This is a GAM GC1523 Full CTB Blue Cine Filter. CTB filters help you achieve proper color balance when using strobes or HMIs under incandescent lighting conditions. These filters will help raise the color temperature to match true daylight (5500 Kelvin).
CTBs are available in various "steps" - each one a different temperature rating. As individual incandescent lamps vary in color temperature and with their operating voltage, CTBs are used to "tweak" the balance between the ambient light and the light source being employed to make the image (strobe, etc).
A full CTB will typically convert a 3200K light source to 5500K, i.e. normal daylight.
Gam GC1523 Full CTB Blue Cine Filter (20x24" Sheet)