key light 
noun
The main source of light in a photograph or film.If you have portable lights, you can dramatically improve your shots using the three-point lighting technique based around a key light, a fill light and a backlight....
- It's a face that, yes, could make you half-believe in the human ideal, make you pass into a foggy romantic movie-trance of studio key lights and shadows with edges so soft you could lay your head down in them and sleep for a week.
- I've always had a soft spot for Crawford in the 1950s, when she was vainly hanging on to the glamour girl image just as her key light got brighter and the camera lens got softer.