Biomedical Engineering Reference
In-Depth Information
for low signal detection, such as luminescence and fluorescence measurement.
The thickness of the optical active area (depletion layer) of back-illuminated
CCD is the same as the front-illuminated CCD. For near-infrared light, because
the absorption of silicon is low, the light may pass through the depletion layer
(n D 4:0) and reflected by the silicon dioxide layer (n D 1:5), causing an etaloning
effect, a constructive and destructive interference pattern that is hard to remove in
postdata processing. This is the main drawback of back-illuminated CCD. Very
recently, a new type of back-illuminated CCD has been produced using a deep-
depletion technique, which substantially suppresses the etaloning effect (Princeton
Instruments, Trenton, NJ, USA).
Back-illuminated deep-depletion CCD has thicker depletion layer, which pro-
vides longer optical path for NIR light absorption, and thus has improved quantum
efficiency for NIR imaging and at the same time prevents the etaloning effect. Back-
illuminated deep-depletion CCD is suitable for extremely low signal detection, such
as fluorescence and Raman measurement. But it suffers higher dark current (about
100 times higher) than the front-illuminated and back-thinned back-illuminated
CCDs. Dark current arises from the thermal vibrations of the silicon lattice. Higher
dark current causes higher dark noise, which is not preferable for low signal
detection. Therefore, back-illuminated deep-depletion CCD requires deep cooling
to suppress the dark current. Now, for most Raman spectral measurements, the back-
illuminated deep-depletion CCD is usually cooled down to 90 to 130 ı C with
either thermoelectric cooler (TEC) or liquid nitrogen. Front-illuminated and back-
thinned back-illuminated CCDs do not require complicated deep cooling because of
low dark current.
All types of CCDs can be UV coated to improve the detection in the UV range.
For signal beyond 1,100 nm, silicon-based CCD is no longer a choice because it
is transparent in this range. Indium gallium arsenide (InGaAs)-based detectors are
preferred.
Further enhancement of the detection can be obtained using an electron-
multiplying CCD (EMCCD), in which the signal of each pixel is amplified
analogous to an avalanche photodiode. Because this amplification is proportional
to the electrons in that pixel and ahead of the signal reaching the shift register,
EMCCD boosts the signals above the read noise. As a result, EMCCD is suitable for
ultralow light, high-frame-rate applications, such as single-molecule fluorescence
spectroscopy and imaging.
For high-speed applications, intensified CCD (ICCD) is the choice. In ICCD,
the CCD is coupled to a microchannel plate (MCP) image intensifier, which is
essentially a multichannel photomultiplier tube. The signal hits the photocathode,
and electrons are accelerated and multiplied in the MCP. The resulting electrons hit
a phosphor screen and are converted to light for the CCD to detect. Therefore, the
intensifier can work as a shutter, gated very rapidly by changing the potential on the
photocathode in MCP. Allowing high-speed gating is the major advantage of ICCD
over EMCCD. However, ICCD is expensive and has limited lifetime.
Scientific CCDs can be exposed with/without a mechanical shutter. For full-
frame CCD, a shutter is usually required. The exposure is controlled by the shutter,
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