What is a front or back surface mirror ?
Wikipedia describes a mirror as
A mirror is an object that reflects light in such a way that, for incident light in some range of wavelengths, the reflected light preserves many or most of the detailed physical characteristics of the original light. This is different from other light-reflecting objects that do not preserve much of the original wave signal other than color and diffuse reflected light.
In sla 3d printing mirrors are used to change the path of lightbeams.
Not a single object does reflect light 100% perfect, so the trick is to get a almost perfect reflection.
If you look at the above drawing, you can see the difference between a front surface mirror and a read surface mirror.
On the front face mirror, the light reflecting layer is added to the front side. Because of that, the light beams get reflected very nicely and theoretically every beam gets exactly 1 reflection.
With a rear surface mirror, there is more than 1 reflection. You get the main reflection, but also 1 or more ghost reflections. This is of course totally unwanted for 3d printing since you want to print very high resolution with ultrathin details. Ghost rays don't play nice with you and the print will certainly get ruined.
How to find out if a mirror is front or rear surface ?
If you have a mirror and want to know if it is a front of rear(second) surface mirror, then you can easily test this as shown above. A first surface mirror clearly shows you the direct reflection at the point where the ballpoint touches the mirror, while the second surface (read surface) counterpart clearly shown you that there is a thick glass layer which makes it unusable for good projection. Most (if not almost all) mirrors in our household are actually rear surface mirrors.