Thanks Steve.
We are a group of students trying to build a small robot to navigate in a small flat arena which has 3 bright light sources (of known intensity/wavelength)at 3 different points (whose co-ordinates are known)on the periphery.The robot has to determine its position and also it should be able to move to a specified co-ordinate when instructed to do so.The whole setup is one dimensional i.e.light sources are on the floor itself.

What we have planned to do is to scan the light sources using photo-transistor(s) mounted on the stepper motor and accordingly record the 3 angles when the sensor receives maximum light intensity(that will be when the sensor is perfectly aligned along each light source).Knowing the co-ordinates of the light sources and their direction the co-ordinate of the sensor can be determined.
This seems to be the only possible way as i don't think there are accurate sensors which can determine the distance from a light source of given intensity/wavelength.

We hope to get some invaluable tips/suggestions.

Thanking you all

Regards

Pete