CCTV CAMERAS AND THE POLARIZING FILTER
BY: Don McClatchie,
Many cameras have a “Back Lighting” switch or control that can reduce the effects of glaring on the camera control, but it may not do enough to eliminate the problem.
Under normal circumstances, a camera with automatic iris or level control looks at the light level entering the camera lens and adjusts the iris for the correct light level. However, when the lens sees a bright glaring spot, the automatic iris is tricked into thinking that the picture is brighter than it is, and so it closes down the iris making the picture look much darker than it really is. Then to make the picture look correct, the installer adjusts the iris control by opening the iris wider so the picture is correctly illuminated. Of course, this “Blooms” the “CCD” Charge Coupled Device inside the camera. Blooming is the leaking of light information in the CCD chip from one pixel to another around an area that is over illuminated. This leaking causes the loss of picture definition in the area around the bright hot spot.
All reflected light like the type that causes glaring and hot spots have one thing in common: a change in the “Polarization” of that light. Natural light is an electro-magnetic wave that uniformly vibrates in all directions perpendicular to the direction of travel. When natural light strikes a reflective surface, the electromagnetic wave is changed from uniform vibration to vibration in only one direction. This light is now polarized and can be blocked by a polarizing filter.
To eliminate glaring light and prevent it from entering the cameras lens, put a polarizing filter on the front of the lens. The polarizing lens must be rotated to the proper circular orientation to block the reflected light. When installing the filter, look at the picture and determine the best position for mounting. Once installed, the glaring reflected light should be eliminated, and the camera and automatic iris will operate normally. The use of a polarizing filter when needed can save you many return trips to a job site caused by intermittent reflections that may happen only at certain times of the day. If you have any questions about this topic, contact FM SYSTEMS, INC. at: 800-235-6960.
We have a HD focus meter called the HDFM-1 for getting the sharpest focus from any HD-TVI, HDCVI and AHD video camera.