![]() ![]() The Quantum Efficiency slider in the tutorial provides an adjustment range of 20 to 98 percent, and the Photon Flux slider allows selecting incident light levels between 0.0 photons per pixel per second. The product of these three variables determines the signal portion (numerator) of the signal-to-noise ratio, which is weighed against all noise sources that contribute to the denominator term of the ratio. The measured signal from a CCD imaging system, utilized in calculating SNR, depends upon the photon flux incident on the CCD (expressed as photons per pixel per second), the quantum efficiency of the device (where 1 represents 100 percent efficiency), and the integration (exposure) time over which the signal is collected (seconds). When the SNR is recalculated to reflect the binning operation, it is assumed that the signal is the same for each pixel within a group. The binning factor represents the number of pixels that are combined to form each larger pixel. The radio buttons labeled Binning Factor can be selected individually to enable a method of signal-to-noise ratio improvement commonly used with scientific CCD cameras, in which the signal-generated charge from groups of neighboring pixels is combined during readout into larger "superpixels". A large signal-to-noise ratio is important in the acquisition of high-quality digital images, and is particularly critical in applications requiring precise light measurements. Changes made to the factors that directly affect signal level, and to those variables primarily contributing noise to the system, have an inverse effect on SNR that is reflected in the displayed value. During image acquisition with electronic sensors, including CCDs, apparently random fluctuations in signal intensity constitute noise superimposed on the signal, and as the magnitude of noise increases, uncertainty in the measured signal becomes greater. As each variable is changed, the calculated value of signal-to-noise ratio is updated in the left-hand yellow box. Parameters that affect signal-to-noise ratio for a CCD sensor can be varied for the system modeled in the tutorial by using the mouse to reposition any of the sliders located below the display window. The cost depends partly on the technology, but mainly on the market - how many are sold, and what the customer is willing to pay.The tutorial initializes with the display of a graphical plot of signal-to-noise ratio as a function of integration (exposure) time for a hypothetical CCD system with specifications typical of high-performance cameras used in microscopy imaging applications.USB3 is faster, but with short (3-5m) cables. The camera interface limits the frame rate: USB2 is slow, but long (10-20m) cables can be used.Usually this can be accomplished with software. Trigger signals are used to synchronise exposures with sample rotation for tomography.Bright Lenses are designed with different camera mounts, and different detector-lens (flange) distances.Hardware binning increases the framerate. Binning increases the effective area of a pixel, and the light collected.A/D readout determines the Dynamic Dange, and is much higher than the 8-bits (256) intensity levels seen by the human eye. ![]() Read Times are much shorter for CMOS than for CCD technology, where slow readout is favoured to reduce readout noise.Cooling reduces Dark Current, but with modern detectors little is gained below 0 oC because of other noise sources, and long-term radiation damage.In general, choose the smallest, cheapest camera compatible with your requirements, and eventually trade-up if necessary. We make small cameras for beam alignment, and large cameras for tomographic 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). The largest markets are for consumer cameras, industrial or security applications, and biological science, all of which have different requirements to neutron imaging. ![]()
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