Just behind the front set of slits, before the monochromator crystal the white X-ray beam
passes through a very thin aluminium foil set at a negative potential with respect to two thin
wire grids mounted a small distance away either side of the foil. The X-rays stimulate
the emission of electrons which are then accelerated towards the wire grids. The current produced
is of the order of 
and is measured using a Keithley 427 current amplifier. A
potential difference of 
is typically applied across the device.
The maximum power radiated by the orbiting electrons is observed in the orbital plane. The radiation
emitted in the orbital plane is perfectly linearly polarized having its electric field vector
perpendicular to the direction of motion in the plane of the electron orbit [15] [41].
Above and below this plane the radiation is elliptically polarized. The ratio of the horizontal
and vertical components of the radiation is dependent on the angle of the radiation away
from the orbital plane and the ratio 
.
By scanning a narrow slit vertically through the white beam and recording the signal from
this device, a plot similar to that shown in Fig. 
is obtained.
Positioning the slit so as to obtain the maximum signal from the photo-emission device
is equivalent to maximising the proportion of horizontally polarised X-rays in the
transmitted beam although this proportion will also be dependent on the vertical range of
angles accepted by the slit.
Figure: Signal obtained from the photo-emission device when a 
front slit is
scanned vertically through the white beam of radiation. The maximum in the output signal
is chosen due to the higher content of horizontally polarised X-rays in the plane of
the electron orbit.