Page 285 - Fundamentals of Light Microscopy and Electronic Imaging

268 DIGITAL CCD MICROSCOPY

• Interline transfer CCD. In this design, columns of pixels in the parallel register
alternate between imaging rows and masked storage-transfer rows, resulting in a
pattern of stripes across the entire CCD. Interline transfer CCDs are used in cam-
corders and video cameras because they can be read out at video rate and provide
high-quality images of brightly illuminated objects. Improvements in camera elec-
tronics and significant reductions in camera read noise have made this design pop-
ular for low-light applications, including dim fluorescent specimens. After an
exposure, all of the pixels in the image columns are transferred by one step and in
less than 1 ms to pixels in the adjacent masked storage columns; the storage array
is read out while the image array is being exposed for the next picture. Cameras
with interline CCDs that are read out serially from top to bottom are sometimes
called progressive scan cameras, a reference to one of the readout modes of video
CCD cameras. This type of CCD is also very fast and does not require a shutter to
control the exposure, but the dynamic range may be somewhat reduced. The latest
interline detectors include very small pixels and microlenses that cover pairs of
image and storage pixels so that photons incident on the storage pixels are included
in the photoelectron parcels in the imaging pixels.

Note: Interline CCDs for Biomedical Imaging

Because of their speed, lack of requirement for a shutter, and lower wavelength sen-
sitivity (desirable for UV-excitable fluorescent dyes and GFP), present-day interline
CCD imagers give superior performance for cameras used for digital microscopy. In
earlier interline CCD designs of the type used in video cameras and camcorders, the
alternating masked columns of pixels and small pixel size meant reduced light sensi-
tivity and reduced spatial resolution in the image; further, high camera read noise
meant reduced dynamic range, with signals suitable only for 8–10-bit ADCs. The sit-
uation has completely changed in recent years with the incorporation of microlenses
on the surface of the CCD and the inclusion of low-noise camera electronics.
Microlenses are placed on the CCD surface in such a pattern that each microlens
straddles a pair of masked and imaging pixels so that light that would ordinarily be lost
on the masked pixel is collected and delivered to the imaging pixel area. Thus, by using
small pixels and microlenses, the spatial resolution and light-collecting efficiency are
now similar to those of a full-frame CCD. The microlens technology also permits
extending the wavelength sensitivity into the blue and UV portions of the spectrum,
making the interline CCD highly desirable for applications involving short wave-
lengths, including UV light. The most recent interline chips incorporate nonabsorbing
materials in the gating structures on the surface of the chip, boosting their quantum
efficiency to close to 80% for green light, a 50% increase over previous designs.
Since the active pixel area of a typical interline CCD is one-third that of a 6.8 m
square pixel on a full-frame CCD, the linear full well capacity in photoelectrons is
2
reduced to about 15,000 electrons (6.8 0.33 1000). Ordinarily, this would
result in a reduced dynamic range for the camera, but electronic improvements have
also cut the camera read noise in half, down to as low as 5–8 electrons, so the
dynamic range remains comparable to that of 12 bit full-frame CCD cameras. Elec-
tronic improvements have also resulted in increased speed of image acquisition, so
12 bit megapixel images can be acquired at 20 MHz or faster rates, 3–4 times the
rate of existing full-frame CCDs of comparable size.