Supplemental Light
Supplemental light is the light which you bring to the scene. Using supplemental light may give you an amazing control over the photographs. When using supplemental light, you can control the color, direction, intensity, and diffusion of the light. The two main examples of supplemental light are the small dedicated flash units and studio lights.
The built-in flash
The electronic flash was invented in 1931 and changed photography forever. Having an electronic flash built into the camera means that you are never without a light source. Yet, unless it is used correctly, lighting a scene with it might produce a quite unnatural look. The position of the flash directly above the lens aims the light source directly at the subject, which causes a harsh direct light and shadows which are usually inconsistent with any type of natural lighting. It is almost certain to produce red eye as well. The actual flash is small in size, and the smaller the light source, the harsher the light. Using the built-in flash can produce underexposed backgrounds and overexposed foregrounds.
Studio lights
Studio lights come in two flavors: strobe lights and continuous lights. Strobe lights fire an intense short burst of light when triggered, a lot like a camera flash, and continuous lights are either on or off.
Strobe lights
Studio strobe lights come in many different sizes, makes, and models. Multiple strobe lights can be used together to give you complete control over the light. Studio strobe lights must be triggered with a sync cable or a radio slave system that allows the lights to be triggered wirelessly.
The two types of studio strobe lights are monolights and flash head/power pack systems. A monolight is a self-contained unit that plugs directly into a wall outlet. All the controls are on a single unit. Each moonlight may be used as a master or a slave. When the monolight is used as a master, the sync cord plugs from the camera directly into the monolight. When the master moonlight fires, any slave monolights also fire. The output for each of the monolights, master or slave, is controlled independently on the actual light. This allows each of the lights to output different quantities of light. The flash head/power pack studio light system consists of a central power supply with individual flash units attached by their own cables. The sync cable is connected from the camera to the main power supply control unit, and each of the flash heads is adjustable by changing the amount of power going to each of the flash heads.
Continuous lights
Continuous lights are always on, letting you measure the light with the camera’s metering system. This means no extra equipment and it is not necessary to sync the camera with the lights. Continuous lights used to get a bad rap for being very hot, but this is no longer the case. The new fluorescent bulbs don’t produce much heat at all and can be used all day quite comfortably. The continuous light system also has the advantage of being able to use different bulbs should need be. On the other hand, because the light output from the continuous lighting setup is not as bright as that from a strobe light, it can’t be used to freeze action in the same way.
Have a look at other articles describing the influence of lighting conditions on photography:
The Color of Light, The Diffusion of Light, The Intensity of Light, Light Direction, Light types,