Distance – The Other Variable

Performance of even the best video cameras depends on the light that reaches it

• By Aaron Tankersley
• Aug 01, 2013

Zoom cameras allow targets from hundreds of feet away to be viewed up close, while megapixel cameras can cover larger areas with fewer cameras. But is it really that simple?

Savvy integrators and end users know to make decisions based on what they have actually used in the real world, not on product claims. Surveillance professionals are also familiar with camera terminology, such as pixels-per-foot (PPF), which can help evaluate the utility of video equipment to perform an expected task.

For example, a systems integrator might decide how many pixels he or she would need to recognize a person or to read a license plate. Considering that number of pixels, how it relates to surveillance camera resolutions and the size of an area being covered can point to which equipment might provide the needed pixel count, or resolution, to perform at the required level.

However, an often overlooked aspect when specifying cameras to cover large areas is the distance of the target from the camera. This variable especially relates to light collection, specifically how much light is collected by the image sensor, based on the distance of the target from the camera. The inverse-square law explains how light decreases over distance as the amount of light that reaches an image sensor is “inversely proportional to the square of the distance” between the light source and the camera. For example, if the distance is doubled, illumination is reduced by a factor of four.

Much more light is collected if the camera is located close to the target, but if the target is far from the camera, fewer photons of light are sensed. Offsetting the effects of this one simple variable can make the difference between a surveillance system that does the job and one that fails. Technology choices can mitigate the challenges, but at what cost?

The solution may be as simple as locating the camera closer to the target.

Let’s take a look at how the distance of a target from a surveillance camera can negatively impact camera functionality related to light collection.

Distance and Light Sensitivity

Using a standard resolution, video surveillance camera, an operator could read a license plate with a 8MM focal length lens from about 30 feet away (based on the guideline of 50 horizontal pixels-per-foot).

An alternative option is placing a megapixel camera at a distance of 90 feet, for example, where data and power sources are readily available. Using a typical, two megapixel camera at a distance of 90 feet, with the same pixels-per-foot, a 14MM focal length would be needed. At this focal length, and based on typical sensor sizes (for example, 1/2.7”), the light available per pixel at the sensor is reduced by approximately a factor of nine.

Considering the element of light, the lens diameter has to scale along with the distance ratio in order to increase the collection area for light. So, at 300 feet, a lens about 10 times larger would be required, and the lens would need 100 times more light-gathering power to allow the same low-light level sensitivity. There also must be sufficient light to view the required subject, and the smaller pixel sizes make them less sensitive to light, which can further complicate the issue. Unfortunately, these diameters and capabilities are not available with today’s standard lenses.

Variables impacting the effectiveness of surveillance cameras viewing long distances include maximum aperture opening, lens diameter and focal length. These variables make it critical to test the functionality of any camera system before committing to a large installation and making a significant investment.

Capturing useful video images from far away requires additional investment in multiple elements of a surveillance camera system, especially more expensive lenses to provide larger pixel sizes. The ability to see greater detail using a higher-resolution camera does not address the physical requirements of sufficient light to accomplish the task. Light levels diminish across long distances, which can make viewing distant locations, such as along a facility’s perimeter or large outdoor areas, from a camera located hundreds of feet away either impractical or prohibitively expensive.

Challenges of Locating a Camera

The simplest solution to meet the challenge of capturing video across long distances is to shorten the distance. Instead of requiring cameras to view long distances, why not locate the camera closer to the item being viewed?

Here are some challenges with this solution:

• Limitations of IP network reach. IP cameras must connect to the network, but the farthest reaches of an enterprise might be beyond the network’s reach. Locating cameras only within reach of the network presents challenges of viewing distant objects, especially related to decreases in light that reaches the camera as the distance increases.
• No available power. Likewise, a distant perimeter, parking lot or other asset might be located far from any available power source.
• Is trenching practical? Digging trenches for electricity and/or network connectivity is expensive, and it may not be practical based on the environment. For example, what if a protected asset is located on the other side of a lake? What if it’s across a public street? While erecting poles and stringing wires is possible, this approach could disrupt aesthetics, cause environmental concerns, create easy targets for vandalism or be vulnerable to certain weather conditions, such as violent winds.

Combining Solar Power and Wireless Transmission

It is possible to overcome the challenges of locating video cameras to view distant assets by using solar power to operate the camera along with wireless transmission to move the video signal from the camera to a network video management system. This simple approach overcomes the aforementioned technology challenges and the potential higher expense of generating clear images using cameras to view across long distances, especially the problem of not enough light. Shorter distance means more light reaches the camera, allowing even standardresolution devices to capture images clearly without specialized lenses, optics or other additional technology expense.

Wireless access to video is changing how security systems operate at the edge of a network. Installing a surveillance camera no longer requires a video cable and a dedicated power source, and there is no need to run wires through trenches.

New wireless systems can transmit video through the air from remotely-located cameras, up to a half a mile away, to a centralized video hub that is connected to the network and tied into a video management system. The camera remains close to the object being viewed, vastly simplifying image capture and retention, and potentially allows it to perform at a much lower cost.

Solar panels, with five-day battery storage, can provide power to special, low-power, surveillance cameras used to view remote locations. Such an approach greatly expands the coverage area of a video system at an affordable price. Inexpensive, remote IP video surveillance is crucial to securing perimeters and large outdoor areas.

Self-contained camera systems, those powered by solar energy that use low-power, wireless, video transmission in the unlicensed spectrum, are highly mobile, which provides a dynamic and adaptive approach to video surveillance coverage. In the case of a special event that requires additional security and video coverage, cameras can be added on an ad hoc basis, with no need to run wires or pay other infrastructure costs.

For example, if there is a particular problem area on a perimeter, the wireless surveillance camera can be moved to provide coverage of that area on a temporary basis, and then moved somewhere else, if another need arises.

A Simpler, Less-expensive Approach

To design an effective surveillance system, video cameras must be placed appropriately, and a multitude of factors have to be considered such as backlighting, low-light levels and sufficient image clarity.

The proliferation of higher-powered zoom lenses and super-high-resolution megapixel cameras in the market suggests a wealth of new possibilities related to camera placement. These systems promise to see more with fewer cameras and to enable end users to save money in the process. However, every technology has its limitations, and even the best, most up-to-date camera systems are only as good as the quality of the image that reaches the camera, which is a function of how much light reaches the image sensor.

As the distance between the camera and image sensor increases, the amount of light that reaches the sensor goes down. Even the highest-resolution camera cannot see an image clearly unless there is enough light. Too often, system designers overlook the effect of distance on how much light reaches the camera or they design expensive systems to overcome the problem. A simpler approach is to put the camera nearer to the object being viewed. New solarpowered, wireless, video cameras can do just that, resulting in less expensive systems with superior performance.

This article originally appeared in the August 2013 issue of Security Today.