Industry TemplatesFree referencePDF, Excel, Word exportPhotonics and Optics

Optical Coating Types and Typical Performance Reference

This table compares the optical coatings you meet in photonics purchasing, grouped by function: antireflection, metal mirrors, dielectric mirrors, beamsplitters and filters. It helps you choose the right type, and write a spec with the right band, angle and durability requirement.

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Coating	Function	Typical operating band	Typical performance (approximate)	Strength	Limitation
Single-layer MgF2 AR	Antireflection	Visible, centred near 550 nm	About 1.0 to 1.5 percent residual reflectance per surface on glass	Low cost, durable	Narrow band, not enough for demanding systems
V-coat (two-layer AR)	Antireflection	Single laser line, for example 532 nm or 1064 nm	Below 0.25 percent reflectance per surface at design wavelength	Very low loss at one line	Rises quickly away from design wavelength
Broadband AR (BBAR)	Antireflection	Typically 400 to 700 nm or 650 to 1050 nm	Average below 0.5 percent per surface across band	Works across a spectral band	Slightly higher loss than a V-coat at one line
Dual-band AR	Antireflection	Two lines, for example 532 nm and 1064 nm	Low reflectance at both design lines	Handles harmonics and pilot beams	More layers and cost
Infrared AR on Ge or ZnSe	Antireflection	LWIR 8 to 12 um or CO2 10.6 um	Below 1 percent per surface vs more than 30 percent uncoated on germanium	Essential for high-index IR materials	Soft substrates, handle with care
Hard carbon (DLC) AR	Antireflection and protection	Infrared windows	Moderate reflectance reduction with high abrasion resistance	Rugged outer surface for exposed IR optics	Not suited to visible optics
Protected aluminum mirror	Metal mirror	UV to far IR, 250 nm to 20 um range	About 85 to 92 percent in visible, lower near 800 nm dip	Broadband and low cost	Lower damage threshold, dip near 800 nm
Enhanced aluminum mirror	Metal mirror with dielectric boost	Visible, 400 to 700 nm	Often above 95 percent average	Higher visible reflectance than plain aluminum	Narrower band than protected aluminum
Protected silver mirror	Metal mirror	Visible to NIR, about 400 nm to 2 um	About 95 to 98 percent average	Highest broadband reflectance in the visible and NIR	Needs a good protective overcoat, poor below 400 nm
Protected gold mirror	Metal mirror	About 700 nm to 20 um	About 96 to 99 percent in the infrared	Excellent infrared reflectance	Poor below about 600 nm, soft
UV-enhanced aluminum mirror	Metal mirror	About 250 to 450 nm	Above 85 percent in the UV band	Best metallic option for UV	Fluoride overcoats are moisture sensitive
Dielectric laser line mirror	High reflector	Narrow, for example 532 nm or 1064 nm	Above 99.5 to 99.9 percent at design angle	Very high reflectance and high damage threshold	Narrow band, angle and polarisation dependent
Broadband dielectric mirror	High reflector	Typically 400 to 750 nm or 750 to 1100 nm	Above 99 percent average across band	High reflectance across a useful band	Narrower than metals, polarisation splits at large angles
Ultrafast dielectric mirror	High reflector with dispersion control	Centred near 800 nm or 1030 nm	Above 99 percent with controlled group delay dispersion	Keeps femtosecond pulses short	Specialist and expensive
Beamsplitter coating (50/50)	Partial reflector	Visible or NIR band at 45 degrees	Splits near 50 percent reflect and 50 percent transmit	Common interferometer part	Polarisation dependence
Polarising beamsplitter coating	Polarisation selective	Laser line or band at 45 degrees	Transmits P and reflects S with high extinction on transmit	Used in isolators and attenuators	Angle and wavelength sensitive
Bandpass filter coating	Spectral filter	Narrow, tens of nm wide or less	Peak transmission often 85 to 95 percent	Isolates a line or band	Centre wavelength shifts with angle of incidence
Longpass or shortpass edge filter	Spectral filter	Cut-on or cut-off edge at design wavelength	High transmission on pass side with OD 4 to 6 blocking possible	Fluorescence and laser rejection	Edge steepness varies with design

Performance values are typical catalogue ranges for guidance only. Real performance depends on substrate, angle of incidence, polarisation, design wavelength and process. Use your coating supplier curves and a measured witness sample for acceptance.

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How to choose a coating

Start with the job: reduce reflection, reflect light, split a beam, or select a band. Then fix the wavelength band, the angle of incidence and the polarisation, because dielectric designs are optimised for specific values. Finally check the power: high-power and pulsed lasers need coatings with a proven laser-induced damage threshold, and metal coatings absorb more than dielectrics.

Single line laser: V-coat or dielectric laser line mirror
Wide spectrum imaging: BBAR or protected silver or aluminum
Infrared: gold mirrors and IR AR on germanium or ZnSe
Ultrashort pulses: dispersion-controlled mirrors

Metal versus dielectric mirrors

Metal mirrors work over a very wide band and are less sensitive to angle, which makes them good for general instruments and infrared. Dielectric mirrors stack many thin layers with alternating index to reach very high reflectance over a designed band, and tolerate far higher laser power. A V-coat or laser line mirror at 1064 nm may reach about 99.9 percent, but only near its design angle. A silver mirror reaches high average reflectance in the visible but absorbs the rest as heat.

What to put in a coating specification

A good coating spec states substrate, surface, wavelength or band, angle of incidence, polarisation, target reflectance or transmission, laser power or damage threshold if relevant, environmental durability class and surface quality after coating. Reference ISO 9211 for definitions and durability testing, and ask for a witness sample or measured spectrum with the shipment. Without these details the supplier will quote against a catalogue standard and acceptance becomes a disagreement.

Common mistakes

Buying a visible AR coating for a near-infrared laser, or the reverse, is the classic error. Another is ignoring the angle shift: bandpass and dielectric designs move toward shorter wavelength as the angle increases. People also forget that a 1 percent loss per surface across ten surfaces removes nearly 10 percent of the light. Record coating codes on the ERP item so reorders match exactly.

Frequently Asked Questions

What is a V-coat?

A V-coat is a simple antireflection coating, usually two layers, optimised for a single wavelength. Its reflectance curve looks like a V with a sharp minimum at the design wavelength, often below 0.25 percent per surface. It is the standard choice for laser lines but performs poorly away from that wavelength.

Should I choose a silver, aluminum or gold mirror?

Protected silver gives the best visible and near-infrared reflectance, aluminum covers the widest range including the ultraviolet, and gold is best from about 700 nm into the far infrared. Dielectric mirrors beat all three at a single band and high power.

Why does a bandpass filter change wavelength when tilted?

In a thin-film interference coating the optical path through each layer shortens as the angle of incidence grows, so the pass band shifts toward shorter wavelengths. The shift is small near normal incidence but grows quickly beyond about 10 degrees, so specify the angle.

How do I verify a coating on receipt?

Ask for the supplier measured spectrum or a witness sample, and check the reflectance or transmission at your design wavelength with a spectrophotometer. Also inspect the surface for scratches, haze, pinholes and delamination against the drawing requirement.

If coating specs and certificates are scattered across item masters, drawings and supplier emails, we can help you build a private assistant over that data in your ERP. Contact us to talk about your setup.

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