Editing Stokes parameters (section)

== Measurement ==

The Stokes parameters (and thus the polarization of some electromagnetic radiation) can be directly determined from observation.<ref>Jackson, p. 300</ref> Using a [[linear polarizer]] and a [[quarter-wave plate]], the following system of equations relating the Stokes parameters to measured intensity can be obtained:<ref>Stone, pp. 313-317</ref>

<math display=block alt=I L of zero equals one-half times I plus Q, I L of pi on four equals one-half times I plus U, I L of pi on two equals one-half I minus Q, I Q of pi on 4 equals one-half I plus V>
\begin{align}
I_l(0) &= \frac12 (I + Q)\\
I_l(\frac{\pi}4) &= \frac12 (I + U)\\
I_l(\frac{\pi}2) &= \frac12 (I - Q)\\
I_q(\frac{\pi}4) &= \frac12 (I + V),\\
\end{align}
</math>

where <math alt=I L of theta>I_l(\theta)</math> is the irradiance of the radiation at a point when the linear polarizer is rotated at an angle of <math alt=theta>\theta</math>, and similarly <math alt=I Q of theta>I_q(\theta)</math> is the irradiance at a point when the quarter-wave plate is rotated at an angle of <math alt=theta>\theta</math>. A system can be implemented using both plates at once at different angles to measure the parameters. This can give a more accurate measure of the relative magnitudes of the parameters (which is often the main result desired) due to all parameters being affected by the same losses.