Oh, another thing: SCRs exhibit hysteresis behavior. This means that the current needed to switch them ON is higher than the current needed to keep them turned on... so you may have difficulty turning the switch back off once it is on, or you may notice the light level needed to turn it on is noticeably brighter than the one at which it turns off again.
This may or may not be a good thing in your application.
What are you trying to do, if you don't mind me prying?