NeutronOptics - Megapixel CCD and CMOS Detectors

Parameters of CCD/CMOS detectors for Neutron or X-ray Imaging

Big pixels are needed, or the ability to combine (bin) small pixels, since light gathering capacity depends on pixel area.
Big detectors are better for a large Field-Of-View, since efficiency depends on the ratio of the areas of the detector to FOV.
Monochrome detectors don't have colour filters that absorb light, and neutrons or x-rays are colour-blind.
14- or 16-bit readout will increase the number of observable intensity levels, or dynamic range.
Cooled cameras reduce thermal noise, increasing the signal/noise ratio and the dynamic range.
Electron/Photon multiplication is not needed, since the scintillator multiplies a single neutron to many thousands of photons.
Moderate cost is important since the detector is eventually damaged in a radiation environment.

The largest markets are for consumer cameras, industrial or security applications, and biological science, all of which have different requirements to 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). We make small cameras for beam alignment, and large cameras for tomographic imaging.

In general, choose the smallest, cheapest camera compatible with your requirements, and eventually trade-up if necessary.

Our Choice of CCD and CMOS detectors.

Detector	Slim CCD	Slim CMOS	Fast CMOS	Square CMOS	FS60 CCD	VS60 CCD	CMOS7.1	4/3" CMOS	APS-C CMOS	FullFrame	ICON-L
Type	Interline ICX829	Pregius IMX174	Pregius IMX432	Pregius IMX533	Interline ICX694	Interline ICX694	Pregius IMX428	Pregius IMX294	Pregius IMX571	Pregius IMX455	Andor ICON-L
No. Pixels	752 x 580	1920x1200	1600x1100	3000x3000	2759x2200	2759x2200	3208x2200	4144x2822	6248x4176	9576x6388	2048x2048
Diag. mm	8 (1/2")	13 (1/1.2")	17 (1.1")	16 (1.1")	16 (1")	16 (1")	17 (1.1")	23 (4/3")	28 (APS-C)	43 (35mm)	39 (square)
Image mm	6.46x4.81	11.25x7.03	14.4x9.9	11.3x11.3	12.53x9.99	12.53x9.99	14.4x9.9	19.1x13	23.5x15.7	36 x 24	27.6x27.6
Pixel size	8.6 µm	5.86 µm	9.0 µm	3.76 µm	4.54 µm	4.54 µm	4.5 µm	4.6 µm	3.76 µm	3.75 µm	13.5 µm
Q. effic	~75%	~80%	~80%	~80%	~70%	~70%	~75%	~90%	~90%	90%
Fullwell	~40,000 e-	~30,000 e-	~80,000 e-	~50,000 e-	~20,000 e-	~20,000 e-	~20,000 e-	~66,000 e-	~50,000 e-	~50,000 e-	100,000 e-
Read noise	10 e-	7 e-	5 e-	3 e-	5 e-	6 e-	3 e-	1.2-7.3 e-	1.0-3.3 e-	1.5-3.5 e-	2.9 e-
Dark e-/p/s	<0.1@25°C	~1.0@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	.0004@-70°C
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	Δ-70°C
Frame Rate	0.5 fps	18 fps	*120 fps	^20-100 fps	0.2 fps	1 fps	30 fps	16 fps	3.5 fps	3 fps	1 fps
A/D Readout	16-bits	12-bits	12-bits	14-bits	16-bits	16-bits	12-bits	14-bits	16-bits	16-bits	16-bits
Binning	hardware	software	software	software	hardware	hardware	software	software	software	software	hardware
Lens Type	Tam f/1.0	Tam f/1.2	Fuji f/1.4	Fuji f/1.4	Fuji f/1.4	Fuji f/1.4	Fuji f/1.4	M43 f/0.95	M43 f/0.95	50mm f/1.2	50mm f/1.2
Mount	CS-	C-	C-M43-	C-M43-	C-F-	C-F-	C-F-	M43-	M43-F-	F-	F-
Usual Dist	150mm	150mm	200mm	200-500mm	200-500mm	200-500mm	200-500mm	200-500mm	350-500mm	500mm	500mm
Usual FOV	75 x 50mm	100x100mm	100x100mm	100x100mm	100x100mm	200x250mm	200x250mm	200x250mm	200x250mm	200x250mm	200x250mm
Pixel @FOV	150 µm	85 µm	90 µm	35-70 µm	45-90 µm	90 µm	90 µm	60 µm	50 µm	30 µm	100 µm
Trigger	Software	Software	Software	Software	Software	Software/GPIO	Software	Software	Software	Software	Software
Interface	USB 2.0	USB2/GigE	USB3/GigE	USB3	USB2/GigE	USB2	USB3/GigE	USB3	USB3	USB3	Andor
Cost Detect+Lens	1	1	1.5	1.7-2	4	6	5	3	5	9	50

The Andor ICON-L is shown for comparison. The collimation and quality of your neutron beam-line will usually be the limiting factor for neutron imaging, not the camera.

Choosing an Imaging Camera - More is not always Better

A Lens aperture of f/1.0 transmits x2 as much light as an aperture of f/1.4, so fast lenses have advantages.

The lens flange-focal distance limits the choice of available lenses. Mirrorless CMOS cameras have much shorter FFDs (~20mm) than older SLR-type cameras (~45mm).

The FFD can be increased for some lenses, by using extension rings as for Macro photography but that increases the effective focal length and reduces the aperture.

The Optical path length depends on the required Field-Of-View (FOV), the lens focal length and the detector chip dimensions - see: Qioptiq.

The Overall efficiency depends on the ratio of the area of the FOV to the area of the detector, so don't choose a FOV larger than necessary.

More pixels means smaller pixels that collect less light. Resolution will be limited by your beam collimation and scintillator thickness, not the detector.

Pixel area is proportional to light collection. Combining adjacent small pixels (binning) can be used to emulate large pixels.

Quantum Efficiency is just the conversion efficiency, and takes no account of the much more important pixel area.

Full Well Capacity is the number of electrons that can be stored in a pixel, increasing the dynamic range of intensities: it increases with pixel area.

Read Noise is introduced simply by reading out the pixel charge, and is lower for CMOS than for CCD technology.

Dark Current is electron noise due to the temperature of the detector, and is lower for CCD than for CMOS technology. CCDs are better for long exposures (>60s).

Cooling reduces Dark Current, but with modern detectors little is gained below 0^oC because of other noise sources, and long-term radiation damage.

Pixel Filtering by imageJ can also reduce noise, by replacing isolated bright pixels by the average of their surroundings.

Read Times are much shorter for CMOS than for CCD technology, where slow readout is favoured to reduce readout noise.

A/D readout determines the Dynamic Dange, and is usually much higher than the 8-bits (256) intensity levels seen by the human eye.

Binning increases the effective area of a pixel, and the light collected. Hardware binning increases the frame rate.

Trigger signals are used to synchronise exposures with sample rotation for tomography. Usually this can be accomplished with software.

The camera interface limits the raw frame rate. High intensities are needed for short exposures and fast frame rates
     USB 2.0 (theoretical 480 Mbps) is limited in practice to <280 Mbits/s i.e for a 2048x2048x16-bit camera to <4 frames/sec (fps)
     USB 3.0 (theoretical 5 Gbps) is limited in practice to <4000Mbits/s i.e for a 2048x2048x16-bit camera to <64 frames/sec (fps)
   * With <10 ms exposures, ~120 fps is achieved for 1600x1100 8-bit pixels or 1200x1000 12-bit pixels over >10m active USB3.2 (1x1) cables i.e. 1.7 Gbps
   ^ With 50 ms exposures, 20 fps is achieved with the square CMOS camera for 3000x3000 14-bit pixels over >10m active USB3.2 (1x1) cables i.e. 2.5 Gbps
   ^ With 10 ms exposures, 100 fps is achieved with x3 hardware binning in SharpCap for 16-bit pixels over >10m active USB3.2 (1x1) cables i.e. 1.6 Gbps

The cost depends partly on the technology, but also on the market - how many are sold, and what the customer is willing to pay. Mass market products are cheaper.