Color Camera Types

You can use three basic camera types for color acquisition—analog or digital RGB cameras, composite color cameras, and Bayer cameras.

RGB Cameras

RGB cameras deliver the three basic color components—red, green, and blue—on three different wires. This type of camera often uses three independent CCD sensors to acquire the three color signals. RGB cameras are used for very accurate color acquisition.

Composite Color Cameras

Composite color cameras transmit the video signal on a single wire. The signal is composed of two of the following components, which are added together:

• A monochrome video signal that contains the gray level information from the image and the composite synchronization signals. This signal is the same as a standard monochrome video signal, such as RS-170 or CCIR-601.
• A modulated signal that contains the color information from the image. The format of this signal depends on your camera. The three main analog color standards are as follows:
  • M-NTSC (also called NTSC) is used mainly in the U.S. and Japan.
  • B/G-PAL (also called PAL) is used mainly in Europe, India, and Australia.
  • SECAM, which is used mainly in France and the former Soviet Republics, is used only for broadcasting, so SECAM countries often use PAL as the local color image format.

Bayer Cameras

Bayer encoding is a method to produce color images with a single imaging sensor, as opposed to three individual sensors for the red, green, and blue components of light. This technology greatly reduces the cost of cameras.

The Bayer color filter array (CFA) is a primary color mosaic pattern of 50% green, 25% red, and 25% blue pixels. Green pixels comprise half of the total pixels because the human eye gets most of its sharpness information from green light.

Light travels through the camera lens onto an image sensor that provides one value for each sensor cell. The sensor is an array of tiny, light-sensitive diodes called photosites, which converts light into electrical charges. The sensor is covered by the Bayer CFA so that only one color value reaches any given pixel. The raw output is a mosaic of red, green, and blue pixels of different intensities.

When the image is captured, the accumulated charge for each cell is read and analog values are converted to digital pixel values using an analog-to-digital (A/D) converter.

Interpolation, sometimes referred to as demosaicing, fills in the missing colors. A decoding algorithm determines a value for the RGB components for each pixel in the array by averaging the color values of selected neighboring pixels and producing an estimate of color and intensity. This algorithm can be applied for the camera, the image acquisition device, or in the software. This algorithm is included in NI-IMAQ and NI-IMAQ for IEEE 1394 Cameras.

After the interpolation process is complete, the white balancing process further enhances the image by adjusting the red and blue signals to match the green signal in white areas of the image.