Digital
imaging is quite different from photographic imaging in
many aspects. A digital image is made up of many small
square picture elements or pixels. Many of these picture
elements are combined in a two dimensional field to produce
an image. A computer can store the pixel data in a file
and the image can be easily reproduced on a computer.
Printing the image depends on several other factors, however.
The
resolution of a digital image is dependent on two factors.
The first is the number of pixels used to create the two
dimensional field or pixel resolution. The pixel resolution
of a digital image is expressed as horizontal times vertical
numbers of pixels (e.g., 1024X1024). Obviously, the higher
these numbers are, the better the resolution will be.
Remember that resolution is defined as the ability to
distinguish between small objects. And, of course, as
the pixel resolution gets higher and higher, the less
noticeable will be the increase in resolution. When an
image is magnified, pixel resolution becomes quite important
as pixels can be seen in lower pixel resolution images
when magnified. This can create a "blocky" image.
The second factor upon which digital image resolution
depends is the color resolution or color depth. This is
defined as the number of different colors (or shades of
gray for black and white images) that can be assigned
to a pixel. Color depth is determined by the number of
binary digits or bits which are assigned to a pixel by
the hardware or software in a computer. Computers store
data as binary digits of which there are only two choices,
a 1 or a 0. Because of this, the number of possible colors
that can be assigned to a pixel is a power of 2. For instance,
a color depth of 8 bits would give 256 possible colors
2 to the eighth power. 24 bit color depth (224) would
allow 16,777,216 possible colors. This color depth, incidentally,
is considered true color as it provides a greater number
of colors and shades than the human eye can distinguish.
Pixel resolution and color depth are the two major factors
that are considered in the purchase of a computer monitor
and a computer video graphics adapter card.
The
confocal microscope is set up with three different imaging
channels. These are all black and white with an 8 bit
color depth because the photomultipliers only collect
black and white data. Each channel is assigned one of
three primary colors, red, green , and blue (RGB). A scanned
image is in one channel as a black and white and then
observed in color in the combined image. One can also
assign a color lookup table (LUT) to the image. This can
be made by assigning varying amounts of each color to
the RGB composite image. To give the appearance of FITC
as it is seen in the microscope, one might want to increase
the red channel a small amount to give a slight yellow
cast to the green. This situation becomes even more important
when printing an image as the color dyes that are used
for color prints do not look the same as an image on a
video screen.
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