Phototransistor basics

Wide band vs narrow band photo diode

Wide band components can see a wider range of colours / wavelengths, narrow band is more centered on a specific colour / wavelength

Basic typical detect circuits

Common-Emitter Amplifier (often used):

Common-Collector Amplifier:

The current generated in the component when light is detected is very small, but the phototransistor is basically and an internal amplifier which magnifies this current to useful levels.

The phototransistor generates a response proportional to the light received by the component up to a certain light level. When the amount of light surpasses that level, the phototransistor becomes saturated and
the output will not increase even as the light level increases.  By adjusting the load resistor in the amplifier circuit one can set the mode of operation between measuring light level or using the phototransistor as a switch.  Bear in mind that changing the load resistor will likely alter the response time of the device (lower resistance = higher current = faster response often).

Using a phototransistor to measure light level

VCC > RL x Ic