CMOS Cameras
CMOS cameras operate at lower voltages than CCDs, reducing power consumption for portable applications. Analog and digital processing functions can be integrated readily onto the CMOS chip, reducing system package size and overall cost. Each CMOS active pixel sensor cell has its own buffer amplifier, and can be addressed and read individually. A commonly used cell has four transistors and a photosensing element. The cell has a transfer gate separating the photosensor from a capacitive "floating diffusion", a reset gate between the floating diffusion and power supply, a source-follower transistor to buffer the floating diffusion from readout-line capacitance, and a row-select gate to connect the cell to the readout line. All pixels on a column connect to a common sense amplifier.
CMOS cameras are available in either monochrome or color configurations. Monochrome cameras capture images in black and white or grayscale. In color CMOS cameras, a range of colors is generated with varying combinations of different discrete colors. One common technique is sensing the red, green, and blue components (RGB) and combining them to create a wide spectrum of colors. Some styles of color CMOS cameras capture color using multiple chips, where each chip is dedicated to capturing part of the color image, such as one color, and the results are combined to generate the full color image. These cameras typically employ color separation devices such as beamsplitters rather than having integral filters on the sensors.
As with most camera styles, the important specifications for CMOS cameras include horizontal resolution, maximum frame rate, shutter speed and resolution. Horizontal resolution is the maximum number of individual picture elements that can be distinguished in a single scanning line. It is most common to characterize horizontal video resolution corrected for the image aspect ratio, or specify the resolution in the largest circle than can fit in a rectangular image. Thus, for example a 640 x 480 image would be specified as 480 horizontal lines. Frame rate refers to the number of frames that can be captured per unit time, typically expressed in frames per second. Shutter speed is the time of exposure or light collection. Typically, this can be set across a wide range, resulting in very different forms of image composition. Resolution in CMOS cameras refers to the bit numbers and signal levels presented in terms of the analog-to-digital resolution of the image.
