In Chapter 1, I introduced the idea that light could be construed as a stream of particles, which I labelled ‘photons’ for convenience.It turns out that these are real particles, which can be produced,played with, measured, stored, and used for doing things. However,even though photons are in a sense the simplest expression of light, making individual photons is not so simple. Most light sources generate light of a different kind, for which the number of photons is not fixed.
A light bulb, for instance, produces a stream of photons that sprays everywhere. If you looked at the light going in just one direction from the bulb, and then examined just a short temporal section of the beam—a time slot, if you like—then you’d be able to count some photons in that slot. But if you repeated the experiment several times, you’d find that the number of photons was random,sometimes large and other times small. The average number of photons would be fixed, depending on the brightness of the bulb,but you’d never be able to say with certainty how many photons you would measure in the beam at a given time. That’s one of the characteristics of ‘classical light’—light that can be described entirely in terms of waves.
Laser light is also of this kind. The average number of photons in a pulse of laser light can be large, but for any given pulse the actual number of photons will be bigger or smaller than the average.The spread of photon numbers in a pulse is approximately the square root of the average number, so that the relative ‘noise’—the variation in the number of photons in each pulse compared to the mean number over all pulses—gets smaller the higher the average number of photons.
Thus a laser beam has intrinsic intensity noise. This sets a limit on the quality of images you can get with laser illumination.The fluctuations in the laser intensity mean that detecting the separation of two points in an image is imprecise. In fact it is very imprecise for low-intensity light, where the mean photon number is small (so the object is hard to see) and the variation in photon number from frame to frame is large. The only way to get precise measurements is to look for longer, thus increasing the number of photons illuminating the object, and averaging the results over many laser pulses. The relative intensity noise is reduced by this signal averaging, leading to a better-resolved image. The precision increases in proportion to the square root of the number of photons used. This is called the ‘standard quantum limit’, since no classical light beam can beat it.