So we have described photosites, but how does a camera sensor actually work? What sort of magic happens inside a digital camera? When the shutter button is pressed, and the sensor exposed to light, the light passes through the lens, and then through a series of filters, a microlens array, and a colour filter, before being deposited in the photosite. A photodiode then converts the light into an electrical signal produced into a quantifiable digital value.
The uppermost layer of a sensor typically contains certain filters. One of these is the infrared (IR) filter. Light contains both ultraviolet and infrared parts, and most sensors are very sensitive to infrared radiation. Hence the IR filter is used to eliminate the IR radiation. Other filters include anti-aliasing (AA) filters which blur the lines between repeating patterns in order to avoid wavy lines (moiré).
Next come the microlenses. One would assume that photosites are butted up against one another, but in reality that’s not the case. Camera sensors have a “microlens” above each photosite to concentrate the amount of light gathered.
Photosites by themselves have a problem distinguishing colour. To capture colour, a filter has to be placed over each photosite, to capture only specific colours. A red filter allows only red light to enter the photosite, a green filter only green, and a blue filter only blue. Therefore, each photosite contributes information about one of the three colours that, together, comprise the complete colour system of a photograph (RGB).
The most common type of colour filter array is called a Bayer filter. The array in a Bayer filter consists of a repetitive pattern of 2×2 squares comprised of a red, blue, and two green filters. The Bayer filter has more green than red or blue because human vision is more sensitive to green light.
A basic diagram of the overall process looks something like this:
Light photons enter the aperture, and a portion are allowed through the shutter. The camera sensor (photosites) then absorbs the light photons producing an electrical signal which may be amplified by the ISO amplifier before it is turned into the pixels of a digital image.