The Blue Ring Gaffer's Glass is an essential "Tool of the Trade" for professional set lighting. It is designed to allow you to locate the center of your key light or "hot spot" to accurately determine where to position lighting to achieve optimum coverage.
By holding the Blue Ring Gaffer's Glass up to one eye while standing no less than 10 ft away from the source of the light, you can look through the Blue Ring Gaffer's Glass into your key light while your assistant focuses the light to your liking.
This will give you a precise pattern for where your light will be thrown, it also gives you an idea of drop off and voids.
The Blue Ring Gaffer's Glass is also used to check the condition of light filament, and to locate the sun when it is behind clouds. The Blue Ring Gaffer'sGlass is not designed to look directly at the sun. Use it only to locate the sun through clouds.
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As soon as you decide to pursue a career as cinematographer, or as gaffer or G&E crew, this should be part of your tool set. Stop wasting time adjusting lights and wondering where the beam center is, just stand where it needs to be, get your gaffer glass and look directly into the light without loosing your retina. You'll need practice to "read" the clouds blocking the sun and giving a good time stimate for direct sun to come back.