Rhomboid Prisms

Product Overview

Unlike wedge prisms (which deflect beams at an angle) or right-angle prisms (which change direction), rhomboid prisms shift the beam axis horizontally or vertically while keeping it parallel to the original path. This pure translation is critical for avoiding vignetting (partial beam blocking) in compact optical setups, such as microscopy systems where space between components is limited. Our Rhomboid Prisms maintain collimation (beam parallelism) with <0.1° deviation, ensuring the beam’s characteristics (diameter, intensity profile) remain unchanged after translation—essential for precision applications like laser spectroscopy or medical imaging .

Product Features

• Material Selection: Available in Schott glass (BK7 for visible-range applications, >92% transmission at 550nm, ideal for general imaging), sapphire (high hardness—Mohs 9—and high-temperature resistance, suitable for harsh environments like industrial sensors), and IR crystals (e.g., ZnSe for mid-IR, 2-12μm, or Ge for long-wave IR, 2-14μm). BK7 is cost-effective for visible use (e.g., microscope optics), while sapphire is preferred for applications exposed to vibration or dust (e.g., factory automation cameras). IR crystals cater to thermal imaging or gas sensing systems that operate beyond the visible spectrum .

• Precision Metrics: Angular tolerance <2 arcseconds ensures the translated beam remains parallel to the original path—even a 5 arcsecond deviation can cause beam misalignment in long optical paths (e.g., 1m path length would result in a 0.2mm offset at the detector). Flatness PV<1/10λ (at 632.8nm) on all optical surfaces minimizes wavefront distortion, which is critical for laser-based applications (e.g., interferometry) where wavefront errors degrade measurement precision. The two reflection surfaces are polished to a parallelism of <1 arcsecond, ensuring consistent translation across the beam .

• Surface Quality: 20-10 scratch-dig finish (standard grade, suitable for most applications) with optional AR coatings tailored to specific wavelengths. For visible applications, AR coatings reduce reflection losses to <0.5% per surface; for IR applications, coatings on ZnSe or Ge prisms reduce losses to <1%. Blackened non-optical surfaces suppress stray light (stray light <0.5%), preventing ghost images that would degrade image quality. For high-sensitivity systems (e.g., low-light astronomy), a 10-5 surface quality grade is available to further reduce scatter .

• Displacement Range: Standard models offer 1mm to 50mm lateral shift. 1mm displacement is ideal for fine tuning fine-tuning beam alignment in lab instruments (e.g., spectrometers), while 50mm displacement is used in large optical systems (e.g., telescope cameras, where the prism shifts the beam to avoid mechanical components). Custom displacement ranges (up to 100mm) are available for specialized applications, such as aerospace imaging systems. The displacement distance is determined by the prism’s length and refractive index—for BK7 (n=1.5168), a 50mm long prism provides ~10mm displacement .

• Mechanical Stability: Robust construction with a rectangular shape that fits easily into standard optical mounts (e.g., 1-inch or 2-inch lens tubes). The prism’s solid geometry resists vibration-induced misalignment—critical for industrial systems (e.g., conveyor belt laser scanners) or mobile devices (e.g., drone cameras). For high-vibration environments, prisms can be mounted in shock-absorbing holders (with rubber gaskets) that reduce vibration transfer by >80%. The prisms also have high compressive strength (BK7: 800 MPa, sapphire: 2000 MPa), making them resistant to breakage during handling .

Applications

Rhomboid prisms are essential in:

• Laser Technology: Adjusting beam paths in spectroscopy (e.g., Raman spectroscopy, where the prism shifts the laser beam to align with the sample) and microscopy (fluorescence microscopy, where the prism moves the excitation beam to avoid blocking the detector). In Raman spectroscopy, precise beam alignment is critical for detecting weak Raman signals—even a 0.1mm misalignment can reduce signal intensity by 50%. In microscopy, the prism’s translation allows the excitation beam to be positioned without moving the sample, reducing the risk of sample damage .

• Instrumentation: Aligning optical benches (used in lab research to set up laser experiments) and compensating for detector position offsets (e.g., in IR cameras, where the detector may be slightly misaligned with the optical axis). Optical benches use rhomboid prisms to fine-tune beam paths—for example, shifting a laser beam 5mm to align with a mirror or lens. Detector offset compensation ensures the beam hits the detector’s active area, preventing signal loss or distortion .

• Defense: Shifting beams in targeting systems (e.g., tank-mounted laser rangefinders) to avoid obscurations from mechanical components (e.g., gun barrels or sensors). In rangefinders, the prism shifts the laser beam around the gun barrel, ensuring the beam reaches the target without being blocked. This design allows the rangefinder to be integrated into the tank’s turret without compromising targeting accuracy .

• Biotechnology: Positioning excitation beams in fluorescence microscopes (used to image cells or tissues) without disturbing sample alignment. In live-cell imaging, moving the sample to align with the beam can cause cells to drift out of focus or be damaged. Rhomboid prisms eliminate this issue by shifting the beam instead, allowing long-term imaging (hours or days) of live cells with consistent focus. The prisms are also used in flow cytometry, where they shift the laser beam to align with the cell flow path, improving cell detection efficiency .

FAQ

Q: How is displacement distance determined?

A: Displacement distance depends on two key factors: the prism’s length (the distance between the input and output surfaces) and the material’s refractive index (n). The formula for displacement (d) is approximately d = L × (n - 1) / n, where L is the prism’s length. For standard models:

• BK7 prisms (n=1.5168) with L=50mm provide ~10mm displacement.

• Sapphire prisms (n=1.768) with L=50mm provide ~13mm displacement.

• ZnSe prisms (n=2.402) with L=50mm provide ~19mm displacement.

Custom prisms can be designed with specific lengths to achieve the desired displacement—for example, a 100mm long BK7 prism provides ~20mm displacement .

Q: Can they work with high-power lasers?

A: Yes, when fabricated from heat-resistant materials and coated with high-damage-threshold (HDT) coatings. Sapphire or silicon prisms are preferred for high-power use:

• Sapphire prisms handle continuous-wave (CW) laser powers up to 1kW/cm² in the visible range, thanks to their high thermal conductivity (46 W/m·K) and low absorption.

• Silicon prisms handle up to 5kW/cm² in the NIR range (1-6μm), making them suitable for fiber lasers (1064nm) or CO₂ lasers (10.6μm, though ZnSe is preferred for CO₂).

HDT AR coatings (dielectric coatings instead of metal coatings) are used to prevent coating damage—these coatings have damage thresholds >10kW/cm² for CW lasers and >1J/cm² for pulsed lasers. For ultra-high-power applications (e.g., 10kW+ industrial lasers), prisms can be water-cooled to dissipate heat .

Q: Do rhomboid prisms introduce polarization changes?

A: Polarization changes are minimal, but they depend on the incident light’s polarization state and the prism’s material. P-polarized light (polarized parallel to the plane of incidence) maintains its polarization state better than S-polarized light, with polarization rotation <1° for BK7 prisms. S-polarized light may experience up to 3° rotation, primarily due to birefringence (a property where light splits into two polarizations) in the prism material. For polarization-sensitive applications (e.g., polarizing microscopy), we recommend:

• Using P-polarized light to minimize rotation.

• Specifying prisms made from low-birefringence materials (e.g., fused silica, which has birefringence <1nm/cm).