In specialist imaging cameras, such as imagers used in deep space astronomy or in medical diagnostics, a cooler temperature is very important indeed. This is because these cameras and their sensors, working at microscopic or very low light levels, can be affected by dark current.
What is dark current?
Dark current sounds sinister, but it’s just the free electrons that zap around the charge-coupled device (CCD) of the camera sensor. These electrons are freed up by thermal energy and interfere with the signal electrons that come in from the image itself to the pixels, altering and clouding the image.
The good news is that at any given temperature the dark current going into each pixel is constant, even if it does “use up” a lot of space for the data that’s really wanted.
Cooling reduces dark current
As you can imagine, if heat increases dark current, then lowering the temperature reduces it. For some applications, cameras need to be ultra-cooled – sometimes to temperatures around -100C. In astronomy, sometimes images use only a few photons during exposures of anything up to an hour. These cameras need more than just a sprinkling of liquid nitrogen to maintain their low temperatures, so usually they use thermoelectric coolers to reach and maintain their extreme coldness. Under these conditions, dark current is pretty much eliminated. There are cameras at the other end of the temperature spectrum, such as the ones used for combustion analysis, which can withstand temperatures of 700C or more and still produce sharp images.
What if a camera has a high frame rate?
You might think that a fast frame rate means less dark current, as the number of electrons captures increases according to exposure time. This is right, but fast-frame cameras are still vulnerable to dark current and so still need some cooling. In general, dark current doubles with every 6C of temperature rise. This means that just being handled can double the amount of dark current, or being taken from a cool room into sunlight.
Is it just dark current that can affect sensitive cameras and imagers?
There are other factors, including variability and instability. In general, camera manufacturers are looking for ever-increasing frame speeds, especially in area scan sensors used in industrial inspections. These cameras use multiple outputs and channels and their composite data rates can be a billion pixels a second or more. When cameras are multichannel, their different outputs must match in terms of contrast, acuity and brightness, regardless of the ambient temperatures and operating conditions. Otherwise, an effect known as “tiling” can occur due to the mis-matching of outputs.
Put simply, all the outputs are affected by temperature and uncooled cameras usually can’t match their outputs. The outputs can be altered digitally after the feed is done, but this is very expensive and time-consuming, so it’s best to avoid tiling or other temperature-related problems.
Repeatability is also vital. Repeated images of the same field or process must have the same background stability in order to be analysed properly Medical imaging and laser profiling need this type of stability in particular, so an ultra-cooled camera offers continuous, accurate images that are relatively cheap and easily accessible.