There are always small changes (at no additional product cost) that can be made to increase the performance of your machine vision camera and thus to your overall inspection or metrology system. Binning is one example. There has been a trend with latest generation image sensors to increase resolution while also reducing the pixel size at the expense of some other performance parameters. If you do not need all of the resolution provided, but require better signal to noise ratio for instance, then binning is an easy solution. Here we will discuss binning with CCD cameras (interline transfer image sensor technology). For more information on binning with CMOS cameras, please see a previous post.
Binning is the same as adding the charge of 2 or more pixels together. The charge in the target pixel then represents the illumination of 2 (or more) pixels. It is possible to bin pixels vertically by shifting two image rows into the horizontal register without reading it after the first shift. It is also possible to bin pixels horizontally by shifting the horizontal register two times into output node without resetting it after the first shift. Horizontal binning cannot be done on the image sensor; this is done in the digital domain in the image processing. Vertical binning, however, can be done on the sensor level.
With vertical binning, the charge of multiple lines is combined in the HCCD before they are readout. For binning of 4 lines this means vertical transport of 4 lines and after that the horizontal transport of this binned line takes place. After that the cycle of vertical transport of 4 lines and horizontal readout starts again.
Combining of both vertical and horizontal binning leads to square (or rectangular) image binning. For example 2 × 2 binning is a combination of 2× vertical and 2× horizontal binning.
The primary benefit of binning is higher signal-to-noise ratio (SNR) due to reduced read noise contributions and increased signal combinations. CCD read noise is added during each readout event and in normal operation, read noise will be added to each pixel. However, in binning mode, read noise is added to each super pixel, which has the combined signal from multiple pixels. In the ideal case, this produces SNR improvement equal to the binning factors.
With binning you can increase pixel size arbitrarily (but reduce the spatial resolution). In the limit we could even read out the CCD as a single large pixel.
Another use of binning is to increase the frame rate. Since the slowest step in the readout sequence is the digitization of a given pixel, binning can be used to increase the effective total frame rate of a given system. Thus, highly binned, low-resolution images can be obtained when high speed is required.
Example of 2x2 binning with a CCD camera