Digital Camera Technology for Today’s Industrial Imaging Applications

The responsiveness of a camera defines how sensitive the camera is to a fixed amount of exposure. The responsiveness of a camera can be defined in LUX or DN/(nJ/cm²). “LUX” is a common term among imaging engineers that is used to define the sensitivity in photometric units over the range of visible light, where DN/(nJ/cm²) is a radiometric expression that does not limit the response to visible light. In general, both terms state how the camera will respond to light. The radiometric expression of × DN/(nJ/cm²) indicates that, for a known exposure of 1 nJ/cm², the camera will output pixel data of × DN (digital numbers, also known as grayscale).

Gain is another feature available within some cameras that can provide various levels of responsiveness. The responsiveness of a camera should be stated at a defined gain setting. Be aware, however, that a camera may be said to have high responsiveness at a high gain setting, but increased noise level can lead to reduced dynamic range.

Digital cameras produce digital data, or pixel values. Being digital, this data has a specific number of bits per pixel, known as the pixel bit depth. This bit depth typically ranges from 8 to 16 bits. In monochrome cameras, the bit depth defines the quantity of gray levels from dark to light, where a pixel value of 0 is %100 dark and 255 (for 8-bit cameras) is %100 white. Values between 0 and 255 will be shades of gray, where near 0 values are dark gray and near 255 values are almost white. Ten-bit data will produce 1,024 distinct levels of gray, while 12-bit data will produce 4,096 levels.

Each application should be considered carefully to determine whether fine or coarse steps in grayscale are necessary. Machine vision systems commonly use 8-bit pixels, and going to 10 or 12 bits instantly doubles data quantity, as another byte is required to transmit the data. This also results in decreased system speed because two bytes per pixel are used, but not all of the bits are significant. Higher bit depths can also increase the complexity of system integration since higher bit depths necessitate larger cable sizes, especially if a camera has multiple outputs.

This article was written by Chris Brais, Applications Manager at DALSA Corporation in Waterloo, Ontario, Canada. For more information, visit http://info.hotims.com/10968-122.