Hi Maris,
It could well be that you're seeing an aliased version of
mains-frequency modulation (ripple) of your light source. The ripple
causes partial strobing in the light source which is noticable at
high shutter speeds. Try acquiring a continuous high speed movie, at
greater than 120Hz if you can (you might be able to go that fast if
you only sample a partial image - depending on your set up and
IPLab's capabilities).
If you sample at just greater than 120Hz, you'll get a regular
fluctuation that repeats every four frames, if you sample at lower
than that (Nyquist) frequency, you'll get low frequency "beats". At
video(=mains) rate, the picture may be steady as a rock depending on
synchronisation. Also depending on synchronisation between your time
lapse system and mains frequency, the fluctuation period in your
played-back movies might be "random", since the phases might be
drifting with respect to each other.
If it does behave as above, there's a cheap cure:
Use a longer exposure if available, or acquire a running-average
image if available, or acquire a burst of video-rate frames at each
time point and average them later. These methods allow you to smooth
the ripple out over time (like your eyes do - I presume that every
thing looks alright through the eyepiece) you'll have to experiment
with these methods to find an acceptable level of smoothing.
and an expensive preventative:
If you're using an incandescent light source, buy a regulated DC
supply (to replace the rheostat that's probably used in your system)
which will remove the mains ripple before it hits the bulb.
Ray
. At 4:51 pm +0000 6/6/00, Maris Handley wrote:
>Hello Everyone,
>
>We are doing time lapse experiments with a Nikon microscope (using
>IPLab software).
>When we play the movie back we are seeing large changes in light
>intensity (almost
>random...not getting brighter continuously or dimmer continuously).
>Our exposure time
>isn't being changed. I believe this is a problem with the light
>source stability,
>but is there something else that might be causing this? Has anyone
>else seen this,
>and what did you do to solve the problem?
>
>Thank you for any help,
>Maris
>
>
>Maris Handley
>Dana-Farber Cancer Institute
>Boston, MA 02115
>(617)632-3179
--
Ray Hicks
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