Electro-Optical Sensors

Electro-Optical (EO) sensors are used in a wide variety of satellite applications.  EO sensors take light energy and turn it into electrical energy.  There are a variety of sensors that do this.  Some image while other provide the centroid of a spot of light.

Scientific imaging is largely done with CCDs.  FPAs can fill the same role as CCDs with many advantages (such as less noise and therefore sensitivity to lower signal levels).  However, fewer people have experience with FPAs the engineering community than CCDs.  Also (I believe) FPAs are more expensive.

LECs and Quad Cells are typically used as position sensors.  LECs use a single photodiode with 4 electrical pickoffs to return the centroid of a spot.  Quad cells use 4 photodiodes each with 1 electical pickoff for centering applications and centroiding applications.  Each sensor returns 4 voltages/currents which are used to calculate the centroid.

Note that for a Quad cell to provide fine resolution centroid information the light spot must be “large” compared to the sensor itself.  A tight light beam (like a laser) creates a small spot and the only information returned is the quadrant where the light is.  A broad light beam where the spot occupies 50% of the sensor face, for example, will create signals from all 4 quadrants for a centroid calculation.

For more information see the Electro-Optical Sensors article on the wiki.