Getting the Reflection
An image is only seen when light reflects of the object in question. The reflection of that light is then perceived by the eye, which your brain can then interpret as the image. When you are measuring light you often first go for the incident light, but instead the important light is usually the light that is reflected off of the object. This light is called reflected light.
Interpreting Reflected Light
Determining reflected light is based on the object that the reflected light would come off of. The reflected light is directly correlated to the kind of surface you are dealing with. This extends to that surfaces texture, material, color, position, as well as the amount of light near it and the kind of light source that is projecting.
A white card has a lot of reflected light, which is why it is often used as another light source in many set ups. This is the primary feature of the white card’s role in the three point lighting scheme that is often used for sit down interviews. A carpet will likely have little reflected light, while a sky blue wall may have a lot. One of the difficulties with using bright green screens is that it has both a high level of reflected light and that light ends up being tinted, which transfers the green screen chroma key effect from the wall onto the subject.
Reflected Light Meter
To measure this reflected light you use a reflected light meter. You take the reflected light meter and you point it directly at the object you are trying to read the light off of. The reflected light meter stays in line with the camera position when it is pointed at the object so that you can clearly interpret what the camera will be reading.
You have to be much more particular with a reflected light meter than you would in other light reading situations as you have to stay in direct line of the reflected light. It is not advisable to use the reflected light meter in very open sections or where you are receiving a direct light source near to the meter. This is going to disrupt the reading, which really should just pick up what is coming off of the object.