I want to build a laser detector using a photoresistor and a pen laser. Ideally, I would like to increase the surface area that detects the laser beam, perhaps through refraction or diffusion. I'm also wondering if it's possible to either filter the light, so that it will only detect a red laser, or somehow implement using a photoresistor that is only sensitive to laser light. Has anyone implemented something similar before ?
Red notch filters are available that will help you only detect red light, but you still have to cope with red ambient light.
The usual solution is to modulate the light, like the 38kHz used in IR remote controls. It's then easy to spot a signal that's varying at that frequency and distinguish it from background levels. With better modulation you can distinguish which laser source illuminates the detector.