A gray card is a middle gray reference, typically used together with a reflective light meter, as a way to produce consistent
image exposure and/or color in film and photography.
The guide number for an electronic flash measures its ability to illuminate the subject to be photographed at a specific film
or sensor sensitivity and angle of view. A higher guide number indicates a more powerful flash.
Derived from "Go Between" or "Goes Before Optics" is a physical template slotted inside, or placed in front of, a lighting
source, used to control the shape of emitted light.
(Source: http://en.wikipedia.org/wiki/Gobo_%28lighting%29). Gobos are used with Optical Snoots
Light modifier of gold color. Used in portrait and product photographies to give a warm tone to the picture.