CCDs are very versatile devices and their readout pattern can be manipulated to achieve various effects. One of the most common effects is Binning. Binning allows charges from adjacent pixels to be combined and this can offer benefits in faster readout speeds and improved signal to noise ratios albeit at the expense of reduced spatial resolution.
Single Pixel Readout vs 2x2 Binning
To understand the process, let us compare the process of single pixel readout versus 2 x 2 binning shown. If we consider a spot of light evenly illuminates the four pixels of our miniature CCD. The CCD has a light sensitive region of just four pixels and a readout register depicted in blue at the bottom of the CCD. The light signal induces a charge of 20 electrons in each of the four pixels as shown by their shading and the numbers in the bottom right hand corner of the pixel.
1.The light falls evenly on the four pixels and creates a charge of 20e in each of the four pixels.
2.The first operation is to shift the charge down one row. The charge from the lowest pixels gets shifted into the readout register.
3. For single pixel readout, the charge in the readout register is shifted to the right and into the readout amplifier. In the binning operation the charge is shifted down again and the charge from the first row is added to the first row in the readout register.
4. For single pixel readout, the first pixel is readout while the readout register is shifted again to shift the charge in the second pixel into the readout amplifier. In the binning operation the summed charge from two right two pixels is shifted into the readout amplifier.
5. In the single pixel readout, the next row is shifted vertically into the readout register. In the binning operation the readout register is shifted again to sum the charge from the 4 pixels in the readout amplifier before being readout.
6. In the single pixel readout mode, the readout register is shifted to the right again to readout the next pixel. Binned operation is now complete.
7. In the single pixel readout the readout register is shifted to the right again to readout the final pixel.
It is important to highlight the main differences in the two readout schemes. In the first we achieve the full spatial resolution the sensor offers. In the Binned example we have reduced the 4 pixel pattern to a single pixel and hence lost spatial resolution. However, the binned operation takes less steps to readout the sensor and hence is faster. Typically, binning 2x2 is twice as fast; this is achieved by having to shift the readout register only every 2 vertical shifts. If we were binning 3x3 or 4x4 on a CCD then the readout would be respectively 3 and 4 times faster.
The binned example also highlights how binning improves signal to noise ratio. If we assume our CCD has a readout noise of 10e. Then in the single pixel example each pixel is readout with a noise of 10e hence we achieve a signal to noise ratio of 2:1 (20e/10e). Even if we subsequently sum the four pixels in a computer after readout the signal to noise ratio becomes 4:1. In adding the four pixels we sum the signal (4 times 20e i.e. 80e) and the noise is added in quadrature i.e. square root of the sum of the noises squared (square root of 4 times 10 squared i.e. 20e). In the binned example there is no noise until the signal is readout by the amplifier so the signal to noise ratio is 8:1(80e/10e) i.e. twice as good as the single pixel readout mode.
One of the most common applications of binning is spectroscopy. In spectroscopic CCD systems, a spectral line is typically an image of the slit formed on the CCD. The image of the slit will typically have a high aspect ratio, i.e. very long and thin and orientated perpendicular to the readout register. The signal from a single spectral line can now be binned to achieve the best signal to noise ratio without any deterioration in spectral resolution.