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Ask the Expert: Four Factors to Consider When Selecting a Megapixel camera

Using megapixel technology for security applications is a compelling reason to move away from analog cameras. Of course, you may own a 5-megapixel or higher digital camera at home, but getting this many pixels to run over an Ethernet network in a network camera at a high frame rate requires a lot of processing power, bandwidth and storage.
So be careful to match the appropriate resolution for your application. Luckily, determining your requirements is linear and easy to figure out once you know your objectives.

February 2, 2010  By Robert Moore

Today, you can easily find megapixel network camera models from major manufacturers in 1.3, 2.0 or even higher resolutions. In fact, the border patrol on the U.S./Mexican border reportedly uses 20-megapixel network cameras. If bandwidth and storage limits were no factor, you’d probably choose the most pixels available, but since a 1.3 megapixel network camera has four times the storage and bandwidth requirements of a standard VGA camera (640×480 pixels or .3 megapixels), make sure not to over specify.

And remember that the higher the pixel count the less light that will be available for each pixel.  So if you require a camera in a low-light setting, a higher megapixel camera may not have the lux level for your scene.
There are two factors in determining the pixel count in a scene: the amount of pixels in the image sensor and the lens type used. When aiming for a broad viewing area, you use a wider angled lens. With this wider view you dilute the amount of pixels available in your target area and limit the amount of detail seen after the fact. When monitoring a parking area without a need for license plate information, this won’t matter. On the other hand, if you need to identify a person or a license plate, select a more narrow-angle lens to obtain high detail. Thus, narrow lenses are used more for identification and after-the-fact forensics.
Let’s first tackle the sensor type needed. Here are four considerations:
Are you going to use the megapixel network camera for after-the-fact forensics?
Given your specific application, what number of pixels are required?
Do you require full-color representation of pictures?  Or high frame rates?
Do you seek multiple views from one network camera?
Watching the TV show CSI, your customers may expect extreme (and unrealistic) uses of video technology.  But they should be able to use recorded video after incidents to solve break-ins, determine involvement in fights, identify license plates on cars entering or leaving the property, etc. Some 95 per cent of video systems installed are used reactively, i.e. they aren’t actively monitored by a human, so megapixel network cameras are a great tool for this forensic.
In determining the right megapixel count, know ahead of time what detail you will want to zoom in on.  For example, suppose you are setting up cameras for a teller area and your target pixel count for facial recognition is 60 pixels from ear to ear to identify a person (60 pixels is a UK requirement for admission in court). Measure the distance from the camera to bank customers to mathematically determine the required megapixel count.  The same goes for identifying license plates.  If you know the camera is 100m from the viewed location and need 15 pixels for the height of the license to get character recognition, then you can mathematically determine the sensor required. Online tools are also available to help determine the right megapixel sensor and lens type.
Two drawbacks of regular megapixel technology include the lack of color quality as compared to the human eye, and the fact that at higher megapixels you don’t get full frame rates. For example, a casino may use chips that are pink and red.  A megapixel camera will be challenged to differentiate between the two. In addition, if you need fluid frame rates, megapixel cameras may only offer 5fps or 10fps at higher resolutions. In this case, use HDTV cameras, which offer full 30 fps, color representation that matches the human eye and are based on standards (SMPTE). An additional benefit of HDTV cameras is being able to use a 16:9 ratio versus a 4:3 ratio. If you have a digital or high-definition TV at home you will understand the difference here as regular non-HD programs will show up on your TV with black strips on both sides of the image. The benefit of the wider ratio (16:9) is that you can make sure your pixels aren’t wasted on ceilings, sky or ground, but the viewing area you want to see.
Finally, determine if your customer wants to replace multiple fixed VGA cameras with one megapixel network camera. A practical example is in a retail outlet in which you want to cover three cash registers with one network camera. To do so, you’ll need to find one that can record multiple streams from the same sensor at the same time. Not all megapixel network cameras are capable of this.
Choosing a lens is the final factor.  The more optical zoom incorporated, the fewer pixels required on your sensor.  The tradeoff is a smaller viewing area.  And the contrary would be true for wider angle lenses.
Take these factors into considerations when planning a megapixel implementation and you’ll be on track to making the right decision. And when in doubt, be sure to consult with your supplier, which will likely have plenty of expertise in-house to help with the decision.

Robert Moore is the Canadian Country Manager for Axis Communications. He can be reached at: Robert.Moore@axis.com

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