An IR Led emitter combined with an IR receiver can make an opto-isolator.
By using an opto isolator we can transfer information between circuits without an electrical connection.
In opto isolators the emitter is always an IR LED.
The receiver can be a photo-diode or a photo-transistor.
Photo transistors are transistors using the base as "light" sensor. When light strikes the photo transistor's base, it will conduct otherwise it will insulate.
Photo-diodes, on the other hand, are semiconductors that produce current flow when they absorb light.
There are two types of photo-diodes the photovoltaics and the photoconductors.
The photovoltaics when they absorb light create a voltage difference between the edges.
Photoconductors are reverse biased photo diodes. When they absorb light the reverse biased resistance will decrease.
Notice that a photo-transistor is nothing else than a reverse biased photo-diode + a normal transistor.
Common Emitter is an expression to indicate exactly an electrical configuration where a part on three of BJT transitor (and only that kind of transistors) is used as reference for the other two.
So keep in mind it's impossible to make confusion with other contexts.
But... what's an emitter, or a transistor? why bjt? I'm instead confused!
And you're right if never seen one of them, neither in theory exposition.
Generally a transistor is a semiconductor device which you can regulate voltage and/or current in output with; maybe its name's explanation could give a bit of information more: transistor is the contraction of "transfer resistor".
According to their inventors it's been so called "because it's a resistor or semiconductor device that can amplify electrical signals as they are transferred through itfrom input to output terminals". ...continue reading Common Emitter. Why and how to