Digital Camera Sensors

Behind every good digital photograph is a good camera sensor. In many ways digital cameras are not that different from their film counterparts. The lens still focuses and filters light, the shutter still controls exposure speeds and the flash still plays its illuminating role. The overall idea is the same but, rather than the light being captured and converted to an image via a chemical development process, a digital camera directs light onto an electrical sensor which converts the light into a signal. This signal is in turn transformed into a digital image. The sensor in the camera is responsible for this this task and effectively replaces film for all practical application.

Image Sensor

Light

Light is energy: photons. Photons are packets of charged energy traveling through space. These packets travel in wave patterns of varying intensity - these patterns of photons are collectively called the electromagnetic spectrum. Lower frequency wavelengths we call radio waves or infrared. On the more energetic end of the spectrum we find ultraviolet radiation, x-rays and gamma rays. In between infrared and ultraviolet - at the range from 4-7.5x10¹⁴Hz - we find the rainbow spectrum of wavelengths which are visible to humans.

Light being scattered

Photons of light bounce around the universe interacting with the various substances they encounter. Each new physical interaction changes the energy levels - and color - of the photon until ultimately they become absorbed by one material or another. One place photons can be absorbed is into the photoreceptors in our eye. Different types of photoreceptors respond to different colors of light and allow us to see everything that we can.

The three primary colors of light which our eyes can see are red, green and blue - as in RGB televisions. These colors of light can be combined at different levels to produce any other color. When all three colors are present - and reflected - the color which we see is white. Absence of light - when all or most available light is being absorbed - is black, or shadow. Because these specific frequencies can be combined to form any color these colors are the most important for cameras to record. Camera developers understood this and based photo sensor technology on this principle. Digital camera sensors also record these same colors of light - as well as information about the brightness level - in order to take digital photographs.

Steve Sasson

Bayer arrays

The sensor itself is actually a small, flat circuit board. On one side is a tightly packed grid with thousands - or millions - of photo diodes. These diodes are a tiny silicon crystal connected into an electrical circuit. When a photon hits the silicon crystal it disrupts and releases electrons which create a charge. This charge is stored in a capacitor until it is converted into an analog signal which is then sent to the processing chip. The processing chip of the camera takes the charge in the capacitor and converts it to information about the brightness of the photons hitting that particular part of the sensor grid. This grid pattern of photo sensors is not radically different in design from the first "modern" digital camera built by Steve Sasson in 1975!

Unfortunately the photo diodes and capacitors can not store information about the color of the light which has hit their pixel. In order to determine the colors present in a photograph image sensor manufacturers places a layer of filters over the grid. Each pixel gets its own filter and the filters themselves form a special type of pattern called a Bayer array. A Bayer array is a block of four filters which are arranged in the order of blue, green, green and red.

Blue	Green
Green	Red

Each filter allows only a single color of light to pass through and hit its respective photo diode. The location of the filter is known to the processing chip of the camera and so the camera is able to reconstruct which photo diode is reacting to what color of light at which strength and assemble them as an image. Digital camera sensors can have thousands or millions of photo receptors and this repeating pattern of Bayer arrays - called a Bayer pattern - is the most common filter pattern for digital camera sensors.

Blue	Green	Blue	Green	Blue	Green	Blue	Green
Green	Red	Green	Red	Green	Red	Green	Red
Blue	Green	Blue	Green	Blue	Green	Blue	Green
Green	Red	Green	Red	Green	Red	Green	Red
Blue	Green	Blue	Green	Blue	Green	Blue	Green
Green	Red	Green	Red	Green	Red	Green	Red
Blue	Green	Blue	Green	Blue	Green	Blue	Green
Green	Red	Green	Red	Green	Red	Green	Red

This pattern of filters has been used widely and is a well established method of imaging. Only recently have companies like Kodak began to introduce new and different filter arrays. On top of different filter patterns these new sensors include white light (panchromic) sensing blocks. It has yet to be seen if these new filtering methods actually produce a superior quality of image or if it is really more of a marketing gimmick.

Sensing the Light

As digital cameras have evolved so have image sensors. Filtering technology aside many digital camera vendors tout their preferred type of sensor as being superior. The two most common types of image sensor are CCD (Charge-Coupled Device) and CMOS (Complimentary Metal Oxide Semiconductor). Both types of technology are relatively established and each has characteristic strengths and weaknesses

CCD

CCD sensors work on a relatively simple analog method. Each photo diode acts like a little bucket for light to fall into. While the shutter of the camera is open the diodes fill with light and build charge. The charge for each individual sensor is converted to voltage one by one. After being sent out of the sensor even more circuitry converts the voltage into a digital signal which can be stored and transferred in the form of a digital photo. CCD sensors produce clear images but tend to have high power demands - quickly draining battery life.

Foveon X3

CMOS

CMOS sensors are active pixel sensors. This means that the circuitry for converting photons to voltage and then to a digital signal exists on the same circuit board as the sensors themselves. This can be considered advantageous because the camera requires less hardware overall to convert light into a digital image. Because the electronics for the image processing exist around the photo diodes there is less actual surface area which is photo sensitive. For many years this was seen as a problem and CMOS sensors were considered to be inferior. Today, the technology has progressed where either sensor type can produce relatively equivalent images.

X3 and 3CCD

Two new sensor technologies are currently emerging. Foveons X3 sensors use three layers of pixels as a way to capture color data for Red, Green and Blue at each sensor on the array. 3CCD technology uses a dichroic prism to split the inbound light into its three component colors. This light is then read by three separate CCD sensors. There have been many claims as to how these new types of sensors will affect digital imaging but only time will tell if they make it in the long run.