138 OPTICS FOR ENGINEERS
where
τ
pass
is less than, but close to, 1
τ
block
is more than, but close to, 0
We can call 1−
τ
pass
2
the insertion loss and
|
τ
block
|
2
the extinction. Alternatively, we can
dene the extinction ratio as
τ
pass
2
/
|
τ
block
|
2
. A good insertion loss might be a few percent
or even less, and a good extinction might be 10
−5
. Specic devices will be discussed later.
6.2.2 Wave Plate
Suppose that in Equation 6.20 we change the values of τ to complex numbers such that both
have unit magnitude but their phases are different. This will cause one component of the polar-
ization to be retarded relative to the other. Later, we will see that birefringent materials can be
used to achieve this condition. For now, let us consider two examples. First, ...