Reflector in used both in photography and cinematography, the basic idea is to lighten the subjects with indirect diffused soft light to control the appearance. Appearance of what ? Appearance of the details of the subjects. For example, when the sun is over our head, if you take a picture without using reflector / reflectors / external extra artificial light source; the shadow casting on the faces will remove the details of the face – this is the basic reason why we use Fill Flash in Digital Photography and Film Photography.
Why we use Reflector
But, the above example has too much limitation. For example, for Still Photography; Fill in Flash might solve the issue to some extent. But, in case of shooting a group of people under the sun or in case of cinematography; usage of fill in Flash has too much technical limitation. Furthermore, the luminance of Sun Ray is so high at the noon, Fill in Flash might totally fail or will demand a big light source with nearly same luminance. The easiest way is to use a reflector, which basically reflect the sun rays; so it is not only technically easy solution, but also is cost effective.
The ultimate thing is that, in any given situation; we have to emulate the basic model of light box. As everything can not be put inside a box, however big it is plus it is not desirable to use a white background constantly, so we actually need to follow the basic diagram and principle of studio setup.
Reflector : Technical and Optical Parts
If you ever have went to a professional studio, you must have noticed various kind of white or silver colored plates or umbrella like things. These are basically the reflectors. Yes, an umbrella with silver coating inside can work as a basic version of a reflector. Now thing about the umbrella. It is actually a parabola if drawn as a line diagram. As it is a parabola, it will have a vertex. Your umbrella will look like this :
So, F is the point, where if the light source is placed, it will reflect the lights – This is called Parabolic Lamp reflector. You have seen this thing in the studio. But, in outdoor, what to do with such a thing ? We will use a more flatter version – that is Board reflector. Yes, this board reflectors are also actually a parabola, the incident light is the sun rays which are taken to be parallel incident rays. Now the diagram becomes different :
Q1, Q2, Q3 are virtual points. If we extend the P1-Q1, P2-Q2 and P3-Q3; they becomes three lines. Now forget about the part of line which stops at P1, P2, P3 starting from Q1, Q2 and Q3. So – a bunch of parallel rays are falling and they actually can focus the light rays on the point F. If we make the red F line more flatter, we can increase the distance of this F. This is the basic principle of using reflectors which are sold as camera accessories :
To assist the setup, we need a typical thing – that is an external light meter.