CMOS Sensors

CMOS sensors, known as active-pixels sensors (APS), are another type of sensor unit common in digital cameras. It contains integrated circuitry, arrays of pixel sensors and a photodetector that connects to a transistor in order to transmit data. The process of image creation using this sensor is called the CMOS sensor process and it is one of the most common sensor-based development technologies on the market.

The CMOS sensor descends from the MOS active-pixel image sensor which was invented in the 1960s. Noble and Chamberlain invented separate sensor arrays that utilized MOS output amplifiers. Most other sensors at the time were passive-pixel sensors. They each contained a single photodiode and a transistor. They were arranged two-dimensionally and shared pixels depending on the row. Each column had an amplifier, but since the designs were faulty, they produced a high-level of white noise as well as other output problems, because transmission was slow.

By 1978, active-pixel sensors were being used as the basis for other sensor designs, such as the focal-plane arrays. They used some of the concepts of the APS design. They also primarily kept the photodiodes from being integrated in the output amplifiers. This design is one of the basic reasons why the CMOS sensor is faster than its CCD sensor, even though its design lends to higher noise production during picture taking.

During the early 1990s, Eric Fossum began writing a series of articles on how the CMOS sensor would replace the CCD sensor. At the time, the CCD sensor was being used exclusively in most point-and-shoot cameras and in the emerging digital camera technology. In 1993, the Jet Propulsion Laboratory began development of several devices based on the CMOS sensor design. They had a high output speed, used little power and produced solid imagery. By 1995, the Jet Propulsion Laboratory invested in a separate sub-division of the lab to fund projects on further developing CMOS sensor technology for public use in such devices as digital cameras, web cams and medical equipment.

Speed was a major downfall of the CCD sensor. Since the output was so slow, it took much longer to process a single image than the CMOS sensor. Since there is less lag time between images, a CMOS sensor-based digital camera will allow for a quicker response time to take multiple pictures. CMOS sensors combine many areas of the pixel process where the CCD sensors do not; this allows for a quicker overall response time and process time. However, CMOS sensors are more susceptible to white noise than their CCD counterparts.

Many manufacturers have jumped on the CMOS sensor bandwagon and begun integrating it into their products. Everything from cell phones to digital cameras use CMOS sensors to take pictures. These sensors have also been used by the medical field, the military and the government in projects relating to internal imagery, surveillance and other applications. Radiology in particular has taken a liking to CMOS sensors.

A basic CMOS sensor consists of three transistors and a photodetector. The latter is a photodiode in most cases whereas in others it is a photogate detector. The photogate detectors create less noise when used thanks to double sampling features built-in. Photodiode and photogate design allow for reds, blues and greens to be more vibrant and sharp than the CCD sensor design, however, since they are separate units, depending on the camera, you may find more variances in the subtleties of color when the image is processed.

During the exposure process, light is accumulated in the sensor and then creates a charge on the photodiode. This voltage changes based on the amount of light being captured and then cut off by the shutter. From there, the first transistor resets the sensor and clears the charge. The second transistor buffers the voltage without removing the charge. The third transistor relates the pixel information row-by-row for output. Depending on the camera and the sensor design, four to eight shared rows can be created and output.

The standard pixel array for a CMOS sensor is arranged into rows and columns. Pixels share reset lines so that one one row is being read, another is being reset. Since one row is transmitted at a single time, this allows for the output transmitter to be focused on that one task and that increase image and color quality as well as increasing the speed of transmission.

Since CMOS sensors tend to be less expensive than CCD sensors, digital cameras utilizing the former tend to be less expensive. This means that most beginning digital camera users will use a CMOS sensor-based digital camera. Learning the pros and cons as well as how to utilize your sensor is the best way to take quality pictures. Sensors can mean all the difference in how your camera captures and reproduces your photographs. Investing in a CMOS sensor to start with might be the easiest way to begin taking good pictures. From there, investigating other sensor technology as well as using cameras with other designs might show you just what digital camera will work best for your photography.