Photo resistors (or Light Dependant Resistors), are useful for experimental projects such as a DIY theremin because they’re low cost and easy to get started with.
You can use a LDR with development boards such as the Arduino and Raspberry Pi. One of the big issues with them is that they have to be calibrated to the room you’re in.
So let’s say you are building a performance at home with a certain level of light in the room, and then you take it a dark venue, all of your values have to be set again.
To get around this, many people will want to use something like a proximity or motion sensor, which are able to track objects without being affected by the amount of light in the room.