Charge-Coupled Devices (CCDs)

Military pilots use night vision helmets to see where they are going at night.  At the front of the aircraft are one or more cameras,  In these are charge-coupled devices (CCD) which are basically arrays of millions of solar cells, each connected to a tiny capacitor.  The CCD can be made to intensify the light.   Each element in the array is called a pixel (picture element).  Modern digital cameras regularly have 6 - 8 million pixels.  The picture shows a CCD.

The schematic shows the idea of the photocell attached to a capacitor.

We know that:

• Capacitors have capacitance measured in Farads;
• The charge is proportional to the voltage.

The relationship is:

Q = CV

  Remember that a 1 F capacitor is a very big capacitor.  Real capacitors are measured in microfarads (mF) where 1 mF = 1 × 10^-6 F.

CCDs unload their charge to a computer by shuffling charge along.

Energy in a Capacitor

We have seen how the graph of voltage against charge is a straight line.

Suppose we add a tiny little bit of extra charge, dQ.  The voltage remains very nearly the same.  In adding the tiny bit of charge we to a tiny little job of work, dW. 

As we know that:

 work done (J) = charge (C) × voltage (V)

we can write:

dW = dQ × V

This is represented by the green rectangle.

So if we wanted to raise the charge from 0 to +Q, we would add up all the little rectangles, which becomes the area under the graph.

The area would be given by:

W = ½QV

Since

Q = CV

We can combine the equations to give:

W = ½CV²

and

W = ½Q²

        C

So the CCD owes its success to the physics of capacitors.