How Does A Digital Camera Sensor Work?

Digital Imaging Basics

Photography has the magical capacity to preserve a moment in time. Key to this is the image sensor at the heart of every digital camera. Just as the retina in the human eye captures light and translates it into nerve impulses that the brain can interpret, the sensor captures light and converts it into an electrical signal that is then processed to form a digital image.

In all digital still and video cameras, the sensor is the key component in capturing an image.

• With all types of sensors, the imaging process begins when light passes through the camera's lens and strikes the sensor.
• The sensor contains millions of light receptors or photosites, which convert the light energy into an electrical charge.
• The magnitude of the charge is proportional to the intensity of the light – the more light that hits a particular photosite, the stronger the electrical charge it produces.
• In order to capture colours as well as brightness information, photosites are fitted with red, green and blue colour filters.
• This means some photosites record the intensity of red light, some the intensity of green, and some the intensity of blue.
• The electrical signals from all the photosites in the sensor are passed to the camera's image processor, which interprets all this information and determines the colour and brightness values of all the individual pixels (picture elements) that make up a digital image.

How Cameras Create A Digital Image

How cameras create a digital image. Light from the subject you're shooting is focused through the lens onto the image sensor, which is covered with a mosaic filter to enable it to detect colour and not just light intensity.

1. The electrical signal generated by the sensor may be amplified by analogue electronics before passing through an analogue-to-digital converter to the image processor.
2. After processing, the camera may temporarily hold images in a buffer while it writes them to the memory card.
3. The end result is a complete colour digital image.

A digital image is a long string of 1s and 0s representing all the colored light dots, known as pixels, which collectively make up the image.

Digital cameras employ image sensors, such as CCDs or CMOS sensors, in place of film. CCDs are collections of tiny, light-sensitive diodes that convert light into electrical charges, which are then digitized to create a digital image.

Digital cameras, too, work on the principle of filtering the three primary colors of red, green and blue. CMOS and CCD sensors resemble black-and-white film in that they respond only to the strength of light.

In digital cameras, the light beam is split light into its RGB elements before striking the light sensor, which then reads the strength of each color per pixel, and converts that information into digital data.

Photosites, Pixels And Colour Filters

• You conventionally hear about the number of megapixels (millions of pixels) in a sensor, but strictly speaking the sensor does not have pixels at all, but sensels (distinct photosites).
• What's more, there is not a one-to-one correspondence between sensels in the sensor and pixels in the resulting digital image, for a whole range of technical reasons.
• It is more accurate to describe a sensor as having a certain number of "effective pixels", which simply means that the camera produces images or videos of that number of megapixels.

The most common type of colour filter mosaic in digital sensors, a Bayer array. This is what makes it possible for the sensor to detect colour, not just light intensity.

There are more photosites dedicated to green because the human eye happens to be more sensitive to green light than to blue or red.

• The pixels are colour-blind, just recording the intensity of light they receive.
• To get a colour image you put filters over each pixel so that some receive only the red light from the scene, some only green light and some only blue light.
• There are usually more green pixels than red or blue because your eye is more sensitive to green light and so it's best to have higher resolution data from the green pixels.

CCD Sensors

There are several different types of image sensor. Digital photography arrived in the mid-1980s with the introduction of CCD (charge-coupled device) sensors. These sensors were the first to make it possible to capture images without the use of film, revolutionising photography.

• CCD sensors are composed of an integrated grid of semiconductor capacitors capable of holding an electrical charge.
• When light reaches the sensor, these capacitors, acting as individual photosites, absorb the light and convert it into an electrical charge.
• The amount of charge at each photosite is directly proportional to the intensity of the light that strikes it.
• In a CCD sensor, the charge from each photosite is transferred through the sensor's grid and read at one corner of the array, in the same way that water might be passed along a bucket brigade or human chain.
• This method ensures a high degree of image quality and uniformity because each pixel uses the same pathway to output its signal.
• However, this process is also more power-intensive than the process in CMOS sensors.

CMOS Sensors

Unlike the CCD sensor, which transfers charges across the sensor to a single output node, a CMOS sensor contains multiple transistors at each photosite, enabling the charge to be processed directly at the site.

• For a start, CMOS sensors require less power, making them more energy efficient.
• They can also read off electrical charges at a much faster rate, which is crucial for shooting high-speed sequences.
• What's more, CMOS sensors share the same basic structure as computer microprocessors, which allows for mass production at a lower cost while incorporating additional functions such as noise reduction and image processing right on the sensor.

CMOS sensor technology has continued to evolve. An innovation developed is Dual Pixel CMOS AF technology, which enables each pixel on the sensor to be used for both imaging and autofocus, resulting in faster and more accurate AF performance.

Another development in CMOS technology is the stacked, back-illuminated sensor design. This design places the photodiodes above the transistor layer to improve light collection efficiency, resulting in less image noise and better image quality. Additionally, the stacked structure allows faster data readout, contributing to the camera's high-speed performance.

Sensor Components Inside A Camera

The image sensor is one of the most important components of any machine vision camera.

While a sensor's function is to convert light into an electrical signal, not all sensors are built the same.

• However they are classified, the purpose of image sensors are the same; to convert incoming light (photons) into an electrical signal that can be viewed, analyzed, or stored.
• Image sensors are a solid-state device and serve as one of the most important components inside a machine vision camera.
• The solid-state image sensor chip contains pixels which are made up of light sensitive elements, micro lenses, and micro electrical components.
• The packaging protects the sensor chip and wire bonds from physical and environmental harm, provides thermal dissipation, and includes interconnecting electronics for signal transfer.
• A transparent window in the front of the packaging called a cover glass protects the sensor chip and wires while allowing light to reach the light sensitive area.

In a camera system, the image sensor receives incident light (photons) that is focused through a lens or other optics. Depending on whether the sensor is CCD or CMOS, it will transfer information to the next stage as either a voltage or a digital signal.

CMOS sensors convert photons into electrons, then to a voltage, and then into a digital value using an on-chip analog to digital converter (ADC).

Colour And Monochrome Sensors

For visible light sensors there are two main types; color and mono.

• Color sensors have an extra layer that sits below the micro lens, called a color filter, which absorbs undesired color wavelengths so that each pixel is sensitive to a specific color wavelength.
• For mono sensors, there is no color filter so each pixel is sensitive to all visible light wavelengths.
• For the color sensor example shown above right, the color filter array employed is a Bayer filter pattern.
• This filter pattern uses a 50% green, 25% red and 25% blue array.
• While most color cameras use the Bayer filter pattern, there are other filter patterns available that have different pattern arrangements and RGB breakdowns.
• For some sensors, especially sensors with smaller pixel sizes, additional micro lenses are used to help guide photons into the photodiode.

Global And Rolling Shutter Types

An important function of the sensor is its shutter type. The two main electronic shutter types are global shutter and rolling shutter. These shutter types are different in their operation and final imaging results, especially when the camera or target is in motion.

• All pixels begin and end exposure at the same time but readout still happens line by line.
• This timing produces non-distorted images without wobble or skewing.
• Global shutter sensors are essential for imaging high speed moving objects.
• Exposure timing is different line by line with reset and readout happening at shifted times.
• This row by row exposure produces image distortion if either the target or camera are in motion.
• Rolling shutter sensors offer excellent sensitivity for imaging static or slow moving objects.

Sensor Size And Pixel Size

It's clear that a sensor's megapixel count isn't the whole story. The physical size of the sensor is an important factor.

• Image sensors come in different format types and packages.
• Resolution and pixel size will dictate the overall size of a sensor with larger sensors having either higher resolutions or larger pixel sizes than smaller sensors.
• Knowing the sensor format is important for choosing a lens and optics for a camera.
• All lenses are designed for specific sensor formats and resolutions.
• Note that sensor formats only describe the area of the sensor chip and not the entire sensor package.

Pixel size is measured in micrometers and includes the entire area of both the photodiode and surrounding electronics.

Typically a larger pixel size is better for increased light sensitivity because there is more area of the photodiode to receive light.

If the sensor format stays the same but the resolution increases the pixel size must decrease.

If two sensors have the same total pixel count but one is physically larger than the other, then each photosite on the larger one must be bigger.

Photosites act as "light buckets" and, in the same way that a wider bucket would capture more rainwater than a narrower bucket, a larger photosite captures more photons with relatively less random noise.