Optical Prism: How It Works, Types, and Selection Guide

Refraction and the Geometry of Deviation

Light entering a prism changes speed and direction because the refractive index of glass exceeds that of air. The relationship follows n₁ sin θ₁ = n₂ sin θ₂, where θ is measured from the surface normal. A ray traversing a prism with apex angle A experiences a total deviation δ = i₁ + i₂ – A, with i₁ and i₂ being the entrance and exit angles. At the minimum deviation condition, the ray passes symmetrically, and the refractive index can be calculated directly from n = sin[(A+δₘ)/2] / sin(A/2). This property allows precise measurement of material dispersion.

Material choice governs both the amount of deviation and the wavelength spread. A common borosilicate crown glass (N-BK7 equivalent) has n_d = 1.5168 at 587.6 nm and an Abbe number V_d = 64.17, indicating moderate dispersion. For ultraviolet applications, UV-grade fused silica (n_d = 1.4585, V_d = 67.8) offers higher transmission below 350 nm and lower thermal expansion. The difference in refractive index between the two materials alters deviation by several degrees at the same apex angle, a critical factor when designing a system.

Prism Types and Their Functions

Each geometry exploits either refraction or reflection to achieve a repeatable angular change. The table above highlights the defining apex angle and the primary optical job. While an equilateral prism is optimized for dispersion, a right-angle prism capitalizes on total internal reflection to fold a beam by 90° without coatings when used with collimated light.

Dispersion and Spectral Resolution

When white light passes through a prism, each wavelength refracts at a slightly different angle because the refractive index decreases with increasing wavelength. For a typical 60° equilateral prism made of N-BK7, the angular spread from 400 nm to 700 nm reaches about 0.5°. The rate of change of deviation with wavelength, angular dispersion dδ/dλ, is approximately 0.08°/nm at 500 nm for the same glass. This separation power is sufficient for many compact spectrometers, where a linear detector array captures the dispersed spectrum without moving parts.

The spectral resolution depends on the prism’s base length and the material’s dispersion curve. A longer optical path through the glass increases the physical separation between wavelengths. For instance, a prism with a 30 mm base can typically resolve the sodium D-lines (589.0 nm and 589.6 nm) when combined with a narrow entrance slit. This principle remains the backbone of prism monochromators used in absorbance and fluorescence measurement systems.

Total Internal Reflection and Image Control

Reflective prisms harness total internal reflection (TIR) to redirect light without metal coatings. When light inside glass strikes a surface at an angle greater than the critical angle, 100% reflection occurs. For N-BK7, the critical angle is only approximately 41.2° for the glass-air interface. A right-angle prism receives collimated light normal to one leg, reflects it at 45° at the hypotenuse, and sends it out the other leg with a precise 90° fold. Because the 45° incidence exceeds the critical angle, no reflective coating is needed, preserving high throughput and eliminating metal absorption.

More specialized prisms manage image orientation. A penta prism delivers a constant 90° deviation regardless of small alignment errors and does not invert or reverse the image—two internal reflections, one of which must be coated, achieve this. A Dove prism rotates an image by twice the prism’s own rotation angle, making it useful in laser beam scanning and image derotation. Corner cube retroreflectors return light exactly along the incident path, a property exploited in lunar laser ranging and industrial alignment, with a return accuracy often better than 5 arcseconds when fabricated to tight tolerances.

Specifying a Prism: Material, Size, and Tolerances

Choosing the right prism for a real optical setup requires translating application needs into measurable parameters. The following list captures the critical specifications that appear on a typical drawing or datasheet.

• Material: Match the transmission window to your wavelength. BK7 works from 350 nm to 2 μm; UV fused silica extends down to 185 nm; ZnSe is suitable for 0.6–16 μm infrared systems.
• Surface Flatness: λ/4 at 633 nm is standard for imaging; λ/10 or better is needed for interferometric use to preserve wavefront quality.
• Surface Quality (Scratch-Dig): 60-40 is acceptable for many illumination systems; 40-20 or 20-10 reduces scatter in laser paths.
• Angle Tolerance: ±3 arcminutes is a general-purpose value; precision penta prisms often require ±30 arcseconds to guarantee consistent 90° deviation.
• Clear Aperture: At least 80% of the prism’s face dimension is usually specified to avoid edge defects and mounting-induced distortion.
• Coating: Broadband anti-reflection coatings such as MgF₂ can boost transmission to over 99% per surface; reflective faces may use protected silver for >96% reflectivity in the visible.

Mounting and Alignment Best Practices

A prism’s optical performance can degrade rapidly if it is stressed or misaligned. Practical handling guidelines help maintain the beam quality specified on the drawing.

1. Use kinematic or semi-kinematic mounts that apply gentle spring pressure against the ground edges, not the polished faces. This prevents stress birefringence that would distort polarization.
2. Align the prism with an autocollimator or shear-plate interferometer to verify the exit beam angle matches the design value within tolerance.
3. Clean surfaces with lint-free tissue and reagent-grade isopropanol; avoid dry wiping, which can generate scratches that scatter light.
4. Block unintended TIR and ghost reflections with black-anodized baffles, especially in multi-prism chains.

When a system requires a prism to be rotated, as with a Dove prism for image rotation, the mount must center the prism precisely on the rotation axis. A radial runout of even 0.1 mm can cause beam walk-off that shifts the image on the detector, undermining measurement repeatability.

Troubleshooting Common Issues

Even a well-specified prism can produce unexpected artifacts. Ghost reflections from uncoated surfaces often appear as faint displaced spots; applying a good AR coating reduces their intensity by two orders of magnitude. An apparent beam deviation that exceeds the prism’s angle tolerance frequently comes from a wedged mounting adapter rather than the prism itself—checking the mount with a dial indicator usually identifies the source. In high-power laser systems, bulk absorption can create a thermal lens: a fused silica prism shows far lower thermal drift than BK7, with a thermal coefficient of refractive index about ten times smaller, preserving beam pointing stability under varying load.

Selecting the right prism hinges on understanding its geometry and material. With careful specification and mounting, prisms deliver reliable, alignment-free beam manipulation in instruments ranging from handheld spectroscopes to satellite laser ranging stations.