Rosco Steel Gobo #7702 - Venetian Blind - Size B

You use this "gobo" (go-between) in a light called a source 4, or leko light, or any other projection ellipsoidal fixture like a daylight joker leko, or a LED source 4. It's placed in a gobo holder that matches the size (in this case size b). Then you slide that into your light, there is an opening at the focal point of the light near the center for a drop in gobo. It does not attach to the outside of a lens, and if you did that the pattern would not project in any recognizable way onto a surface. They come in all sorts of sizes.

The aputure spotlights have them already. If you’ve lost them this should fit.

The Rosco Standard Steel Gobo #7702 - Venetian Blind - Size B 86mm is designed for use with ellipsoidal stage lights, not for monolight usage. It is much too small to be used with a reflector designed for monolights.