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Hi Gus, "Incident" and "reflected" are directly related to each other. "Ambient" is only indirectly related the other two terms. Ambient Light:
In photography, this is the light that is present without the photographer adding any additional lighting (usually with a flash or strobe). When you measure the existing luminance level of your subject or scene with a light meter, you are determining how much ambient light you have. Most light meters are designed to measure "ambient" lighting. [There is a very special type called a "flash" meter which is used to measure how much light there will be with a strobe firing. These are usually used by professionals in a studio.] Reflected Light:
As used in the context of metering ambient light, measuring reflected light is done by aiming the light meter at the subject (or scene) to be photographed, from the location of the camera. You are measuring the light reflected by the subject (or scene) toward your camera lens. The internal metering in your camera _always_ measures reflected light. This is also called the "subject luminance." Unless you compensate when making the reading, it is assumed the overall average of the subject (scene) is equivalent to 18% gray, which is what humans perceive as a shade of gray exactly halfway between pure white and pure black. This use of an average works well nearly all the time. Why is it 18% instead of 50%? I believe it is related to the reflectance. That is, something "18% gray" reflects 18% of the light falling on it. That is why I stated it's what humans _perceive_ as halfway between white and black. Incident Light:
Many light meters have a translucent white dome or cover. This is placed over meter sensor for making an incident light reading. Incident light is the light illuminating (or falling on) the subject instead of what is reflected by the subject. An incident reading is made by placing the diffuser over the meter sensor, and aiming it at the camera lens from the location of the subject. Incident light is also called "subject illumination" instead of "luminance." Why make an incident reading? If properly done it is more accurate than a reflected reading, especially if the subject is very light or very dark, and you want to preserve that in the image. Why use reflected readings if an incident one can be more accurate? Since reflected is measured from the camera position, it is much more convenient and almost always accurate enough, even with finicky slide film which is not very forgiving of exposure errors. Indeed, sometimes it is impossible because of the location of the subject. Sometimes if the camera is illuminated by the same amount of light from the same source(s) as the subject you can do it from the camera location and aim in the same direction as if you were at the subject's location. Whether you are making a reflected or incident reading, you are measuring the ambient light. -- John
December 15, 2000
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