There are colour sensor modules for the arduino, they detect colour light within the human vision spectrum, and give you a level of red, green & blue, they are not that expensive.
A while ago it was a popular pic micro thing to use a led as a proximity sensor or pushbutton, a led's capacitance changes with the light falling in it, so if you with a microcontroller light an led, then quickly in software and hardware put the led into a capacitance - timer circuit you can tell how much of its own light is being reflected, therefore you can use it as a prox.
At least I think thats correct, might be worth googling.
That way you can choose what colour, or colours/wavelengths you want to bounce off the target,, and get back a value of conductivity or what ever you call light conductance.
A while ago it was a popular pic micro thing to use a led as a proximity sensor or pushbutton, a led's capacitance changes with the light falling in it, so if you with a microcontroller light an led, then quickly in software and hardware put the led into a capacitance - timer circuit you can tell how much of its own light is being reflected, therefore you can use it as a prox.
At least I think thats correct, might be worth googling.
That way you can choose what colour, or colours/wavelengths you want to bounce off the target,, and get back a value of conductivity or what ever you call light conductance.