Page 294 - Fundamentals of Light Microscopy and Electronic Imaging

COLOR CAMERAS 277

Image storage Zip drives, CD burner and software
Computer maintenance Maintenance is generally high
Slow speed Generally not capable of video-rate image capture/display
Time requirements Time required for image processing after acquisition; com-
puter required for image display, processing, storage

COLOR CAMERAS

Many investigators choose to use color to represent the visual appearance of specimens
seen in the microscope. Although there are some technical difficulties in displaying
color in prints, the increase in information in a color image can be very significant.
Some common color applications include fluorescence microscopy, stained tissue sec-
tions in histology and pathology, and beta-galactosidase and peroxidase stains in speci-
mens viewed by bright-field or DIC microscopy.
Unlike color photographic film, a CCD device by itself is not color sensitive, so the
acquisition of color images requires that red, green, and blue wavelengths be isolated
with filters, acquired separately by the CCD, and then joined together to create a com-
posite color picture. These requirements impose additional constraints that limit the res-
olution of space, time, and light intensity. Therefore, depending on the particular design
and solution, we will see that color cameras tend to be slower, have reduced spatial res-
olution and dynamic range, and produce noisier images than gray-scale cameras. The
designs and solutions for providing color information vary considerably:

• External color filters or color liquid crystal device. A motorized filter wheel rotates
separate red, green, and blue filters into the light path at the location of the lamp or
in front of the camera. The camera acquires separate images for each color. The full
spatial resolution of the chip is maintained using this arrangement, but the speed of
acquisition and display is reduced. To increase speed, a liquid crystal tunable filter
can be used that is transparent and displays RGB colors in very rapid sequence.
• Three-chip design. The camera contains a beam-splitting prism that directs the
image to three separate CCD devices each masked by a color filter. The spatial res-
olution of the CCD is maintained, and the frame rate (important for rapid sequences
and for video output) can be very fast, because acquisition is simultaneous for each
color channel; however, light intensity delivered to each detector is considerably
reduced. Compared to a single-chip gray-scale camera exposed for a comparable
amount of time, the color image is nearly 10-fold dimmer. Gain can be applied to
brighten the color image, but at the expense of reduced S/N ratio and increased
graininess.
• Color microlenses and movable color masks. CCDs can be manufactured with red,
green, and blue microlenses applied in a specific pattern on individual pixels on the
chip. These cameras are fast, and pixels are illuminated at greater intensity than in
the three-chip design, but spatial resolution is reduced. Another solution for indi-
vidual pixel masking features an array of red, green, and blue microlenses (a color
mask) that moves rapidly and in a square pattern immediately over the surface of
the CCD. Each pixel is responsible for providing RGB color information, so maxi-
mum spatial resolution is obtained. This camera design is ideal for bright-field
color photography of histological specimens at low magnification.