FCM coating

[Glaiden 6]

Newbie question - Can anyone please explain to me, what is FCM coating? 

[RikM 2,574]

FCM I don't know but FMC normally full multi coating and is the anti-reflective, anti-scatter etc. coatings in the lens elements, particularly in eyepieces. 'Fully' multi coated means all the air-glass surfaces not just the top face of the eye lens. FMC normally means better performance than simply top coated or nothing mentioned.

[rfdesigner 89]

For reference, Multi-coated means the anti-reflectance works well over a range of wavelengths.  single coated means it will work nearly perfectly at one wavelength then deteriorate away from that

i.e. green is excellent, deep red and deep blue rather less so.

[acey 1,260]

Multi-coated means there are, er, multiple coatings, rather than a single one.

Any lens surface will reflect some light, i.e. fail to transmit perfectly. If you coat a lens then there are two reflections coming off it: one from the surface of the coating, another from the glass underneath. The trick is to make these reflections cancel. The energy of these waves won't just disappear, it will get transmitted through the lens, so the resulting image will be brighter.

The cancellation will happen if the incident and reflected light waves are half a wavelength out of phase. So you want a coating of just the right thickness (in fact, a quarter of a wavelength). But which wavelength? Light is a mixture, so we have to make a choice. Or else use multiple coatings.

In multi-coating we have several layers on top of each other to deal with different incoming wavelengths. These layers all need to be different thicknesses (each of them a quarter of the desired wavelength) and also different refractive indices (so that there will be a reflection at each layer interface). That produces a better result, but with more effort, hence cost.

The colour you see on a lens doesn't necessarily tell you all that much. More important is the brightness of the reflection: the brighter it is, the poorer an "anti-reflection" job it's doing. A perfect lens, if such a thing could be made, would show no reflection at all: all the light would be getting transmitted through it without loss.

Any decent eyepiece will be "fully multicoated", i.e. each reflecting surface has multiple coatings to improve light transmission. But there's a lot more to an eyepiece than coatings, and generally you get what you pay for.

Edited August 27, 2014 by acey

[Glaiden 6]

Thanks for answers.