have a lower dispersion coefficient (ν d_d ≈ 67) and are not as effective as flint glass.

（2） Calcium fluoride (CaF ₂) crystal prism: high transmittance scenes for deep ultraviolet and far-infrared

Calcium fluoride crystals are one of the few materials that can maintain high transmittance in the deep ultraviolet (120nm) to far-infrared (10000nm) wavelength range (transmittance ≥ 70% in the 120nm wavelength range and ≥ 90% in the 10 μ m infrared wavelength range), and have extremely low refractive index (n_d=1.4338) and low dispersion characteristics. They are suitable for extreme wavelength scenarios such as deep ultraviolet lithography and far-infrared spectroscopy analysis, but have low mechanical strength (Mohs hardness 4) and are prone to moisture release, requiring special coating protection.

Typical applicable scenarios:

Deep ultraviolet lithography and semiconductor manufacturing: such as the optical prism of EUV (extreme ultraviolet) lithography machines, which uses high-purity calcium fluoride crystals (purity ≥ 99.999%) to achieve high transmittance (≥ 60%) in the 13.5nm extreme ultraviolet band. It is the core optical component of EUV lithography technology and is used for the manufacturing of 7nm and below process chips. Currently, there are no other materials available to replace it; The detection prism of the deep ultraviolet spectrometer utilizes the deep ultraviolet transmittance of calcium fluoride crystals to analyze the spectral characteristics of the material in the 120-200nm wavelength range for purity detection of semiconductor materials.

Far infrared imaging and detection: such as the condenser prism of far-infrared (8-14 μ m) thermal imagers, which uses calcium fluoride crystals to converge the far-infrared radiation of the target object onto the detector, achieving accurate imaging of low-temperature targets (such as -50 ℃ to 100 ℃), suitable for aerospace, low-temperature physics research and other scenarios; The dispersive prism of the far-infrared spectrometer covers the 5-25 μ m far-infrared band, analyzing the vibrational spectra of polymer materials and inorganic compounds for material structure research and quality control.

Not applicable scenarios:

In a humid environment (relative humidity>60%), calcium fluoride crystals are prone to deliquescence (white powder appears on the surface), resulting in a decrease in light transmittance. They need to be used in a dry environment or coated with moisture-proof coating;

Scenes with frequent mechanical collisions (such as portable devices) have low crystal hardness and are prone to cracking and edge breakage due to collisions, which can affect optical performance.

（3） Lithium niobate (LiNbO3) crystal prism: electro-optic and acousto-optic functional scenes

Lithium niobate crystal is a typical electro-optic crystal with excellent electro-optic effect (refractive index changes with external electric field), acousto-optic effect, nonlinear optical properties, and good transmittance (≥ 85%) in the visible to near-infrared band (400-2500nm). It is suitable for scenarios requiring electro-optic modulation, acousto-optic deflection, or nonlinear optical functions and is a key component in optical communication and laser technology.

Typical applicable scenarios:

Optical communication system: such as the electro-optic modulation prism in fiber optic communication, using lithium niobate crystal to change the refractive index of the crystal through an external electric field, achieving intensity and phase modulation of laser signals (1310nm, 1550nm band), ensuring signal transmission rate (≥ 100Gbps), suitable for long-distance fiber optic communication networks; The deflection prism in the optical switch utilizes the electro-optic effect of lithium niobate crystal to control the on/off and switching of the optical path, achieving the routing selection of optical signals.

In the field of laser technology, such as the electro-optic Q-switching prism of Q-switching lasers, which uses lithium niobate crystals to change the polarization state of the laser through the electro-optic effect, achieving compression and energy enhancement of laser pulses, and outputting high peak power (≥ 1MW) laser pulses for laser marking, laser distance measurement and other scenarios; The prism in the acousto-optic deflector utilizes the acousto-optic effect of lithium niobate crystal to control the deflection angle of the laser beam (deflection range ≥ 10 °) through ultrasonic waves, and is suitable for laser scanning, laser display and other equipment.

Not applicable scenarios:

In high temperature environments (>200 ℃), lithium niobate crystals undergo phase transitions at high temperatures, leading to the failure of their electro-optical properties;

Strong laser power scenarios (>10W/cm ²) are prone to light damage (such as surface scratches, internal defect propagation), which affects modulation accuracy.

3、 Optical Plastic Prism: Lightweight and Low Cost Batch Application Scenarios

Optical plastics (such as PMMA, PC, PS) have the advantages of low density (PMMA density 1.19g/cm ³, about half of glass), strong impact resistance (PC impact strength is 200 times that of glass), easy injection molding (complex shapes can be mass-produced), and low cost. However, their light transmittance (PMMA visible light transmittance ≥ 92%, close to glass), temperature resistance (PMMA long-term use temperature ≤ 80 ℃), and stability (easy aging and yellowing) are lower than optical glass and crystals, making them suitable for lightweight, low-cost, and batch application scenarios without extreme environmental requirements.