Donuts are used in the color frame slot of an ellipsoidal reflector spotlight  to sharpen gobo projections.  Similar to the way the human eye sharpens focus when it squints, a donut reduces the aperture and sharpens the output of the fixture.  There is some reduction in brightness to achieve this increase in sharpness but this is often a tradeoff that designers are willing to make. 

City Theatrical Donut for S4 SL SH and PC

For use with the following Ellipsoidals:

Source Four

Strand SL

Shakespeare

Pacific (19-50 degrees)

Features: Used to help bring templates into perfect sharpness

Durable black powder coat finish

Fabricated from light gauge steel

Dimensions: 6 1/4" x 6 1/4"Center hole: 3 3/16"