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.
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Donut 6.25″x6.25″
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City Theatrical Donut 6.25 In X 6.25 In S4, Sl, Sh, Pc 3-3/16 in Center Hole