The Photoelectric Effect in action: electronic cameras

The photoelectric effect -- a single packet of light/energy giving its energy to a single electron -- is the basis for electronic cameras. Let's look at two different types of cameras in detail:

• ordinary CCD cameras
• image-intensified cameras

The first type uses the photoelectric effect once; the second type employs it twice, in order to build up a stronger signal when light levels are low.

CCD cameras

The term CCD stands for Charge-Coupled Device. It is basically a thin wafer of silicon with an array of electrodes glued to its front (or back).

• Look at some pictures of real CCDs

The basic idea is that silicon, a SEMI-conductor, has two types of electrons:

1. valence electrons are stuck to individual atoms
2. conduction electrons are free to move throughout the wafer

When a photon strikes the CCD, it may excite a single valence electron into the conduction band, allowing it to move freely. The conduction electrons can then be shepherded to a readout amplifier by the electrodes.

• See this page for more details.

This works fine in ordinary situations. But when the light levels are very low, CCD cameras are overwhelmed by noise. If only one photon strikes the camera during a frame, producing one electron, but 10 electrons are knocked free by thermal motions of the atoms during the same time, the picture will be lost.

Image-intensified cameras

Is there any way to amplify the light signal, so that it will stand out above the noise? The answer is "yes" -- in fact, there are several schemes to do it.

• An introduction to different types of image intensifiers

One of these methods involves MicroChannel Plates, or MCPs.

• A nice little tutorial on MCPs

Here's the basic idea:

1. a single photon strikes photocathode at the front of the MCP
2. the photon knocks free one electron via the photoelectric effect
3. the electron is accelerated away from the photocathode by a strong electric field (several thousand volts over a space of about one millimeter!)
4. as the electron shoots downward, it runs into the walls of a little tube, or channel
5. each time it strikes the walls, it knocks free several other electrons, which each accelerate and hit the walls, building up a large bunch of electrons
6. the electrons finally strike a phosphor plate at the bottom of the MCP
7. the phosphor converts electrons into photons, in a sort of inverse photoelectric effect
8. a big bunch of photons strikes an ordinary CCD just below the MCP, forming an image

A typical intensifier can turn a single input photon into tens of thousands of output photons! The ratio between the number of photons striking the front of the MCP and the number of photons coming out the back end is called the gain of the device. Expensive MCPs may have gains as large as 50,000.

The big advantage of intensified cameras is that they show lots of detail even at low light levels. For every one incoming photon from the scene, the MCP produces thousands of photons. Thus, the signal-to-noise ratio is much higher than in an ordinary camera when light levels are low.

                S/N in situations with little input light
====================================================================
                     ordinary camera            intensified camera
--------------------------------------------------------------------
 
 number of 
  photons                 2                              2
  from scene

 
 Signal = 
 number of                2                         20,000
  electrons
  produced


 Noise =
 number of
  electrons               5                              5
  from thermal
  noise
---------------------------------------------------------------------

 Signal/Noise             0.4                        4,000

=====================================================================

The disadvantage of these systems is that they can saturate and "white-out" when they encounter lots of light -- such as those found outside during the day, or under a streetlight.

For more information

• Information on intensified cameras and thermal cameras at How Stuff Works.

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.