Date: Thu, 23 May 2002 12:37:29 -0500 (CDT)
Subject: Re: Lookin at Laser Discs
We have used a Canon Optura Pi quite successfully for motion
studies. It has two very useful features, settable shutter speed and
progressive scan, which are not available on cheaper models.
Shutter speed determines the time that the CCD gathers an image,
exactly as in a camera. It can be manually set and locked with camera
controls, through a menu in the viewfinder. As others have noted,
1/250th second or even 1/500th works well for projectiles, free fall,
etc. The penalty is that you need more light to get a good image.
To be compatible with the US television scheme, the CCD is read out in
two 'fields' which together constitute a 'frame' of the full image, at
a frame repetition rate of (almost) 30/second. Standard TV receivers are
interlaced, in that each field consists of alternate scan lines
written down the tube, followed by the remaining scan lines written
from the next field. In a progressive scan system, all the lines of
the frame are written sequentially down the display.
When a fast-moving object is viewed with an interlaced scan you can
see two images of the object, one in each field, slightly
displaced. Progressive scan avoids this problem, except for a slight
shape distortion, because all the lines in the image are taken almost
simultaneously. A progressive-scan camera is capable of reading out its CCD
in this fashion, avoiding a second cause of blurring.
Video-capture software, like VideoPoint capture, give you the choice
of using the interlaced image at full resolution, a de-interlaced
image at half the effective resolution, or a progressive scan image at
full resolution. For motion studies the progressive scan image taken
with a fast shutter speed is clearly superior.
Video can be a neat tool for demos and student labs. I hope this
clarifies some of the issues.