... let me add my 2cents...

As far as I understand about "interlace", it is not an advantage for
image capture, but for exposure.
There are two main reasons why to use interlace:

First:
------
When the TV CRT needs to produce a complete frame, the idea is that the
whole image should have a flat image brightness. A TV CRT has a (focus
adjustable) image dot size, that by the phosphor issue, the center of
the dot is brighter than the surrounding, like a flashlight spot. By
this reason, the real center of a raster image line is brighter than the
borders.

To avoid this "image darkness" to be visible, the image should be
assembled with raster lines as much close as possible, so the "center"
of raster line 56 would be overlapping the darker border of the line 55.
Problem is that line 56 also has a darker border, even being not so
bright as its center, it will overlap the bright part of line 55.
Because it happens so fast (to your eye), line 56 border will actually
replace part of the line 55 image, so the image loses quality.

To eliminate this problem, the interlace counts with your eyes.
Overlapping image "odd lines" to "even lines" in one single frame scan,
your eyes will see quality loss.  If keeping the same lines position,
but first exposing only the "odd" lines complete and than the "even"
lines, the phosphor will not suffer the strong overlap effect, and your
eyes either, so your brain will "see" a better quality image.

The interlace is only useful when the "phosphor image decay" has certain
relationship in time with the scan time itself... Suppose that you
increase your TV scan frequency from 30 fps to 120 fps (repeating 4
times the same whole interlaced frame), the interlace will lose effect,
since the raster will be much faster than the phosphor decay, so the
overlap will happens even with interlace.

Second:
-------
Phosphor decay and scan time.

The phosphor decay is one of the major elements to any CRT quality. A
long decay creates a vivid and bright image, but with blur and lose in
definition, since overlaps happens all the time. A short decay creates a
sharp and well focused image, but lose in flat brightness over the
entire image, since when the raster is being formed at the bottom of the
image, the top is already decaying and getting dark, so scintillation is
the effect.

Today's TV and image monitors are not working only in dark family rooms
with all lights off and all the family members quiet. Today they are
installed outside, ambient light all around, so they need image as
bright as possible.  The most economic way to produce a bright image is
by increasing the phosphor decay time, but remember, it loses quality
and sharpness.

One way to increase phosphor decay and eliminate the blur effect, is
interlacing. It gains instantly 50% of image quality, since the vertical
raster will be split in time, so no more overlapping. The horizontal
raster still a mess, but your eyes can notice an image improvement.

So, I guess, the CCD chip capture all the image elements at once, close
the electronic shutter, and deliver to you in interlaced mode, just
because *you want this way*. There is no advantage to the CCD chip to do
this, the advantage is to the CRT add your eyes.

Wagner.

> #4) A corolary to #3 - When a CCD camera gives interlaced output, is the
> shutter only open once per frame, or once per field? In other words, can
> the interlacing cause motion blur problems?