Inside our eyes there are cone-shaped cells, which are sensitive to some colors, called “primary colors”: red, green and blue. The other colors that we perceive are just combinations of these primary colors. In photography, the red, green and blue corespondents of light expose the corresponding chemical layers of color film. The Foveon sensors work as our eyes; they have three sensor layers that measure the primary colors, as shown in this diagram. If these color layers are combined, it will result a digital image, formed by a mosaic of pixels of uniform colors that are so tiny that it appears uniform and smooth. The only cameras that provide Foveon sensors are Sigma SD9 and SD10 digital SLRs.

The Current Color Filter Array Sensors

The other cameras have sensors that measure only the brightness of each pixel. A color filter array is positioned on top of the sensor to capture the red, green, and blue components of light falling onto it. This way only one primary color is measured by each pixel, while the other two colors are just estimated, based on the surrounding pixels. As a result of these approximations, the image sharpness is reduced. However, as the number of pixels in current sensors increases, the sharpness reduction becomes less visible. Also, as the technology develops, many refinements are made to increase photography quality.

Active Pixel Sensors (CMOS, JFET LBCAST) versus CCD Sensors

To understand the term of digital sensors we have to imagine an array of buckets collecting rainwater. Digital sensors work the same way buy they consist of an array of pixels collecting photons, the minute energy packets of which light consists. The light sensitive photodiode converts the number of photons collected in each pixel into an electrical charge. For a camera to be able to process the values into the final digital photography, the electrical charge is converted into a voltage, amplified and then converted again to a digital value, using analog to digital converter.

In CCD (Charge-Coupled Device) sensors, the pixel measurements are processed sequentially by circuitry surrounding the sensor; while in APS (Active Pixel Sensors) the pixel measurements are processed simultaneously by circuitry within the sensor pixels and on the sensors itself. Capturing images with CCD and APS sensors is similar to image generation on CRT and LCD monitors respectively

The most common type of APS is the CMOS (Complementary Metal Oxide Semiconductor) sensor. CMOS sensors were initially used in low-end cameras but recent improvements have made them more and more popular in high-end cameras such as the Canon EOS D60 and 10D. Moreover, CMOS sensors are faster, smaller, and cheaper because they are more integrated (which makes them also more power-efficient), and are manufactured in existing computer chip plants. The earlier mentioned Foveon sensors are also based on CMOS technology. Nikon’s new JFET LBCAST sensor is an APS using JFET (Junction Field Effect Transistor) instead of CMOS transistors.

11 Responses to “Sensors”

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