Dear Charlie:
I am using a monochrome camera (Panasonic BP310) with an 8mm AI lens from
Computar enclosed in a standard Pelco environmental housing to look at vehicles
at a traffic junction.
My problem is that during the day when the sun is shining brightly, the
picture looks very grainy and with little contrast. However, when the day is
overcast or towards the evening, the picture gets real sharp and with good
contrast i.e.. black is really black.
What is happening...I have adjusted the lens ALC and Level to no avail. I
have also changed cameras and lenses to no avail.
Second problem is that when the road is wet, the car headlights gets
reflected into the camera and the smearing is real bad. Is there a way round
this problem?
Sincerely, Too Bright and Still in the Dark. Dear Too
Bright,
Thanks for the excellent question, or should I say list of questions.
Although your information is rather in-depth, I have a couple of questions to
help make sure I give you good advise. I have included these as well as some
comments and potential suggestions with your letter below. I hope that you can
read it OK.
First things first. When you say that you have adjusted the ALC and the Level
to no avail, what method and what tools are you using to adjust such. Grainy,
flat images usually denote a video signal from the camera of less than one volt
peak to peak (1vpp). This is usually due to the iris being closed too far down
during peak daylight hours. It could also point to the opposite direction since
your image comes clear at night. Let me know the answer to my question and we
can go from there. As for the ALC, this sets the speed of the iris response. You
should have it set somewhere between 10 and 12 on a clock scale on top of the
pot.
Head light glare can be cut down with the use of an Infrared Cut filter. This
will cut down the glare and ultimately reduce or remove any vertical smear. You
must be careful however, that you don't cut the sensitivity of the camera down
too much by adding the filter. If this is the case, your night time image will
become equal to what your current daytime image is.
At any rate, I hope that this helps you out a little. Get back to me with the
general info and I should be able to give you some better answers. I am in your
service.
Sincerely, Charlie P.
Dear Charlie:
Thank you for replying to my e-mail. I do not use any special tools for
adjustment of the ALC & Level pots on the lens. What I did before mounting
the camera/lens outdoor was firstly to set the ALC to the AV or average
position. Next, I would disable AGC on the camera and point towards the
In the day, I would have a colleague station in the control room viewing the video on a monitor. When the picture becomes grainy, I would then try adjusting the ALC between AV and Pk and also the Level pot to see if it improves.....as usual it does not help. I understand from most people that the pots on the iris lens is very sensitive and should normally not be adjusted. On the other hand, I do not know of the proper way of making iris adjustment...most lens/camera manuals would say something like adjust for proper picture.
Once more, Too Bright and Still in the Dark.
Dear Too Bright,
Sorry for the delay in responding to your previous email and question.
However, the answer to your question is rather lengthy and I wanted to dedicate
the right amount of energy to it to be able to give you the best advise.
First, we need to start with the operation of the camera and auto iris lens.
Although most people think that the lens responds to light, it has no ability to
judge light. In actuality, it monitors the video output of the camera. The video
signal is designed to be one volt peak to peak. Too much video signal will cause
all sorts of problems, such as image wash out, horizontal tearing, even vertical
roll in some cases. Too little video signal has a whole separate set of
symptoms, such as grainy or snowy picture, flat or little contrast images, dark.
The catch 22 come in to play with just a little more understanding. The more
light that is focused on the CCD, Chip, imager, tube or whatever you want to
call it, the bigger the video signal will be coming out of the camera. The less
light that is focused on the imager, the smaller the video signal. Consequently,
the sole purpose of the auto iris lens is to monitor the video signal coming
from the camera and to open or close according to the increase or decrease of
the video signal from it's one volt peak to peak perfection.
OK, let's go into the operation of the lens that you are working with. First
you have a level adjustment. This is what we call the zero adjustment. The
objective is to adjust the level pot so that the camera produces a one volt peak
to peak video signal under all conditions. Obviously, if you don't have it set
properly, you will end up with either too much or too little video signal and
the problems that go with them.
Next you have an ALC adjustment. The ALC (Auto Light Control or Auto Light
Circuit) controls the speed of the iris. If this is adjusted wrong, the iris
will respond too quickly and could actually cause the iris to go into a flutter.
A flutter is where the image appears to get bright, then dark, then bright, et
cetera.
Now for the kicker... You must have an oscilloscope or a hand-held video
meter in order to set the balance of the lens properly. You cannot, as you have
found out the hard way, eye-ball this adjustment. You must have the right test
equipment and follow a set
1. Set the camera in such a position as to be viewing an evenly, very well lit area. You don't want extreme shadows or extreme white walls as you will be trying to set the video signal output for an average. To many dark shadows will cause extreme depletion in the video signal while extreme white walls will cause an overload.
2. Run the video output from the camera to the input of an oscilloscope or a hand-held video meter. Loop it through to a monitor or to a 75 ohm load. The signal must be terminated to insure proper levels.
3. Turn the ALC off by adjusting the pot to the full counter clockwise position.
4. Adjust the level pot until you have a one volt peak-to-peak composite video signal coming from the camera. Composite is measured from the sync line (bottom most portion of the signal) to the average across the top of the video.
5. Turn the ALC pot to the 10:00 AM position on the pot or leave it off completely.
Your level is now set. You will have a good, even day picture and the best image available at night as determined by the sensitivity of the camera and the amount of available, reflective light.
I hope this helps you out. I know that it is probably not the answer that you were looking for, but it is the right answer. If you still continue to have grainy images at night or washed out during the day, my only other suggestion is that you take a serious look at the camera that you are using. Grainy or no night image would represent too little sensitivity on the camera's part. Washed out image during the day could represent too much sensitivity to IR or hot light. If I or my staff can be of any additional service to your CCTV needs, please feel free to holler. We are in your service.
