Whether being used for data transmission or as part of an alarm circuit, phototransistors are sensitive light detectors that can rapidly detect and amplify an incoming signal. The most important part of phototransistor selection is getting one that does the job. For example, if it's a telecommunications circuit, the risetime must be short enough to support the data rate; if it's a lowlight detection circuit, the amplification should be high and the dark current should be low. Once the phototransistor is selected, the next task is to choose the matching light emitting diode (LED).
Select a phototransistor based on the manufacturer's specifications. Specifications can be found at manufacturer's websites or at an omnibus website such as Globalspec.
Find the spectral response of the selected phototransistor. Most manufacturers will provide a curve, but all will at least quote the wavelength range and the peak wavelength.
Select an LED that matches the peak wavelength of the phototransistor for maximum circuit performance. For silicon phototransistors, the peak wavelength will be in the near infrared, say from 800 to 900 nanometres (0.8 to 0.9 millionths of a meter).
Select a maximum luminous intensity for the LED. It should be bright enough so the phototransistor will operate at its peak response. The intensity will be reduced by beam spreading, reflection and scatter, and absorption along the path.
Many suppliers offer LEDs and phototransistors as matched sets. As above, select the phototransistor first, then see if pre-matched LEDs are available.