photosite

What is a photosite?

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When people talk about cameras, they invariably talk about pixels, or rather megapixels. The new Fujifilm X-S20 has 26 megapixels. This means that the image produced by the camera will contain 26 million pixels. But the sensor itself does not have any pixels, the sensor has photosites.

The job of photosites is to capture photons of light. After a bunch of processing, the data captured by each photosite is converted into a digital signal, and processed into a pixel. All the photosites on a sensor contribute to the resultant image. On a sensor there are two numbers used to define the number of photosites. The first is the physical sensor resolution which is the actual number of physical photosites found on the sensor. For example on the Sony a7RV (shown below), there are 9566×6377 physical photosites (61MP). However not all the photosites are used to create an image – the ones that are form the maximum image resolution, i.e. the maximum number of pixels in an image. For the Sony a7RV this is 9504×6336 photosites (60.2MP) used to create an image. This is sometimes known as the effective number of photosites.

The Sony a7R V

There are two major differences between photosites and pixels. Firstly, photosites are physical entities, pixels are not, they are digital entities. Secondly, while photosites have a size, and are different based on the sensor type, and number of photosites on a sensor, pixels are dimensionless. For example each photosite on the Sony a7RV has a pitch (width) of 3.73µm, and an area of 13.91µm2.

How do camera sensors work?

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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.

Cross-section of a sensor.

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).

sensor-colour1
Filtering light using colour filters, in this case showing a Bayer filter.

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.

Why camera sensors don’t have pixels

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The sensor in a digital camera is equivalent to a frame of film. They both capture light and use it to generate a picture, it is just the medium which changes: film uses light sensitive particles, digital uses light sensitive diodes. These specks of light work together to form a cohesive continuous tone picture when viewed from a distance. 

One of the most confusing things about digital cameras is the concept of pixels. They are confusing because some people think they are a quantifiable entity. But here’s the thing, they aren’t. Typically a pixel, short for picture element, is a physical point in an image. It is the smallest single component of an image, and is square in shape – but it is just a unit of information, without a specific quantity, i.e. a pixel isn’t 1mm2. The interpreted size of a pixel depends largely on the device it is viewed on. The terms PPI (pixels per inch) and DPI (dots per inch) were introduced to relate the theoretical concept of a pixel to real-world resolution. PPI describes how many pixels there are in an image per inch of distance. DPI is used in printing, and varies from device to device because multiple dots are sometimes needed to create a single pixel. 

But sensors don’t really have “pixels”. They have an array of cavities, better known as “photosites”, which are photo detectors that represent the pixels. When the shutter opens, each photosite collects light photons and stores them as electrical signals. When the exposure ends, the camera then assesses the signals and quantifies them as digital values, i.e. the things we call pixels. We tend to use the term pixel interchangeably with photosite in relation to the sensor because it has a direct association with the pixels in the image the camera creates. However a photosite is physical entity on the sensor surface, whereas pixels are abstract concepts. On a sensor, the term “pixel area” is used to describe the size of the space occupied by each photosite on the sensor. For example, a Fuji X-H1 has a pixel area of 15.05 µm² (micrometres²), which is *really* tiny.

A basic photosite

NB: Sometimes you may see photosites called “sensor elements”, or sensels.