you could use an ultrasound distance sensor instead, like the - SHARP 2Y0A02 F 9Y - which has a working range of 20-150 cm (~8 .. 60 inch)
(and I assume throughout the world) has a safety sensor
Since nearly every garage door opener in the US (and I assume throughout the world) has a safety sensor, I concluded someone must know how they work.
Some discussion that might be relevant here: http://arduino.cc/forum/index.php/topic,100804.0.html
I think many of those sensors are Infrared beam modulated at 38KHz. You could build you own with IR LEDs, a 555 timer to drive them, and typical 38KHz receivers like these:http://goo.gl/2uwYM
Do those allow a continuous 38Khz signal? Most IR receivers require some period of off time in the signal
Most IR receiver ICs are designed to reduce their gain when they detect continuous 38kHz from a compact fluorescent light bulb. Their gain is at max when the IR pulses are in bursts of data.