Pellin-Broca Prism
A constant-deviation dispersing prism that always directs a chosen wavelength at exactly 90° regardless of the wavelength being selected — simply rotate the prism to choose which color exits at 90°. The standard prism for monochromators, UV harmonic separation, and wavelength-selective beam routing where a fixed output direction is required.
Angles
90°, 75°, 135°, 60°
Output deviation
Always 90°
Wavelength selection
By prism rotation
Reflections
1 TIR + 1 refraction
Overview
- A four-sided prism with angles of 90°, 75°, 135°, and 60° — equivalent in optical function to a combined 30°–60°–90° dispersing prism and right angle prism
- Light entering through the AB face undergoes TIR at the BC face, then exits through the AD face — always at 90° to the entry direction
- The wavelength exiting at 90° is selected by rotating the prism — the prism is mechanically rotated to tune the wavelength, just as a grating is rotated in a monochromator
- The output direction does not change as the prism is rotated — only the wavelength exiting at 90° changes, making alignment of downstream optics permanent
- Used for separating laser harmonic wavelengths (e.g. 1064 nm, 532 nm, 355 nm from an Nd:YAG laser) where each harmonic is routed at 90° by choosing the appropriate prism angle
- Named for French optical instrument maker Ph. Pellin and professor André Broca who developed it in the early 20th century
Key Features
Constant 90° output
Unlike an equilateral prism where the output angle changes with wavelength selection (requiring re-alignment of downstream optics), the Pellin-Broca always directs the selected wavelength at 90° to the input. This fixed output geometry is the defining advantage for monochromators and spectrometers where the exit slit position must remain fixed.
Rotation-based wavelength selection
Rotating the prism about an axis parallel to the refracting edges sweeps different wavelengths through the 90° exit direction. The rotation is smooth and continuous — enabling scanning spectrometers where the prism is motor-driven to scan a spectrum across a fixed exit aperture or detector, just as a diffraction grating is rotated in a grating monochromator.
Harmonic separation
Used extensively in pulsed laser systems to separate the fundamental and harmonic wavelengths produced by nonlinear frequency conversion (e.g. 1064 nm, 532 nm, 355 nm, 266 nm from Nd:YAG lasers). Each harmonic can be directed at 90° by setting the prism angle — cleanly separating co-propagating harmonic beams without the ghost reflections of dichroic mirrors.
Brewster angle UV versions
UV fused silica Pellin-Broca prisms can be designed with entry and exit faces at Brewster's angle for the design wavelength — eliminating surface reflection losses entirely for p-polarized UV laser beams. This is particularly valuable at 240 nm and below where AR coatings have limited performance and the Brewster angle condition provides better transmission.
Design and Construction
Geometry
Face layout
- Face AB (entry): light enters here; typically AR-coated or at Brewster angle
- Face BC (TIR): 90° internal angle; TIR deflects beam 90° inside the prism
- Face CD (intermediate): transmits internal beam to face AD
- Face AD (exit): dispersed wavelength exits here at 90° to AB
Tolerances
- Angle accuracy: ±30 arcsec standard; ±5 arcsec precision grade
- Surface flatness: λ/4 standard; λ/8 for high-resolution spectroscopy
- Surface quality: 20-10 standard for laser use
Coating options
Entry/exit face options
- BBAR AR — for broadband spectroscopy instruments
- V-coat — for specific laser wavelengths
- Brewster angle cut — for UV laser systems; no coating needed for p-polarization
TIR face
- Uncoated — TIR for standard visible/NIR glass prisms
- Protected aluminum or silver — for applications where TIR acceptance angle may be exceeded
Optical Materials
Standard glass
Visible & NIR
- N-BK7 — standard visible monochromator prisms; Abbe V=64; moderate dispersion
- N-SF10, N-SF11 — high-dispersion flint glass; better wavelength resolution in compact monochromators
UV-grade
- UV Fused Silica — primary material for UV laser harmonic separation (193–355 nm range)
- CaF₂ — deep UV below 193 nm; VUV spectroscopy applications
Specialized materials
Laser-grade
- Suprasil (synthetic fused silica) — highest laser damage threshold; for high-power UV laser systems
- Corning HPFS — premium homogeneity fused silica for lithography and high-precision spectroscopy
Wavelength Options
Deep UV
- 185–350 nm
- UVFS / CaF₂
- Brewster or BBAR
Visible
- 400–700 nm
- N-BK7 / N-SF10
- BBAR or V-coat
NIR
- 700–2500 nm
- UVFS / N-BK7
- NIR BBAR
Applications
Spectroscopy
Scanning monochromators
The defining application — rotating the Pellin-Broca prism scans successive wavelengths through the fixed exit slit of a monochromator. The constant 90° output allows the exit slit, collimating mirror, and detector to remain stationary while the selected wavelength is continuously tuned across the spectrum.
Laser
Harmonic beam separation
Separates co-propagating harmonic wavelengths from pulsed Nd:YAG lasers (1064/532/355/266 nm) — each harmonic exits at 90° at the specific prism angle for that wavelength, enabling clean separation without dichroic mirror ghost reflections or bandwidth restrictions.
UV Optics
UV laser wavelength isolation
Brewster-angle UV fused silica Pellin-Broca prisms separate UV laser harmonics with maximum transmission for p-polarized beams — critical in excimer laser spectroscopy and UV material processing where reflection losses from conventional optics must be minimized.
Research
Optical parametric systems
Used in optical parametric oscillator (OPO) and amplifier (OPA) systems to separate signal, idler, and pump wavelengths — routing each wavelength at 90° for independent measurement, blocking, or further processing without the bandwidth limitations of dichroic mirrors.
Education
Optical laboratory instruments
Standard component of teaching-laboratory spectrometers where the constant-deviation property simplifies calibration — the exit slit position is fixed at 90° and only the prism angle calibration changes with wavelength selection.
Solar
Solar spectroscopy
Solar spectroscopes and coronagraphs use Pellin-Broca prisms to scan solar emission lines — the fixed exit aperture allows a telescope eyepiece or camera to remain fixed while successive solar spectral features are brought into view by prism rotation.
Why choose Pellin-Broca Prisms
Fixed 90° output
Output direction never changes during wavelength scanning — downstream optics, slits, and detectors remain permanently aligned regardless of prism rotation angle.
Rotation-based tuning
Simple mechanical rotation selects the wavelength — more robust, repeatable, and contamination-resistant than tuning by mirror or grating rotation in equivalent instruments.
Clean harmonic separation
Spatially separates laser harmonics without dichroic mirror ghosting or bandwidth limitations — a cleaner solution for multi-harmonic laser systems than filter-based approaches.
Brewster-angle UV efficiency
UV fused silica versions with Brewster-angle faces achieve near-100% transmission for p-polarized UV beams — critical for high-power UV laser systems where coating damage and reflection losses are unacceptable.
Frequently asked questions
Here are some common questions about achromatic lens.
The Pellin-Broca's geometry combines a 60° dispersing section with an internal TIR reflection at a 90° face. The 60° section provides dispersion; the TIR face redirects the beam 90°. When the prism is rotated, the angle at which each wavelength hits the 60° dispersing faces changes — but the TIR face always deflects the internally traveling ray by exactly 90°. The result is that whichever wavelength happens to travel at the correct angle through the 60° section (determined by the prism rotation) always exits at 90° to the input face.
For a scanning monochromator where the exit slit must be fixed: always use the Pellin-Broca — its constant 90° output eliminates the need to physically move the exit slit as the wavelength is changed. For a static spectrograph where all wavelengths are recorded simultaneously: the equilateral prism is simpler and more efficient, spreading the full spectrum across a detector array. The Pellin-Broca is preferred for any application requiring wavelength selection with a fixed exit aperture.
