In photonics since 1999. Benelux technical support, answer within one working day.
Coating a lens for the band you will actually use
An uncoated glass surface reflects about four per cent. A broadband antireflection coating brings that below half a per cent, but only inside its design band. Outside it, the coating can be worse than no coating at all, which is why the wavelength belongs at the top of every optics enquiry.
THE SHORT ANSWER
An uncoated glass surface reflects about four per cent. A broadband antireflection coating brings that below half a per cent, but only inside its design band. Outside it, the coating can be worse than no coating at all, which is why the wavelength belongs at the top of every optics enquiry.
| Uncoated reflectance per surface | About 4 % |
|---|---|
| Broadband AR reflectance | Below 0,5 % average |
| Typical VIS-NIR band | 400-1000 nm |
| Available coating range | 266 nm to 6000 nm |
| Effect of 10 surfaces uncoated | Around 34 % light lost |
| Ghost images | From residual surface reflections |
Roughly four per cent of the light reflects at every air-glass interface. One surface is a rounding error. A ten-surface assembly loses about a third of the light and scatters the lost portion around the inside of your instrument.
How antireflection coatings work
A coating adds layers whose thickness makes the surface reflections cancel by interference. Because the mechanism is interference, it is wavelength dependent. A broadband coating spreads the cancellation across a band and accepts a compromise everywhere; a V-coat optimises one wavelength almost perfectly and ignores the rest.
The specification that gets left out
Standard catalogue coatings are usually specified for the visible or the visible and near infrared. Order a lens for a 1064 nm system without saying so and it will arrive correct against its own datasheet and reflect several per cent of your beam. Worse, that reflection travels back along the beam path toward the laser.
Coating capability extends from the deep ultraviolet at 266 nm out to 6000 nm in the infrared. It only helps if the request names the band.
Ghosts and glare
Residual reflections do not simply vanish. They form faint out-of-focus images and a general veiling haze that reduces contrast. In a system where contrast is the measurement, coating quality is not an efficiency question but an accuracy one.
Handling
Coatings are thin and soft. If the optic will be cleaned regularly, ask for a protected coating and accept the small performance penalty, because a scratched coating performs far worse than a slightly compromised one.
When to use it
- Any system with several air-glass surfaces, where losses compound
- Low-light or photon-starved measurements
- Laser work, where a residual reflection can travel back into the source
- Imaging systems suffering ghost images or veiling glare
What to watch out for
- A coating optimised for the visible may reflect strongly at 1064 nm; state your wavelength, not your application
- Broadband coatings compromise across the band; a single-wavelength V-coat does better at one line
- Coatings have their own damage thresholds, often lower than the substrate
- Cleaning damages coatings, so specify a protected coating if the optic will be handled
Still deciding?
Tell us the wavelength, the aperture and what you are measuring. We answer with a part number, a price and a lead time, usually the same working day.