Large pixel CCD and megapixel CMOS cameras for neutron imaging.

Camera makers often emphasise the advantages of megapixel CMOS cameras, which are by far the most popular and usually the most suitable for commercial applications of optical imaging; they are also less expensive to make. CMOS cameras have advantages, but also disadvantages. Both CCD and CMOS cameras are optimized for imaging directly with light. The largest markets are for consumer cameras, industrial and security applications, and biological science, all of which have different requirements to neutron imaging. Expensive cameras are produced for biological science, but such cameras have few advantages for neutron imaging. We use cameras designed for amateur astronomy, where the technical requirements are closest to those needed for neutron imaging (longer, low noise exposures with high dynamic range). See Why buy a camera instead of making one. Finally, neutron imaging has a radiation background that eventually damages or destroys the detector, and otherwise negates many of the advantages of expensive cameras.

Choice of CCD and CMOS detectors.

Detector Slim CCD Slim CMOS HiRes CMOS Fast CMOS Square CMOS # FS60 #VS60 CMOS7.1 4/3" CMOS APS-C CMOS Full-Frame
Type Interline
ICX829
Pregius
IMX174
Pregius
IMX183
Pregius
IMX432
Pregius
IMX533
Interline
ICX694
Interline
ICX694
Pregius
IMX428
Pregius
IMX294
Pregius
IMX571
Pregius
IMX455
Resolution pixel 752 x 580 1920x1200 5472x3648 1600x1100 3000x3000 2759x2200 2759x2200 3208x2200 4144x2822 6248x4176 9576x6388
Image diag. mm 8 (1/2") 13 (1/1.2") 15.9 (1") 17 (1.1") 16 (1.1") 16 (1") 16 (1") 17 (1.1") 23(4/3") 28(APS-C) 43(35mm)
Image area mm 6.46x4.81 11.25x7.03 13.13x8.75 14.4x9.9 11.3x11.3 12.53x9.99 12.53x9.99 14.4x9.9 19.1x13 23.5x15.7 36 x 24
Pixel size µm* 8.6 x 8.3 5.86 x 5.86 2.4 x 2.4 9.0 x 9.0 3.76x3.76 4.54 x 4.54 4.54 x 4.54 4.5 x 4.5 4.6 x 4.6 3.76x3.76 3.75 x 3.75
Quantum effic* ~75% ~80% ~79% ~72% ~80% ~70% ~70% ~75% ~90% ~90% 90%
Fullwell e- ** ~40,000 ~30,000 ~15,000 ~80,000 ~50,000 ~20,000 ~20,000 ~20,000 ~66,000 ~50,000 ~50,000
Read noise e- ** 10 7 4 5 3 5 6 3 1.2-7.3 1.0-3.3 1.5-3.5
Dark c. e-/pix/s <0.1@25°C ~1.0@45°C ~2.0@45°C ~2.5@45°C 0.001@-20°C 0.0004@-10°C 0.0004@-10°C 0.03@-10°C 0.002@-20°C 0.003@0°C 0.003@0°C
Peltier Cooling uncooled uncooled uncooled uncooled Δ -35 °C Δ -27 °C Δ -35 °C Δ -35 °C Δ -35 °C Δ -35 °C Δ -35 °C
Read time (s)*** 0.5 41 fps 18 fps 69 fps 20 fps 1 to 3 0.1 to 1 30 fps 16 fps 3.5 fps 0.5
A/D Readout** 16-bits 12-bits 12-bits 12-bits 14-bits 16-bits 16-bits 12-bits 14-bits 16-bits 16-bits
Binning h,v x1 x2 x4 x8 software software software software x1 x2 x4 x8 x1 x2 x4 x8 sofware software software x1 x2
Mount CS-mount C-mount C-mount C-mount C-or F-mount C-or F-mount C- or F-mount C- or F-mount C- or F-mount F-mount F-mount
Trigger signals Software Software Software Software Software Software GPIO software Software Software Software
Interface*** USB 2.0 USB3/GigE USB3/GigE USB3/GigE USB2/USB3 USB 2.0 USB 2.0 USB 3.0 USB 3.0 USB 3.0 USB 3.0
Relative cost 1 1 1.5 1.5 2 3 6 4 3 5 8

     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.

    Our small cameras for fast beam alignment are very efficient; 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 detector itself, the efficiency of the camera is proportional to the ratio of the CCD to scintillator area.