Re: Connectix QuickCam problem

Bill Robertson (Bill.Robertson@crc.doc.ca)
Wed, 10 Apr 1996 14:33:41 -0400


Thanks to everyone who took the time to answer my question about
using Connectix QuickCam's under fluorescent lights. The consensus
is that all QuickCam's suffer from the problem I described and that
the solution is to change the light source from fluorescent to
incandescent.

In article <31627903.84032201@crc-news.doc.ca>, bill.robertson@crc.doc.ca wrote:

> The problem with both my PC QuickCams is how they behave under
> fluorescent light vs. incandescent or natural light. Under
> fluorescent light, with a well lit static scene and brightness set to
> automatic or manual (under either QuickMovie or CUSeeMe), the captured
> video continuously fluctuates getting brighter and dimmer about once
> per second. Under incandescent or natural light the problem
> disappears and the brightness of the captured video remains constant.
>
> Have I got 2 bad cameras? Is there an automatic gain control time
> constant somewhere that I didn't adjust?

--
Andrew O'Neall <a-oneall@uiuc.edu>

No, it's not a camera problem. Mine does the same thing under flourescents. In a room under regular light bulbs, the problem goes away, and the captures look great.

--
vphone@connectix.com (VideoPhone Technical Support)

You might also try turn the Brightness setting to manual and not automatic. Although, you have probably already tried this, just thought I'd suggest it just in case

--
bseburn@vaxxine.com (Brian Seburn)

I would agree with putting the Brightness on manual. The problem I was finding was as simple as what I was wearing. If I had a shirt on that had white or light coloured sections on it, and the camera was on automatic, it changed everytime I move almost. I would presume the fluorescent light was being reflected and the camera was compensating for it. having it on manual stopped the problem. I have since installed a incandesent light about the camera and turn out the fluorescents when I want a good picture. Works great!

--
frankper@tiac.net (Frank Perreault)

This is because the camera does not have a mechanical shutter to regulate the amount of light coming into the camera. Instead it uses software to to simulate a shutter. You see a flucuation because of the 60 times a second blinking of flourecent lights and the simulated shutter oscillation (the brightness setting) of the camera which ranges from once every 4 seconds to 1/2000th of a second.

--
Rich Hinz <rich@ncn.com>

I can tell you my quickcam and all of them I have seen tend to "fluctuate" in flourecent lighting. I don't think it's your camera but at the same time have no suggestion for correctingh it. At least you know the camera's not broke :)

--
ode <ode@charlie.acc.iit.edu>

Could be theres electromagnetic intereference from the choke in the flourecent light. is it close to the camera? I have flourecent lighting from the celing and dont have any problems

--
pbb@sys.uea.ac.uk

I'm in a lab working on capturing live video. Testing each frame some were slightly darker than others. I looked around for help and found that fluorescent lighting pulses at a certain rate (about 55 hertz I think). You can think of the light it is emmitting as a sort of square sine wave. It just *appears* constant because our eyes can't pick up a flicker at that rate.

The camera is reading each frame at a rate (say 20 hertz (fps)). At some frames the fluorescent light with "be at the crest of lighting" and some it will be at the trough (no light emitting). This is where the conflict arises.

Anyway I went to my Prof. and he said "Yep, thats correct but we have got incandescent lights (the problem above is why)". As it turns out, there is also a very minor impuity with cameras. Only about 2% on our IndyCams but enough to muck up my work a bit. If you're just displaying the video then this 2% will make next to no difference at all.

--
Rolf Mathison <76376.3224@CompuServe.COM>

Your cameras are fine. Unfortunatly flourescent lights DO change brightness 120 times/sec. The phosphors can respond quickly enough so that depending on exactly when the image is sampled the brightness level can vary a great deal. The cameras automatic brightness control tries to respond but as soon as the next sample is taken a new level is introduced, which accounts for the flickering effect. You don't see this effect with your eyes because your eyes use relativly slow, averaging type of sampling. This is also why movies, which are 24 still frames per second appear as motion... Incandescent lamps are also driven by the 60Hz. power line, but the slow response of the filament (a lot like your eyes) average out the changes so they appear constant. Try slowing down the frames/sec sampling rate, this may minimize the effect.

--
"Jay Shoup" <jashoup@indiana.edu>

The reason they have this problem is two fold. First fluorescent light actualy flickers, your eye does not see thie because the human eye (actualy brain) is not able to see this flicker. (it is called persistance of vision). The Quickcam IS able to see this. What is happening is that the if the QC takes a picture at the same moment they light is flickering (ie going black) there is no light. This causes the QC to begin adjusting the level to a brighter setting. Just as this is begining to happen another photo is taken by the QC and it now has bright light, so the QC begins to darken to compinsate for the brihgt light it just saw.

There are two methodes that can be used to correct the problem, the first is not under your control.

(1) Adjust the frame rate of the QC so that the BF (Beat Frequency) is such that it does not appear in the video frame. (This is a function of the camera and the software working together and as far as i know is NOT adjustable)

(2) Remove the incadesant light source that is causing the problem. (a) Two light sourses that you need to look at are; The room lighting, and any possible light from a TV or your Video monitor. Both of the later can cause this SCAN RATE problem.

Your best best is to just add light from an incadesent source.

--
leo@adi.com.au (Leo Possajennikov)

I also have this problem with Mac QuickCam under fluorescent light.

--
-- 
W.J. (Bill) Robertson          bill.robertson@crc.doc.ca
CRC/VPRB/DRX Box 11490 Stn. H, Ottawa, ON CANADA K2H 8S2
Voice: (613) 998-2819                Fax: (613) 998-9648