You can improve the infra-red sensitivity of the Lego light detector by using an IR filter on top of the sensor. This filter can be obtained by cutting a non-exposed ("black") piece of color slide film - quite economical for a rather good IR filter !
The space between two perforations has just the right width to create the tip you will carefully insert along the sensor. This protects the sensor from the light coming from the LED (this trick comes from Ralph Hempel). Simply placing a flat piece of film on the detector doesn't work at all because of this parasitic light. Fold the filter over the sensor and secure it either with a small piece of tape or with a Technic Lego brick as shown on the photo.
Using my modified light detector for infra-red proximity detection with my IR-lamp, I get a 50% increase in sensitivity in moderately lit places and a much better working in brightly lit ones.
Note that by folding the film over the LED instead of the
sensor, you enhance the value range you can get from your detector,
as described by Ralph