Page 263 - Fundamentals of Light Microscopy and Electronic Imaging

246 VIDEO MICROSCOPY

(a)

(b)

Signal voltage
(c)

Vertical sweep
Figure 13-10
Vertical resolution in video. For a test pattern of 3 horizontal black bars whose width is equal
to that of a raster line and whose spacing is equal to 2 lines, the resolution and contrast of
the lines on the monitor depend on the position of the bars with respect to the raster scan
lines of the camera. The raster lines are the white horizontal stripes, and their width is
determined by the diameter of the scanning electron beam. (a) With bars perfectly aligned
with the raster, maximum resolution and contrast are obtained. (b) With bars straddling two
raster lines, there is no resolution, and the region occupied by the bars appears a medium
gray. (c) With the bars at an intermediate location (the usual case), the lines are resolved,
but the contrast is reduced, the bars appearing medium gray. For this reason, the average
vertical resolution is usually calculated as 0.7 of the maximum number of display lines of the
camera, or 486 0.7 340 lines.

Nyquist sampling theorem for preserving signal resolution and is described in greater
detail later in this chapter.) For a 100 , 1.4 NA oil immersion objective, the diffraction
spot radius is 24 m (0.24 m 100) at 546 nm. Thus, an additional magnification of
2.9 is required for proper sampling (70/24 2.9). This can be obtained by inserting a
4 relay lens (also called a TV lens). In CCD video cameras with small 5–15 m diam-
eter pixels, the magnification requirement is considerably reduced (40–125 magnifi-
cation). In this case, a 100 objective would be sufficient to provide the necessary
magnification, and a 4 TV projection lens would not be required.

Temporal Resolution

Time resolution of video is generally fixed at the camera scan rate of 30 frames/s (60
fields/s). This is very fast compared to the typical rate of full-frame digital CCD cam-
eras of 1 frame/s.

Resolution of Light Intensity (Dynamic Range)

The saturated signal divided by the noise signal gives the camera’s dynamic range. The
intrascenic dynamic range of most vidicon-type tube cameras is 70:1 to 100:1. The