measuring reflectance of surfaces

Hi , I need to measure the reflectance intensity having different surface ( 940nm IF) approximately one meter distance . Using an IF transmitter and a photodiode with an operational amplifier achieves measuring 50cm , is that sending pulses at a certain frequency and putting a band-pass filter can remove noise from ambient light , and gain reach, send pulses at a given frequency It makes it easy with the PWM output arduino , but as I do for the photodiode just take those frequencies ? ... for example the TSOP4038 receiver takes signals modulated 38khz , but gives a digital output (0 and 1) and I need an analog output , not digital, to deduce the intensity with which the surface reflects infrared ... Any suggestions ? Sorry for my bad english.