Philo has developed a new NTX LEGO sensor that is worth taking a look at.

“I built this proof of concept of a light barrier using NXT light sensor and an optic fiber. Any small object passing between the end of the fiber and light sensor phototransistor can be easily detected, even if they have different color or reflectivity.

The fiber I used comes from the RIS 2.0 kit, but similar ones can also be found in some ExoForce sets. The fiber is maintained in front of the LED and the other end is routed to face the sensor. You must take care to avoid any gap between the connector holding the fiber and the sensor to prevent direct light bleeding from LED to phototransistor. The fiber must be applied on the LED for best light coupling. The LED light is greatly attenuated by the fiber (the red glow proves that!). If you use the barrier in bright ambient light, you must shield it properly! ”