🚀 go-pugleaf

In article <jecome01.715798061@starbase.spd.louisville.edu> jecome01@rigel.spd.louisville.edu (Eric Comer) writes:
>	if the lens caps is place on a ccd camera and an image is capture,
>	why aren't all of the pixel values 0 (black)?  when the image is
>	normalized it seems like there are faint light diagonal lines
>	throughtout the whole image.  the image acquired was a gray scale
>	image the min value was 0 and the max value was 6.  any ideas?

Noise, mostly.  There may also be imperfections in the CCD -- a
perfect chip, in fact, is fairly rare (and expensive), particularly at
higher resolutions.  Finally, there is a charge accumulation (dark
current) that occurs even when the shutter is closed or the lense cap
is on.

Excerpting from the Photometrics Series 200 manual, "[all] electronic
circuitry generates noise.  Photon shot noise, dark current, and read
noise are primary CCD camera noise sources.

"Photon shot noise is a funamental property of the quantum nature of
light...

"Dark current, or thermally generated charge, cannot be distinguished
from photon induced charge.  Dark current can be measured and
subtracted, but its noise component cannot be ignored...

"The third primary source of noise, read noise, sets the detection
limits of a CCD camera.  Read noise is generated by the on chip output
ampilfier...

"Three types of integration measure the charge that has integrated on
the CCD:  exposure, bias, and dark integrations. ..."

There's a whole lot more to this, obviously, but that should at least
give you some ideas on what's going on.

The bottom line to all of this is that you have to be certain to
remove the effects of the bias, dark current, and, possibly, light
source non-linearities and artifacts when you grab an image.

							spl

-- 
Steve Lamont, SciViGuy -- (619) 534-7968 -- spl@dim.ucsd.edu
UCSD Microscopy and Imaging Resource/UCSD Med School/La Jolla, CA 92093-0608
"There was a time when tyrants feared/The new world of the free.
Now freedom is afraid and shrieks/At tyranny." - "Brave New World," A. Macleish