Reducing noise and increasing camera frame rate through binning – on sensor binning versus digital binning

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.  Perhaps there are low light levels in the system and you need to improve image quality. Binning which is adding the charge of 2 or more pixels together can both increase signal to noise ratio (SNR) and frame rate. Higher signal-to-noise ratio is achieved due to reduced read noise contributions and adding  signals (pixels) together. By adding pixels together the noise component will be reduced due to averaging. Because fewer pixels are processed with binning, a higher camera frame rate can be achieved to increase the system throughput. 

But there is a difference between on sensor (charge domain) binning and digital binning in the camera which can affect which improvements are achieved. The differences are listed in the table below.

Differences between on sensor and digital binning:

When the binning is done off the sensor and in the digital domain, the signal increases but the noise floor (read noise) also increases.  If the binning is done on the sensor, the signal increases but the noise floor stays the same. With imaging in low-light, on sensor binning can offer dramatic improvements in image quality over digital pixel summing.

Figure 1 shows a microscope image taken at full resolution on the left and 2×2 binning on the right. The full resolution image is dark and low contrast whereas the 2×2 binned image is sharp and bright. Notice that while the resolution of the 2×2 binned image is ½ that of the full resolution image, the field-of-view is identical. Thus, binning allows the user to trade off resolution for sensitivity. [source: Quantum Imaging]

Figure 1. Left side full resolution; Right side 2×2 binning. (Source:  Quantum Imaging)

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. But, the frame rate improvement is only realized when binning is done on the sensor as when in the digital domain the entire sensor is already read out. Thus, highly binned, low-resolution images can be obtained when high speed is required. 

Binning is also used occasionally to provide physically large pixels when needed in some optical configurations. Furthermore, binning is sometimes used to give a larger full well capacity for the sensor. In some sensors, it is possible to bin pixels with, say 100,000 electron full well together in a 2×2 binning mode yielding a new full well capacity of 4×100,000 or 400,000 electrons. This higher full well can be very useful in applications which are photon shot noise limited due to high background illumination. [source: Quantum Imaging]