Testing Polarizing Filters on HD CamerasBy Ethan Ace, Published May 30, 2014, 12:00am EDT (Research)
Sun glare is particularly challenging for surveillance cameras. Even top cameras struggle with such conditions.
One technique suggested is using polarizing filters. Simply put, these filters pass certain polarizations of light, while blocking others, reducing sun glare at specific angles.
To test how polarizers worked in real world scenarios, we tested 720p cameras in three configurations:
- Auto-iris lens with polarizing filter
- Auto-iris lens, no filter
- P-iris lens, no filter
We tested these three cameras outdoors on a bright, sunny day, to see how the polarizing filter impacts performance.
Here are our key findings from this test:
- Properly aligned polarizing filters can reduce sun glare significantly compared to unfiltered P-Iris and DC auto iris lenses, with the ability to see subjects through glass otherwise obscured by glare and reflection.
- Small changes in angle of incidence impact filter effectiveness due to the narrow polarizations passed by the filter.
- Low light performance was marginally degraded with the filter on, with little practical impact.
- P-iris lenses provide a marginal improvement in visibility over standard DC auto iris lenses, but glare and reflection is still a significant issue.
Polarizing filters can definitely help to reduce or eliminate sun glare in outdoor scenes. However, camera placement and angle of incidence need to be carefully considered if they are to be used, as polarization will only remove glare from narrow angles, not the entire scene. Cameras should be placed with this in consideration, in locations where repeatable angles are most likely in order to maximum the effectiveness of the filter.
Filter Mounting Types
There are two common types of filter mounts available:
Slip ring: This type of mount attaches to the front of a standard lens via a ring secured with set screws. Slip ring mounts and filters are available in
multiple sizes to accomodate different lenses. Since the filter is mounted to the front of the lens and is essentially a flat piece of glass, focus of the lens remains nearly unchanged. Pricing varies from about $20 to $50 USD for sizes suited to CS mount lenses, but may run over $100 for larger filters used in photography.
C-Mount: C-Mount filters add an adapter between the lens and camera with a rotating ring built in to align the filter. This allows for more integrated package with less chance of the filter being bumped or removed in installation. However, since these adapters add distance between the lens and imager, focus is affected. C-Mount adapters sell for about $100+ online [link no longer available], not including the filter, generally another $50-100, making for a more expensive package overall.
We used a slip ring filter from Midwest Optical Systems in this test. In this video we show how the filter is installed, with a threaded ring mounted to the lens:
In this video we show how to properly align the filter for glare reduction, and show angle issues. In this example, we are able to reduce glare on either the windshield or side windows of the car, but not both. Because of this, shots with filtered cameras should be carefully considered for best effectiveness.
Positioning the cameras to view the front-right of the vehicle, the windshield of the car brightly reflects the sun in the low left corner, with sky and clouds reflected across the rest of the glass. The camera with the polarized filter can see our subjects skin tone, black shirt, and facial structure, while the other two cameras only see the glare of the sun and reflection of the sky off the windshield.
Next, we moved to a frontal shot of the car, with significant windshield glare. Using the polarizing filter, windshield glare is nearly eliminated and we can clearly see our subject in the car. The unfiltered cameras, both standard and P-Iris, are unable to see him, though the P-Iris lens is marginally better.
Low Light Impact
We tested the polarizing filter at night to see what impact, if any, it had on low light performance. In a low light scene, about 1 lux, the image is noticeably darker with the filter attached, seen in this overview shot with our subject in the car:
However, practical differences are small, seen in the image below. While the chart is slightly darker, it is no less legible with the polarizing filter on, and details of our subject are similar.
All cameras were tested using 1/30s max shutter at 10 FPS with quantization ~28 average. All other settings were left default.
Cameras used in this test and their current firmware version:
- Polarized 720p: Bosch NBN-733V, firmware 5.90
- Standard 720p: Bosch NBN-733V, firmware 5.90
- P-Iris 720p: Samsung SNB-5004, firmware 1.13_131218
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