Re: Infrared intensity varying electric resistance

Thank you for the question. A standard diode has a current - voltage 
characteristic such that current flows relatively easily in one direction 
(+V) and very poorly in the other direction (-V). The only significant 
difference a photodiode makes is that its current or voltage is effected 
by the amount of light with energy greater than or equal to energy gap of 
the diode junction.  So the voltage (if you do something to hold current 
constant) or more typically the current (if you hold the voltage at some 
constant 0 V to some negative voltage - called reverse bias condition) 
varies continuously as a function of the photons (quanta of light, of 
infrared energy in your question) being absorbed. The only problem which 
arises is when one applies a voltage equal to or greater than 
the "breakdown voltage". At that point the diode starts to draw excessive 
current which dwarfs any photovoltaic effect.

http://en.wikipedia.org/wiki/Photodiode


http://www.rp-photonics.com/photodiodes.html