fill light 
noun
A supplementary light used in photography or filming that does not change the character of the main light and is used chiefly to lighten shadows.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....
- This is the fill light and helps soften the shadows.
- Shots of Nicole were backed with masses of flowers, the cutaways of O'Brien were dramatically lit with what appeared to be coloured gels on the fill light.