By Vlado Damjanovski.
Everything started in 1969 with the discovery of the Charge Coupled Device (CCD), by George Smith and Willard Boyle from Bell Labs. Their discovery, initially thought of being used as a memory device, proved to be so valuable to the imaging industry today, that they were honoured with a Nobel Prize for Physics in 2009, 40 years after their invention.
Interestingly enough, the CCD principles are based on the photoelectric effect, which were explained by Albert Einstein back in the early 1900-es, for which Einstein too received his Nobel prize in 1921, not for his theory of relativity, like many would have believed.
Photoelectric effect explains how light energy (photons) are converted into electrical signal in the form of free electrons. You may have seen this effect demonstrated in some National Geographic or science museums souvenir shops, a glass sphere within which a free floating cross hangs, made of silver foils. When you turn a strong light shining onto this glass sphere, the little foil-cross starts spinning in the opposite direction of the light. Light has enough energy to eject electrons from the silver foil collectors so that they create action-reaction effect forcing the cross to spin. Something similar is happening in today’s imaging chips, be that CCD or CMOS.
The base material of an imaging chip is silicon, a semiconductor that “decides” it’s conductive state based on electric field around it. The name semiconductor in electronics indicates exactly that – a material that sometimes acts as a conductor (like copper) and sometimes as insulator (like rubber).
When light photons “hit” the silicon area (which we call picture element – or short “pixel”) it frees up electrons proportional to the light intensity. The more light you have – the more electrons will be ejected from the silicon pixel. This is basically how light information (a scene projected by a lens onto the surface of the imaging chip) is converted into electrical signal. This is a very useful process for converting light information into electrical information. CCD chips can hold the amount of charges (electrons)by manipulating voltage near the pixel.
When a voltage is applied to the area, it becomes like a magnet for the electrons which tend to stay where there is stronger voltage. When a next door pixel gets the same voltage applied too, and the active pixel starts loosing the voltage, the charges get attracted to the higher voltage and they get transferred to that pixel. This is, in a very simplistic form, charge coupled transfer of electrons.
Or, in practical terms, transferring of the electronic image representation from the chip out to the read-out stage, where electrons are converted to voltage, which then make up the video signal. The inherent problem of this process is that electrons are also produced in the silicon by the temperature surrounding it.
The laws of physics are such that state any temperature above absolute zero (-273 C) can generate (eject) electrons in the silicon. The higher the temperature – the more thermally generated electrons. This is the biggest problem in electronics in general – and we call it “noise”.
Noise can only be avoided at absolute zero which is not a temperature you can achieve in your kitchen, but only exists in labs. From the information point of view – noise is unwanted signal because it does not represent the actual image information projected on the chip. Even at normal room temperature there are thermally generated electrons, let alone in the middle of a sun baked hot area. The hotter the environment where cameras are installed is – the more noise is generated by the temperature itself.
Every three degrees of temperature increase doubles the noise. This is especially noticeable in darker areas in a picture, which is unfortunately the most common area of interest in surveillance applications.
For the above reasons, cameras used in astronomy for example are cooled down with special coolers to more than 45 C below the ambient temperature. The cooler the imaging chip is, the better details will see in low light, and astronomy certainly looks for even the tiniest speckles of distant stars, hence noise can easily mask such a detail.
In CCTV however, it is not practical to cool cameras this way, although attempts have been made by some manufacturers for cooling their cameras with Peltier solid state coolers. This, unfortunately, brings the price up and such cameras have not become popular.
There is yet another factor that influences the noise dramatically, not just the temperature, but the pixel size. So the trick is to find the best compromise of pixel and chip sizes. We would like to have imaging chips with more megapixels, but we don’t want to increase the chip size too much as it is expensive. Yes, we want to fit as many pixels as we can in the smaller chip, but this has practical limits too. This is one of the biggest problems with current imaging chip technology. You can only fit so many pixels in a tiny area such as your little fingernail chip size.
Pixels are like little buckets collecting electrons. As described, electrons are generated by the light projected from the lens, but also from the temperature. If the buckets are larger, then electrons produced by the light information will be much more relative to the thermally generated electrons. If the buckets are smaller, the light produced electrons will be much less relative to the thermally generated electrons which will be seen as more “noisy” picture.
With today’s technology, a “good size” pixel is around 6 x 6 micrometers (um), which is typically what you will find on most good Digital SLRs (DSLRs). But in smaller compact cameras these pixels get reduced to 1.5 um. If you compare the area in square micrometers – this is 36 squared micrometers compared to 2.25 squared micrometers, as ration of 16 times. And just for your visual comparison, an average human hair diameter is around 100 micrometers.
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