Shining Light on CCTV

By Khali Henderson

Installing CCTV cameras is a standard and effective component of any comprehensive security system, but its contribution is diminished or enhanced by the venue’s lighting conditions.

The Illuminating Engineering Society of North America (IESNA) wrote the book on lighting. Recently, its Security Lighting Committee issued guidelines (G-1-03) for security lighting that include recommendations on lighting for photographic surveillance. (An online course also is available.) According to the guidelines, the most important illumination factors are the type, quality and quantity of available light.

Type. The type or source of light can be natural, such as the sun or moon, or electric, such as incandescent, fluorescent or high-intensity discharge.

At one extreme is the incandescent lamp, which is the least expensive but also the least efficient in converting energy to light. Its main benefits are low cost, good color rendition and ease of installation.At the other extreme is the low pressure sodium lamp, which is fairly expensive to purchase, has no color rendering ability, but is extremely efficient at producing light. The other light sources fall between these extremes. For CCTV surveillance, color rendering can be more or less important depending on the need to identify a person’s appearance in detail.

Quality. The quality refers to the light’s compatibility with the image sensor, pixels in today’s CCD cameras. Characteristic differences in the spectral response among the sources can affect the image.

Quantity. The intensity of light needed by the camera imager.

This can be further reduced or attenuated by light reflecting off surfaces in the scene. The amount of light falling or reflected on a scene can be determined with an illuminance meter, according to the IESNA, which recommends readings be taken from the location where the camera lens will be positioned and in the direction in which it might be pointed.

The sophistication of today’s equipment can compensate for lowlight scenarios, says lighting designer Doug Paulin, owner of Lighting Forensics and a past chairman of the IESNA Security Lighting Committee.

CCTV manufacturers specify the required amount of illumination — also known as minimum scene illumination — for their cameras in lux. This value relates the actual energy per unit-area falling on a surface to what the human eye perceives. A sunny day would be about 100,000 lux while a full moon at night would be about 0.1 lux. For cameras, CCTV experts say the lower the lux rating, the lower the light levels at which it will produce an acceptable image. Thus, cameras with low lux ratings are said to be more light-sensitive. Monochrome cameras generally are more sensitive and have lower lux level ratings than color cameras and, therefore, will produce acceptable images in lower light conditions than will color cameras.

While lux ratings offer some indication of the camera’s sensitivity, they are not always useful in side-by-side comparison, says Ramon Duran, vice president of product development for Samsung CCTV. He explains that manufacturers assign lux ratings based on non-standard measures. So, while one would assume that a camera requiring a minimum scene illumination of .05 lux would be twice as sensitive as one requiring .1 lux. However, Duran says this is not necessarily the case as there are two other variables to consider.

One is the lens aperture. “If one is f-stop 1.0 and the other is 1.4, you can’t compare” the lux rating because one camera is set to let in twice as much light as the other, he says. Another variable in calculating the lux level is how manufacturers measure the light output as expressed in units called IRE, with black equal to 0 and white equal to 100. Ramon says the logical approach is to use a baseline of 50 IREs, but some cameras use 25 or 30, making it more difficult to compare apples to apples.A similar problem can occur when considering reflectance, light reflected from the surface back to the camera.

These discrepancies are a running concern for the security industry, and at the behest of the FBI, the Security Industry Association is considering creating standards for scene illumination, says Hunter Knight, director and chair of SIA’s technical committee, which is developing standards for video quality. In a February meeting of interested parties, the lighting question was raised, he says, but so far it is not officially part of the agenda.

Duran says, given the misinformation, it’s important that sample cameras are obtained to check their performance under actual lighting conditions.