What kind of video equipment do stage managers use during blackout to determine when actors are onstage and ready for stage lights?
I'd prefer simple, wired, systems with a dedicated display. But if it's cheap (<$100USD), a wireless system is OK.
Run of the mill night vision (infrared) security camera with zoom. A couple infrared flood lights on stage to light the camera sensor up with.
It's not unlike what you'd use to post security cameras on someone's house if they had a big yard they wanted to cover and needed a little more IR than the lights built into the camera can provide on their own.
Might want to be careful with wireless. Wireless probably entails WiFi, and the transcoding and networklatency could be undesirable. Ideally you want to pull the video signal straight off the back of the camera.
In line with MNicolai, if you're looking for specifics, we use a security unit from Harbor Freight. They don't carry the model we have anymore, but this looks promising for you.
Followup note, tho - watch out for cable length and interference issues on those low-end analog cams. Don't skimp on cheap cable and connectors if you need to extend beyond the 25' or whatever comes in the box. and keep all that packing matl til you are sure that the 200' you needed to run doesn't flummox the camera/monitor combo.
Almost any video camera, without an IR filter will work as long as you run a IR source somewhere in the house. All CCDs are extremely sensitive to IR . Great way to find out if your tv remote is working or not. Point it at a camera and see if it flashes at you.
A great solution that's also incredibly cheap is old handicams with an AV out. Most of them had IR and a hardware IR switch to be On/Off/Auto, a long zoom, and are available at every thrift store and eBay.
I like them because the IR filter that usually exists on security cameras is slow and physical because they're designed to move twice a day - sunrise and sunset. When there's a blackout that happens, it takes the camera a few seconds to flip into "night mode" by removing that IR filter, by which time, your scene change is already long over. Also that physical filter moving a dozen times a night, especially with cheap cameras, those motors wear out.
The handicam's On switch (vs auto) makes the IR filter be removed all the time, so it'll probably look overexposed except in the dark, but saves the auto switching time. An ND filter can help with the light during the scenes and shouldn't affect the IR too much.