Large pixel CCD and megapixel CMOS cameras for neutron imaging.

• More pixels mean smaller pixels. Light gathering capacity depends directly on pixel area, and this is more important than small differences in quantum efficiency. Resolution in radiography is determined in practice by the resolution of the scintillator (50-100µm), beam collimation and the distance between object and scintillator, while an optical camera is limited in principle only by the wavelength of light (~0.5µm).

• CMOS cameras have fast readout and low readout noise, 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 (1-30s) exposures needed with neutrons. Even though dark current is mandatary for compliance with the EMVA-288 standard, CMOS camera manufacturers often don't quote it because it is much higher than for CCDs; it is not the same as "temporal dark noise", which is read noise for msec exposures. The new Sony "StarVis" CMOS chips reduce dark current - by using very small pixels !

• CMOS high speed readout is obtained with on-chip circuits to read each pixel independently; the analogue-digital converter is then simpler, usually only 12-bit. Pixels cannot be binned before readout, as with a CCD to increase sensitivity and reduce noise. CMOS cameras are inexpensive because circuitry is on-chip; they are ideal for megapixel consumer cameras, except for low-light conditions. A modern high efficiency Sony Pregius CMOS camera (Point Grey) is included for when intensity is sufficient to permit short exposures and high frame rates (<40f/s).

• Both CCD and CMOS cameras can use Peltier cooling to reduce thermal noise, but because the total noise is lower for CCDs, the dynamic range is higher. 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). The dynamic range with CMOS is usually much lower, especially with high gain and longer exposures.

• Electron or photon multiplication is sometimes used to boost low light signals, with "single quantum" detection claims. But a single neutron capture produces up to 160,000 photons; even if only a small percentage are captured by the detector, further multiplying photons does not improve statistics. Multiplication is useful when imaging light, or 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 "scientific CMOS" (sCMOS), again mainly used for biological imaging. But sCMOS is also useful for neutron imaging with high flux sources when very short exposures are required.

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

#   The VS14 and VS60 are faster readout cameras with the same 2/3" or 1" 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 LF40+ is no longer available, but can be replaced in the same camera box by the A383L+ which has similar performance.
     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 2.0 is also sufficient for the 1920x1200 12-bit Sony Pregius CMOS camera up to 10 frames/sec (fps) over >10m active cables
     USB 3.0 is limited in practice to ~4000Mbits/s i.e for a 2048x2048x16 bit camera to ~64 frames/sec (fps) with short ~3m cables.

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 using 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. This explanations is simplified, but summarises 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 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; the chips are also small, limiting imaging area 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 matched to our smaller cameras.