This "Blue Gel"...is like see through Plastic...comes in a Roll, and is used by Television Camera Operators to soften Lighting, by either hanging this "Blue Gel" over the Studio Lights....or...by just putting a pit of this "Plastic" in front of the Lens....which I have done. I am just experimenting with it...and have no idea if it has a use in Mainstream Photography...I am...a little...unorthodox....and can't even guarantee, that I have stated the correct use for "Blue Gel"