Prisms confuse buyers because the names describe shapes while the jobs describe geometry. Two prisms with the same triangular outline can do opposite things to an image, and two with completely different outlines can be interchangeable in a given instrument. The reliable way to choose is to start from the job — deviate, fold, erect, rotate, displace or split — and let the job name the family. The families below are the thirteen families documented in the manufacturer's catalogue on this site, set out on the optical prisms page.
Right Angle Prism: The Ninety-Degree Fold
The right angle prism is the ninety-degree fold of the prism family. Light enters one leg face, reflects from the hypotenuse by total internal reflection, and leaves through the other leg at 90 degrees to its original path. Used the other way — entering the hypotenuse — it folds a path back parallel to itself. Because the reflection is internal and total, the hypotenuse needs no coating in folding service, and because the part is one piece of glass it holds its geometry through handling that would knock a mirror pair out of alignment. Coat the hypotenuse with a partially reflective film instead, and the same blank becomes a beam splitter. That is the general form. What this site's catalogue records is narrower: a stock listing named as a dielectric-coated right-angle splitter prism — the record holds the listing's name and its dielectric coating, and does not state which face carries the coating, so the coated face is confirmed in the enquiry. It is that listing which anchors the right angle prisms page here.
Penta Prism: The Deviation That Cannot Drift
The penta prism deviates a beam by exactly 90 degrees using two reflecting faces whose included angle does the work. Its defining property is forgiveness: within a useful range, tilting the prism does not change the deviation, because the geometry of the two reflections cancels the tilt. Surveying instruments, alignment telescopes and rangefinders use penta prisms precisely where a constant, calibration-grade 90 degrees must survive a product's whole life. The image is neither inverted nor reverted by the penta's two reflections, which also makes it safe in viewing paths. A half pentagonal variant applies the same constant-deviation property at 45 degrees, for compact layouts.
Cube Beam Splitter: The Splitter as a Block (General Form)
In general optics usage, a cube beam splitter is two right-angle prisms cemented hypotenuse-to-hypotenuse with the splitting coating at the cemented interface. The result is a beam splitter with the mechanical personality of a block of glass: it drops into a mount, keeps its faces parallel to the system, and splits the beam into transmitted and reflected arms at 90 degrees. Polarising versions separate the beam by polarisation state instead. That textbook construction is not, however, what this site's catalogue records: the catalogue carries a prism group named Cube, and for that group it records the name only — no construction and no splitting application — so the textbook construction cannot be inferred from the group name, and supply of a cube-form splitter would have to be confirmed separately in an enquiry.
Roof, Porro and Pechan: Image Erection
Viewing instruments have a problem lenses create: the objective forms an inverted image, and the user wants it upright. Erecting prisms solve it by geometry. The Porro prism — and the Porro pair in classic binoculars — folds the path twice and restores the image to its correct orientation while shortening the instrument. Roof prisms carry a roof edge, two faces meeting at a precise 90-degree ridge, which completes the erection in a slimmer in-line housing; the Schmidt-Pechan prism combines a Pechan section with a roof section to erect the image in compact binoculars and sights where a Porro pair will not fit. The roof edge is the precision feature of the family: its angle error shows directly as image doubling.
Dove Prism: The Image Rotator
The dove prism is the family's moving part. A long trapezoidal prism used in reflection, it rotates the image at twice the prism's own rotation angle as it turns about the beam axis — rotate the dove 45 degrees and the image turns 90. That property makes it the standard image rotator and de-rotator: in instruments that must hold an image steady while the housing turns, a dove prism driven at half speed does the holding. Its length and its sensitivity to collimation are the prices; a dove only performs in a parallel beam, so it belongs in the collimated sections of an instrument, never in a converging one.
Rhombic, Brewster, Wedge and the Specialists
The rhombic prism displaces a beam sideways without changing its direction — a pure offset, used to adjust interpupillary distance in binocular instruments and to set parallel paths in interferometers. The Brewster prism works at Brewster geometry for low-loss deviation inside laser systems, where every reflection loss is budget. The wedge prism, covered in depth on the wedge prisms page, deviates by a small precise angle and, in counter-rotating Risley pairs, steers beams. Paul prisms sit at the specialist end of the documented family list, and special-shaped prisms cover the geometries that production instruments standardise on but catalogues cannot name in advance — they are quoted from drawings.
Choosing: Start From the Job
Deviation that must never drift: penta. A simple fold or a fold that also splits: right angle. A splitter that must stay aligned: a cube form (the general form above — not a listing recorded in this catalogue). An image to erect in a slim housing: roof or Schmidt-Pechan; in a classic housing: Porro. An image to rotate: dove. A beam to offset: rhombic. A small angle to trim or steer: wedge. A laser path to deviate with minimal loss: Brewster. Naming the family in the enquiry — with leg or aperture dimensions, the glass, and the coating per working face — is what turns a prism conversation into a quotation. The full family list and the sourcing notes are on the optical prisms page.
Frequently Asked Questions
Which prism keeps a 90-degree deviation constant?
The penta prism. Its two-reflection geometry makes the deviation insensitive to small tilts, which is the property alignment and surveying work asks of it.
Which prism rotates an image?
The dove prism, at twice its own rotation angle — and only in a collimated beam.
What is the difference between Porro and roof prisms?
Both erect the image. Porro pairs fold the path in a wider housing; roof (including Schmidt-Pechan) designs erect in a slim in-line housing, at higher manufacturing precision.
Technical References
For the general prism statements in this guide: Edmund Optics, Introduction to Optical Prisms (prism types, functions and beam deviation) and RP Photonics Encyclopedia, Wedge Prisms (deviation relation and Risley pairs).
A Short Note on Prism Materials and Coatings
Two specification habits apply across every family above and are worth collecting in one place. Material first: state the glass your design assumes. The source catalogue records no material grade for its prism listings, so the glass for a prism order is a specification to confirm in the enquiry, not a default to assume; unusual bands or thermal duties make that confirmation more important, not less, and the enquiry should name the band when it does. Coating second: the working faces decide. Faces working by total internal reflection are left uncoated and must be kept clean and uncemented; faces that transmit take an anti-reflection coating when the light budget matters; faces that split or reflect at partial angles carry the film that defines the split. A prism enquiry can walk its own faces — this one reflects internally, this one transmits, this one splits — and the quotation is then written against the faces as stated; it takes four sentences to write.