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Issue link: https://securitysales.epubxp.com/i/166764
Te important concept to understand is a camera is designed to give you a certain number of pixels, which is efectively video information. Changing the lens or the zoom level of a lens gives you a diferent FOV, but it does not give you any more pixels. Te pixels are constant. So widening the scene effectively dilutes the video information of any particular pixel. Here is one way to conceptualize it: Consider the number of pixels you have with a given camera as a cargo net made of elastic. Each square in the net represents a pixel. If you widen the view of the camera, you efectively stretch the net. You have widened the pixels, but remember a pixel is only a single value, so now you've stretched that single value over more of your scene. In practical terms, that means you will have less information about a specifc object at a given distance. As an example, a narrow FOV may give you enough pixel information to allow you to read a person's baseball cap at 100m, while a wide FOV of that same person may not contain enough information in the diluted pixels to be able to determine if the person is even wearing a hat. So it is a planning decision to determine whether it is more important to view a wider area at the expense of more detailed video information, or to restrict the size of the FOV and gain that data. Te mistake is not understanding the tradeof being made. 6. GETTING AN INTERESTING VIEW We all like to be entertained, but that isn't necessarily the best approach when setting the FOV of a security camera. A common mistake is to include "interesting areas" in the FOV, versus areas of interest. A low camera FOV may allow for better identifcation of specifc details related to the target (face, clothing details, etc.), but can prove less effective in detecting and tracking than a higher camera FOV. For example, consider a situation where a high trafc sidewalk runs next to a facility's perimeter fence. Oftentimes the camera's FOV is set to include this high trafc region. However, if the area of interest is not the sidewalk itself, but rather the region between the sidewalk and the fence, the argument is why the sidewalk would be included in the camera's FOV. Every movement on the sidewalk now becomes the responsibility of the monitoring guard or security software to detect and assess as a possible threat. Te same can be said of roads or areas of water that occur in the background. If the area is not of interest from a security standpoint, then care should be taken to consider whether the FOV should include the region. 7. NOT UNDERSTANDING CAMERA MISSION A wide feld of view will dilute the available pixels, causing a loss in information about a specifc target, as shown in this example with the same target viewed at 50m with two different FOVs (4CIF, 480 X 320). To preserve the maximum amount of pixel information, the width of the FOV should be no wider than what is required to effectively monitor the scene. A fnal aspect of the FOV is understanding the intended mission of the camera. In practice this rarely goes beyond the setup of the detection area. However, as cameras become higher in resolution, there are more missions to consider. For detection, it is typically better to have a FOV that is taken from a higher vantage point. Tis helps avoid occlusions, allows for better understanding of the target's current track and potential trajectory. However, a higher vantage FOV is not always conducive to facial or clothing identifcation, where the face or clothing graphics are seen as at an angle, or blocked by a hat. Te opposite holds true, where a lower FOV results in better identifcation, but potentially poorer tracking and detection. Again, the mistake is not whether to have a high FOV or low FOV, it is not understanding the benefts and drawback of each, and not using the camera's mission to drive the most appropriate height when setting the FOV. Eric Olson is Vice President of Marketing at Phoenix-based video analytics developer PureTech Systems. He can be contacted at eric.olson@ puretechsystems.com. SEPTEMBER 2013 / SECURITYSALES.COM / 143