Megapixel CMOS cameras are not really suitable for neutron imaging.

• More pixels mean smaller pixels. Light gathering capacity depends directly on pixel area, and this is more important than relatively small differences in quantum efficiency. Resolution is determined in practice by the resolution of the scintillator (~100µm) by beam collimation and the distance between object and scintillator. An ordinary camera is limited in principle only by the wavelength of light (~0.5µm). A Kodak report explains When digital cameras need large pixels.

• CMOS cameras have fast readout, designed for short exposures (0.01-0.1s) in daylight, but at the expense of higher thermal electronic noise (dark current) which becomes important for the longer (1-10s) exposures needed with neutrons. Our CCD cameras use Peltier cooling to further reduce thermal noise, and longer read-times to reduce readout noise.

• Dynamic range is higher for large pixel, low noise CCDs. High dynamic range means that with a single exposure both low and high intensity features can be seen, by choosing the perceived intensity range of 256 levels out of a possible 65,536 (16-bit).

• Electron or photon multiplication is sometimes used to boost low light signals, with "single quantum" detection claims. But a single neutron capture already produces up to 160,000 photons; even if only a small percentage are captured by the detector, further multiplying photons does not improve neutron statistics. Multiplication is useful when imaging light, or even with x-ray scintillators, but electron or photon multiplication increases noise and reduces dynamic range.

• An effort has been made to combine the low noise qualities of CCDs with the fast readout of CMOS, resulting in so-called "scientific CMOS" (sCMOS), again mainly used for biological imaging. But sCMOS is also useful for neutron imaging with high flux sources where short exposures are possible.

CCD/sCMOS detectors. We can fit the most expensive cameras, but price increases rapidly for often minimal practical advantage.

#   The VS14 and VS60 are faster readout cameras with the same Sony EXview HAD II (ICX) CCDs as the less expensive FS14 and FS60.
     Our larger "Kodak" (KAI) CCDs are read out more slowly to reduce noise. They are also used in much more expensive cameras with faster readout.
     The collimation and quality of your neutron beam-line will usually be the limiting factor for neutron imaging, not the camera.

*   Light capture per pixel is proportional to the pixel area and its quantum efficiency (at 525nm)
     Fullwell Capacity is the number of electrons that can be stored without overflow (blooming)
     Fullwell Capacity is also proportional to pixel area, but also depends on anti-blooming design
     Noise can be "Dark current" due to thermal energy, or "Read noise" due to electronic readout
     Dark Current can be reduced by cooling for long exposures. Modern Sony ICX CCDs have exceptionally low dark current.
**  Dynamic range is the ratio of Fullwell capacity to Noise, & is lower than the 16-bit (65,536) readout
     Dynamic Range in Decibels DR= 20log (Fullwell capacity/Noise)dB eg typically DR= 5,000= 74dB
     Dynamic Range is often exagerated by neglecting dark noise (true only for very short exposures)
     Note that electron- or photon-multiplied cameras generally have a low dynamic range eg DR= 1,500= 64dB
     A high dynamic range means that contrast between slightly different intensities is better, important for imaging
*** USB 2.0 is limited in practice to ~280Mbits/s i.e for a 2048x2048x16 bit camera to ~4 frames/sec (fps)
     USB 3.0 is limited in practice to ~4000Mbits/s i.e for a 2048x2048x16 bit camera to ~64 frames/sec (fps)

In our "interline" CCDs, charge accumulated by photo-sensitive columns is quickly transferred to adjacent storage columns. The advantage is that smearing is avoided during readout, so a mechanical shutter is not needed. Some of the chip area is used for storage, but that is compensated by the use of micro-lenses over every pixel. Sony sensors are typically ~75% sensitive to the mainly green (540nm) light emitted by neutron scintillators, and at that wavelength there is little advantage for more expensive "back-illuminated" CCDs. These explanations are simplified, but summarise why we use "slow" readout CCDs for neutron imaging.

Of the high-end cameras, the PCO.2000 uses the same Kodak KAI-4022 interline CCD as our LF40+ and has similar imaging performance, except that it uses more expensive electronics with faster readout. Our VS60 is a less-expensive fast readout CCD alternative to the CIS2020 sCMOS camera, whose main advantage is the even faster readout for lower readout noise (but higher dark current, which limits exposure times). The sCMOS camera is then best suited for fast imaging on high flux sources, while our cooled CCD cameras are best suited for lower flux imaging with longer acquisitions (>1s).

Our small cameras use Sony EXview HAD CCDs, which are very efficient, with low noise and fast readout; the chips are also small, limiting light capture but permitting the use of inexpensive fast C-mount lenses. Our f/1.0 lens is twice as fast as an f/1.4 lens. More important than the efficiency of the CCD itself, the efficiency of the camera is proportional to the ratio of the CCD to scintillator area, so our smaller CCDs are ell matched to our smaller cameras.