Coating families are easy to confuse because they are all thin-film stacks built in the same chambers. What differs is the function: some coatings maximise transmitted light, some remove a band, some produce color, and some protect the substrate from the environment. This matrix sets out how to pick between them and which numbers belong in the specification.
Buyer's Guides in practice
This article collects the practical guidance we give most often when customers ask us to specify this range, together with the numbers behind each recommendation.
Anti-reflective coatings: more light, less glare
An AR coating exists to reduce reflection at a surface. Uncoated optical glass reflects roughly four percent per face, and a two-sided window therefore loses almost eight percent of the light while producing the ghost images that users notice first. A single-layer magnesium fluoride coating brings each surface to about one and a half percent; a broadband multilayer stack reaches below half a percent, and below a quarter of a percent on precision work. Specify the band, the average and worst-case reflectance, haze, and the acceptable color of the residual reflection. In most systems the second surface contributes as much reflected light as the first, so a specification that coats only one face gives up most of the available improvement.
IR-cut coatings: separating visible light from near infrared
Silicon sensors respond to near infrared, which would otherwise wash out color. An IR-cut coating reflects that band with an interference stack, keeping the part cool and the transition steep - typically 10 to 30 nanometers - with visible transmission above 90 percent and blocking of OD2 to OD4. The catch is angular: the edge shifts toward shorter wavelengths as the incidence angle grows, so a wide-angle system needs either a wider transition, an absorptive element, or a relaxation of the color tolerance across the field. Specify the cut-on wavelength, the blocking depth, the edge steepness, and above all the angle of incidence at which the curve is measured.
Color and dichroic coatings: producing a wavelength, not removing one
Color filters made by coating rather than by dyeing hold their color permanently, because there is no organic colorant to bleach. The design defines a passband, a transition and an out-of-band blocking level, and the same process can place several colors on a single plate or print a logo in a defined color. Tolerance splits into two systems: the spectral tolerance, typically plus or minus five nanometers on a dominant wavelength and two nanometers on a precision edge, and the geometric registration, typically plus or minus 0.05 mm for patterned parts. Both belong on the drawing, along with the acceptance angle and the maximum flux the part will see.
Quartz, sapphire and IR window coatings
Windows in the ultraviolet, in high-energy laser paths, or in the mid infrared are built on materials that behave differently from ordinary optical glass. Fused silica transmits from about 185 nanometers and tolerates high laser energy; sapphire adds hardness and reaches to about 4.5 micrometers; germanium, silicon and zinc selenide cover the thermal bands. Coatings on these substrates must control absorption rather than only reflectance, because absorbed energy becomes heat and heat becomes a focus shift. Specify the laser damage threshold at the operating wavelength and pulse length, and verify it on witness samples from the same material lot rather than relying on a certificate alone.
Durability and environmental coatings
The functional stack is only half the specification; the outer surface decides how the part behaves in the field. Hard oxide layers pass pencil and steel wool abrasion tests, which matters on any surface a user touches. Hydrophobic and oleophobic top coats make rain and fingerprints shed instead of smearing, and are usually specified with a water contact angle above 100 degrees plus an abrasion durability figure, because the layer is consumed by wiping. Anti-static layers reduce dust attraction where cleanliness matters, and barrier layers prevent alkali migration in humid environments on alkali-containing substrates.
Matching the coating to the application
The selection order that works reliably is: identify the band the system must pass, identify the band it must block, decide the acceptance angle, then choose the coating family that satisfies all three at an acceptable cost. Only after that does it make sense to choose the substrate, because the substrate changes the effective optical thickness of every layer. The mistakes that recur are using a visible-only AR coating in a system that also works under 850 nanometer illumination, using a dichroic IR-cut in front of a wide-angle lens, and coating one face when the rear surface is the dominant source of flare. Each of those is a specification error rather than a manufacturing one, and each is resolved by writing the application into the drawing.
Selection data at a glance
| Parameter | Typical value | Notes |
|---|---|---|
| Reflectance per surface | R ≤ 0.5 % average, R ≤ 0.2 % achievable | 420-680 nm |
| Transmission gain | up to 99 % single side, > 99 % both sides | Versus 92 % uncoated B270 |
| Band options | 400-700, 400-1100, 900-1700 nm, dual band | Broadband or tuned |
| Substrate | B270, ultra-clear, borosilicate, quartz, acrylic | 0.5-12 mm |
| Haze | ≤ 0.3 %, typically ≤ 0.1 % | Haze meter |
| Abrasion | pencil hardness 9H, steel wool 0000 pass | Hard oxide stacks |
| Adhesion | tape test 4B or better | Cross-hatch |
| Environmental | 85/85 humidity, salt spray, UV stable | Outdoor grades available |
| Coating pattern | full face, aperture only, or masked | Margin 0.5-1 mm typical |
Frequently asked questions
What reflectance can an AR coating actually reach?
Under 0.5 percent average per surface is routine, and under 0.2 percent is achievable across the visible band with a multilayer stack. Total transmission above 99 percent is possible when both faces are coated.
Should I coat one side or both?
Both, whenever the budget allows. The uncoated rear surface usually reflects as much light as the front coating removes, so leaving it bare gives up most of the benefit and often introduces visible ghosting.
How durable is the coating on a touch screen?
Hard oxide stacks pass a 9H pencil test and 0000 steel wool abrasion, and top layers can be added for fingerprint resistance. Durability under daily fingerprint oil exposure is normally limited by the cleaning method, not the coating.
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