First and Second Surface Mirrors

Mirrors are part of most people’s everyday life, from the bathroom mirror, to a car’s rearview mirror, and so on. What most people may not realize is that there are more than one type of mirror, or at least more than one way for a mirror to be silvered.

Most mirrors people use on a daily basis are what are called second surface mirrors. This is where the back of the mirror is coated with the reflective material. This lets the glass protect the reflective coating so that it does not get damaged. Since the reflective coatings are actually quite fragile second surface mirrors are used anyplace where they might be exposed to weather, people’s fingers, and so on. The only problem is that in order to be reflected, light must pass through the glass to strike the reflective material in the back. Since this means the light is traveling through the glass, it gets refracted (bent) just a bit. There is also some loss of light from the light hitting the glass. For most daily use this it not very important. After all, we are just trying to shave, put on makeup, or checking to see if the lane next to us is clear. We’re not taking accurate measurements in a lab when traveling in our car (or at least we shouldn’t be!)

But when using a more advanced instrument, such as a telescope, that  distortion and light loss can be crucial. So almost all reflecting telescopes (and more advanced optical systems) use First Surface Mirrors instead. On a first surface mirror the reflective coating is put right on top of the glass instead of behind it. This means there is no distortion from the glass (as long as the glass surface is properly shaped) and no loss of light. The difference can be quite dramatic. A Second Surface Mirror reflects only around 80-85% of the incoming light (sometimes less) while a First Surface Mirror typically will reflect 94% to as much as 99% of the incoming light, depending on the coating.

There is a price, however: First Surface Mirrors are very vulnerable to the elements. Just touching them with your finger can damage the delicate coating, and while a telescope may be used outside, it should not be left there or left exposed with its dust caps off when not in use. A big issue with solar energy researchers who want to concentrate light with mirrors is that they would like to use the more efficient first surface mirrors but need coatings that can reflect the light and survive the elements. Some additional coatings can be added on top of the mirror to strengthen them but not effect their optical quality. Even without exposure to elements sometimes some damage can creep into a first surface mirror around the edges over the course of a few years.

The way to tell the difference between the two types of mirrors is to touch them gently with a pencil tip. If the tip seems to touch the reflected tip it is a First Surface Mirror, if there is a gap it is a Second Surface Mirror.

A note about mirror coatings: Many folks refer to the coatings a ‘silvering’ and this dates back to the days when mirrors were actually coated using silver. But silver is expensive and prone to tarnishing, even in Second Surface Mirrors (you can see this in many older mirrors). Most mirrors made today are coated with aluminum, which does not tarnish. There are some mirrors coated with silver these days, but this coatings are used because silver is excellent for reflecting infrared light.

A final note about so-called ‘1-way mirrors’, the favorite of cop-drama interrogation rooms and the people who watch focus groups in action. Strictly speaking there is not really such a thing as a one way mirror. What is used is a window with a mirrored coating that reflects much of the light, but not the majority. These actually only work when the light in the room with the ‘mirror’ side is more brightly lit than the observing room. If you watch the cop-dramas you may notice this is one thing they get correct as the observing room is kept rather dark. If you were to light up the other room the “one-way mirror” would act merely as a poor window.

Spectrum Scientifics