[Editor's Note: This is the first in a series of three articles addressing the growing technology of megapixel video surveillance cameras. Upcoming installments (to be published on SecurityInfoWatch.com during March 2007) will cover topics of digital zoom versus mechanical pan/tilt/zoom, and the relationship between megapixel cameras and storage/bandwidth needs.]
Megapixel Surveillance Cameras: Worth the Price?
Why are so many end users switching to megapixel technology, or at least examining this technology at tradeshows and in project bids? The main reason is price. That probably sounds confusing since the conventional wisdom is that a reliable, industrial-quality megapixel network surveillance camera is three or even four times more expensive than comparable low resolution (4CIF) security cameras. How can the megapixel camera be cheaper?
Video Surveillance Goals
Before we take up price, let's first discuss why an end user would even consider megapixel? This is the most important question because in some instances megapixel cameras may not be appropriate. Before you look at any camera or NVR/DVR combination you have to initially define the goals for your video system. For simplicity, we narrow the security goals down to three categories:
1. General Surveillance: These are applications; often live viewing, where you don't need detail on recorded video. For instance, you may be watching a road and looking for traffic jams but do not need to read license plates. Or you may be looking to see what a crowd is doing but do not need to recognize faces. You may simply want to detect when someone is in a restricted area so you can respond immediately.
2. Forensic: These are applications where you need to see, record, and recognize images like license plates and faces, so you can go back "after the fact" and determine exactly what happened.
3. High Detail: Applications where you need to read a license plate but also read the model of the car. In a retail or banking context, you need to clearly see the customer's and employee's faces as well as identify the currency in their hands.
By assigning a category to your application you are, in fact, defining the resolution requirements. With older surveillance technology, resolution was defined by its TVL (television lines, a.k.a. total video lines). Over the last few years the market has evolved to defining resolution by the total number of pixels. This is more objective when comparing products that have different resolutions. So, when an image is 640 x 480, that is actually 640 (horizontal or columns) pixels x 480 (vertical or rows) pixels, for a total of 307,200 pixels, or approximately 0.31 of a megapixel (million pixels).
But resolution can only be defined by pixel if you determine how wide an area those pixels will be covering. How do we determine that? It's all about pixels-per-foot.
Coverage Defined by Pixels-per-foot
For general surveillance applications, you need approximately 20 pixels per foot. Forensic applications require about 40 pixels per foot. and high detail applications demand at least 80 pixels per foot.
Once you have defined the type of application you have, and you know how wide an area you have to cover, you can figure out if megapixel is right for you. An example will make this much clearer. Let's say you want to cover a parking lot with forensic detail and the lot is 100 ft. wide. For simplicity, we will only discuss here the width (horizontal field of view). For a true analysis, you need to factor in the width and depth.
100 ft. x 40 pixels/ft. = 4,000 pixels, which is what you will need to cover that 100' area and recognize license plates and facial detail. Once you have this, the rest is easy.
The next step is to determine what resolution cameras you want to use. You determine this by dividing the number of pixels you need to cover the area (in this example, we've determined that count to be 4,000 pixels) by the number of horizontal (columns) of pixels provided by your camera.