In photography we call these patterned flags a “cookie”. They are very useful for adding a little life to a backdrop, or creating a scene.
In the theater, they are called “gobos” which stands for GOes Before OpticS the gobo is physically located between the light source and the lights optics. It is actually inserted into the light’s housing. To avoid being burned up and ruined by the intense heat of the lights, theatrical gobos are generally made of metal.
In photography, we don’t place a gobo between the light source and the light’s optics. Instead we place it in front of the light. It is between the light source and the area we want the shadow to fall upon. So, though it does the same basic thing, it technically isn’t a gobo. In photography we call them cookies.
The term “cookie” comes from the repetitive “cookie cutter” patterns that most cookies have.
The major difference being that since a gobo comes before the light’s optics; it is easier to control the focusing of the shadow images. They are much sharper than what we get with a cookie.
Cookies can be made of most any materials ranging from metal to paper and everything in between. We have this flexibility since the cookie isn’t subjected to the heat that a gobo has to endure.
In this picture of a cat the cat appears to be snoozing with the late afternoon sunlight shining through the window. In reality, it was shot with a hard light source and a cookie cut to resemble the Tee shapes of the window crossbars.
Sometimes the use of a cookie is not hidden and is very obvious, other times it’s hard to tell.
Question; is this a real scene of a girl looking out a window? Or is it a studio shot with a hard light source and a cookie? There’s no way to know, and that’s the whole point (I suspect the latter).
Obviously – since they are smaller – it is easier to modify studio lights with flags, dots, and so on… but the sun can be controlled too!
Flags can be put between the subject and the sun to either block the light or cast a shadow. Natural objects, like a building, trees and so on can be used as well.