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Dark Noise
A light sensitive detector generates a signal even in complete darkness. A photocathode emits thermally generated electrons. This signal is given in electrons per unity area and time. See also EBI.
CCD and CMOS sensors have leakage currents that are accumulated for each pixel and detected during the readout. The leakage current is characterized by its mean value (electrons per area and time) and its statistical fluctuation. The fluctuation increases – when a normal distribution is presumed - proportionally to the square root of the number of electrons and is called dark noise. Cooling the sensor or reducing integration time allows to reduce leakage current and consequently also dark noise.
Dark Signal Noise
See dark signal
Dark Signal Uniformity
A dark signal is generated by each CCD and CMOS sensor even without any input light. Together with the transfer characteristics of the readout electronics each pixel may have its own dark signal. The dark signal uniformity describes this systematic signal variation. The temporal signal fluctuation needs to be considered separately.
Deflection Speed
see sweep speed
Deflection Unit
see sweep unit
Distortion
Optical and electro-optical imaging systems might have different magnification depending on the spatial position on the active area. This results in a non-proportional image. Pincushion and barrel distortion are typical for tapers and streak tubes with large photocathodes.
Dual Sweep
Streak tubes may provide two pairs of deflection plates in an orthogonal arrangement that allows dual sweep operation. Typically the first deflection plates close to the photocathode are used to generate a fast sweep with high deflection frequency. The second deflection plates close to the phosphor screen are used to for a slow sweep speed at lower frequency. If no second sweep is required, the second deflection plates can remain without drive voltage.
