Page 259 - Fundamentals of Light Microscopy and Electronic Imaging

242 VIDEO MICROSCOPY

show when the light intensity is inadequate, properly adjusted, or too
bright (saturating). At the microscope, the lamp power supply is adjusted
or neutral density filters are used to make this adjustment. If the illumina-
tion intensity is not in the optimal range, the camera will not be able to uti-
lize its full range of gray levels to represent the image.
2. Adjust the camera offset and gain (offset first, gain second) to regulate the
black and white levels in the image. It is important that the camera, not the
TV monitor, be used to optimize the gray-level settings, because the TV
adjustments do not affect the signal sent to the VCR or printer. An in-line
image processor capable of giving numeric values for features in the image
is useful for making gain and offset adjustments. Alternatively, a dedicated
signal analysis unit, such as the RasterScope available from Dage-MTI
Inc. (Michigan City, Indiana) or an oscilloscope can be used. In the
absence of these quantitative aids, the adjustments must be made by eye.
3. Adjust the brightness and contrast of the TV monitor to optimally display
the captured image. Adjustments to the monitor affect our impression of
image quality, but monitor adjustments do not influence the signal
recorded on a VCR or sent to a computer. The TV’s adjustment dials for
contrast and brightness act in the same way as the gain and offset dials on
the camera controller. The brightness (offset) is adjusted first to distin-
guish black from the darkest grays in the image, followed by the adjust-
ment of contrast (gain) to bright image features to a value close to white,
but not so bright as to reach saturation. Since video monitors usually
exhibit a of 2 or more, bright features tend to be differentially enhanced
while darker gray features are buried in a black background. It is therefore
important to examine the image of a white-to-black gray scale and adjust
the contrast and brightness settings so that all of the gray levels are repre-
sented equally on the monitor. Repeat the operation, using a specimen of
buccal epithelial cells examined with a 40 lens in phase contrast optics.
4. Adjust the vertical hold to correct the registration of raster lines. The two
fields comprising a video frame are traced separately on the monitor. Peri-
odically, you should check that the two fields are interlaced properly—
that is, that the spacing between raster lines on the monitor is uniform. To
do this, hold a magnifier up next to the screen and bring the raster lines
into sharp focus. Adjust the spacing of the lines with the vertical hold con-
trol on the TV monitor until the spacing is uniform. Be sure to check dif-
ferent areas on the monitor to obtain the best setting.

For a detailed description of the procedures used to adjust a video camera
and monitor, see the writings of Sluder and Hinchcliffe in Video Microscopy
edited by Sluder and Wolf (1998).

VIDEO ENHANCEMENT OF IMAGE CONTRAST

The camera controls for gain and offset can be used to increase the dynamic range and
contrast of low-contrast signals, even signals invisible to the eye (Allen, 1985; Allen et