Of Prizes, Einstein and Digital Photography

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My two favorite topics are photography and physics as readers of this blog can infer. So it was with great interest that I read recently of the awarding of the Nobel Prize in Physics to Willard S. Boyle and George E. Smith, the inventors of the CCD. A CCD is the sensor found in, among other things, digital cameras. At this point I should just say camera. Film cameras are now the exception not the norm. I still love and shoot daily with my Leica M7.

The CCD was invented in 1969 and only a few years later, in 1975 the first prototype was created at Kodak by Steve Sasson. On the  prototype Sasson:

“shoehorned in a portable digital cassette instrumentation recorder.  Add to that 16 nickel cadmium batteries, a highly temperamental new type of CCD imaging area array, an a/d converter implementation stolen from a digital voltmeter application, several dozen digital and analog circuits all wired together on approximately half a dozen circuit boards.”

It also took 23 seconds to record a crude 100 line black and white images on to cassette tape (remember those!).

1975 Prototype for a digital camera

1975 Prototype for a digital camera

What does Einstein have to do with any of this?

I am so glad you asked! The CCD invented by Boyle and Smith used a silicon plate with millions of light sensitive photocells. The CCD accumulated light induced charges over it’s surface and these charges were read out at the edge of the light sensitive area. In essence this process makes use of the photoelectric effect theorized by none other than Albert Einstein (probably while relaxing on the beach). Einstein postulated that light was actually made of discrete packets called photons. This helped explain a certain phenomenon.

If a light shines on a sheet of metal, it will dislodge electrons from the metal. If the light is of low intensity (that is, low brightness) and low energy (say red light), then a few electrons of low energy will be ejected from the metal. By increasing the brightness of the light, we would classically expect the electrons ejected to more energetic. However, this does not occur. Instead, what happens is that some electrons are ejected but they are still of low energy.

Now if we increase the energy of the light, say to blue light, then we find that the ejected electrons are now high in energy. In turn, low intensity blue light will eject a few high energy electrons and high intensity blue light will eject many high energy electrons.

The relevant observation for photography (notice I did not say digital. It is a given.) is that the energy (or color) of the light determines the energy of the ejected electrons. The intensity of the light does not effect the energy of the electrons (color), but the number of ejected electrons. Boyle and Smith took advantage of this phenomenon in creating the CCD.

photons

For this Einstein won the Nobel Prize in 1921.

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