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Glossary of Terms
101
The quantum efficiency of a detector is determined by several factors that include: (1)
the material’s intrinsic electron binding energy or band gap, (2) the surface
reflectivity and thickness and (3) the energy of the impinging photon. It should also
be noted that QE varies with the wavelength of the incident light, as illustrated by the
fact that standard “front illuminated” CCDs generally have a peak QE of 45-50% at
around 750 nm. Back-thinned CCDs typically have improved QE curves, compared
with their “front illuminated” counter-parts, that produce peak QE’s in the 80-85%
range. Additionally, the QE response of “front illuminated” devices can be improved
by coating the chip with a fluorescent dye that converts UV light to longer
wavelengths where the quantum efficiency of the CCD is higher.
Readout Time
The readout time of a CCD is the interval required to move the charges from their
photo-sensitive locations to the readout register, sample and amplify the charges and
then digitize them into discrete digital data points. Included in this readout time is the
correlated double sampling (CDS) technique, which generally requires more
processing time per pixel compared with other less accurate measuring methods. It
should be noted that faster readout times increase the total system noise thereby
reducing the effective system dynamic range. See also Correlated Double Sampling
and Dynamic Range.
Responsivity
Responsivity is the absolute QE sensitivity given in units of amps/watt. CCDs are
typically characterized by performance factors such as QE, counts and gain (specified
in electrons/count) instead of responsivity.
Saturation Level
The maximum signal level that can be accommodated by a device is its saturation
level. At this point, further increase in input signal does not result in a corresponding
increase in output. This term is often used to describe the upper limit of a detector
element, an amplifier, or an ADC.
Spectral Response
Most detectors will respond with higher sensitivity to some wavelengths than to
others. The spectral response of a detector is often expressed graphically in a plot of
responsivity or QE versus wavelength.
Time Interval
The elapsed time between the start of one accumulation to the start of the next
accumulation. The Time Interval, Integration Time and Readout Time of the CCD
detector have the following relationship:
t
interval
≥
t
integration
+ t
read