Digital Imaging
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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