Another property of artificial light that is important is color temperature. This imparts a particular mood to the human observer. It is also important for the optimization of camera performance although cameras can correct for color temperature using white balance. Color temperatures are quoted on the absolute Celsius scale in Kelvin (K) (0K being absolute zero, the lowest temperature possible, 273K being freezing water, and 373 being boiling water) and are characterized as follows:
• 3,000K — Warm: friendly and intimate (restaurants and hotel lobbies)
• 3,500K — Neutral: friendly and inviting (showrooms and offices)
• 4,100K — Cool: neat, clean and efficient (classrooms and hospitals)
• 5,000K — Daylight: bright, alert, exact coloration (galleries or medical exam areas)
Incandescent and halogen lights are used for a warm environment, mercury vapor lamps cover the neutral through daylight range and metal halide fixtures are used for the cool and daylight temperatures. Fluorescent lamps and LEDs are available to cover all of these temperature moods. High- and low-pressure sodium lamps output light with color temperatures between 1,750K and 2,000K.
While the CRI describes the quality of light, quantity of light is measured in lumens (a 100 watt incandescent bulb emits about 1,700 lumens) and the intensity of light, or illuminance, is measured in lux (lumens per square meter) or footcandles (lumens per square foot). (A rule-of-thumb conversion is 1 fc = 10 lux — more accurately 10.764.) It is this measure that we refer to when we speak of light levels. The following are measures of natural light levels in footcandles:
• Direct Sunlight 10,000 fc
• Full Daylight 1,000 fc
• Overcast Day 100 fc
• Twilight 1 fc
• Full Moon 0.01 fc
• Quarter Moon 0.001 fc
• Starlight 0.0001 fc
• Overcast Night 0.00001 fc
Reflection of Light
Neither we, nor a camera imaging sensor, receive all of the light that hits an object, since some light is absorbed and some is reflected towards us. Different materials have different levels of reflectance, measured as a percentage:
• Asphalt 5%
• New Concrete 40%
• Old Concrete 25%
• Red Brick 25%
• Grass 40%
• Snow 95%
Natural light is all around us (even if we are in the shade) but exterior artificial light tends to be directional — it is less expensive to focus the light on the object that needs to be illuminated than to broadcast it uniformly. Lighting fixtures in parking lots direct the light vertically down onto the horizontal asphalt surface so that we can see tripping hazards; however, this minimizes the intensity of light reflecting horizontally from a vertical object, such as a face or signage, towards the human eye — or a horizontally-pointing camera lens.
For security applications, fixtures that spread the light horizontally, without producing glare, improve our ability to identify and recognize people and actions.
Uniformity and Glare
Uniformity of lighting is related to the contrast of lighting levels in adjacent areas and is measured as the ratio of the highest light level to the lowest light level. A uniformity ratio of 4:1 or lower is recommended for parking lots and walkways.
Glare is a very high light level that directly impacts the eyes — or a camera image sensor — and “blinds” the person or camera to surrounding activity. Glare imparted to those intent on unauthorized intrusion may deter their activities, but most communities have strict ordinances against light “spilling” from commercial properties into residential areas — light trespass. When glare is directed at responding security forces, they have difficulty identifying activity. When glare impacts employees and visitors, they have a sense of insecurity.
At nighttime, we switch on artificial lighting to illuminate our environments so that we can see, be seen and continue our regular activities. As described above, color rendition and color temperature are two properties of artificial light that are used to define our lit environment. Another element of lighting systems that need to be considered are starting & re-strike times and capital & recurring costs.
Starting & Re-strike Times: We are accustomed to switching lighting in our homes on and off with instant reaction, but some industrial lamps produce light from an arc discharge (large spark). This technology requires elements to warm up after switch-on and before the discharge can be made. In addition, they must cool down if the power is interrupted before the arc can be re-struck.
In some applications, this is of little concern; however, to security, a momentary power outage can leave security forces and cameras blind for up to 20 minutes. Some designs of discharge fixtures incorporate a second lamp that does not need any cool-down time before re-striking.
While incandescent, fluorescent, halogen and LED lamps switch on and off instantly, metal halide, mercury vapor and sodium lighting takes up to 8 minutes to reach full output when switched on from cold and up to 20 minutes on re-strike when hot.
Capital & Recurring Costs: The cost of lighting includes up-front capital costs as well as recurring expenses. The ratio is approximately 8 percent capital, 4 percent maintenance (cleaning and bulb replacement) and 88 percent electrical costs. Obviously, the efficiency, or efficacy of the lamp (measured in lumens per watt) and its life expectancy has a large impact on the overall cost of lighting. The table below provides comparisons between different types of lamp efficacy and expected life (there are 8,760 hours/year, 8 hours/day is 2,920 hours/year).
Depending on the operating environment, cleaning of lamps is important since dirt will reduce the efficacy of a lamp by 5 percent per year in an office and 20 percent per year in an industrial environment. Aging of the lamp can also reduce its efficacy by up to 20 percent during its lifetime.
Lighting Levels and Security
While it is difficult to identify the direct correlation between nighttime artificial light levels and crime levels, studies in Crime Prevention Through Environmental Design (CPTED) show that criminal activity is less prevalent in open, well-lit areas.
A study by Rensselaer Polytechnic Institute (RPI), published in 1996, measured the “sense of security” felt by people in response to the implementations of different lighting designs in various environments, such as the exterior of a garden apartment complex, a loading dock area and retail parking. The qualitative findings were that installations with increased lighting levels and improved lighting uniformity produced a higher “sense of security” and, in some cases, more cost efficient lighting.
Lighting and Video Cameras
Faceplate Illumination: Of the light that is reflected from an object being viewed, some is absorbed by the lens of a camera before arriving at the sensing element. Most camera manufacturers will specify the light sensitivity or minimum illumination, in lux or footcandles, required at the camera faceplate to produce full or usable video, given a lens with a particular f-stop. If the actual lens being used has a higher f-stop, less light will be available at the image sensor. If you are in any doubt about the available light for a camera application, it makes sense to request a demo unit and test it in the actual environment that it will be used.
Megapixel Cameras: Megapixel cameras are a boon to many security applications and are needed to save us from the embarrassment of the low-quality images from convenience store hold-ups that appear on the nightly news. However, the image sensors in megapixel cameras require a higher intensity of light than conventional cameras to excite all of the pixels, and the lens needs to be of much higher quality. Be aware and ensure that the correct light levels and accessories are specified to maximize the megapixel camera’s performance.
Low Light and Video Motion: Another essential for adequate light levels is where camera image display and recording is being initiated by video-analyzed motion detection. When the level of natural or artificial light decreases — day passing to night or interior lighting being reduced on a timer or by a standalone motion detector — the camera element may still receive enough light to produce a snowy or pixilated image; the “snow” may be interpreted by the camera as motion so that the image is recorded at all times and “real” motion alarms are lost — along with your video storage budget!
Glare and Light Contrast: Video sensing elements do not normally handle high contrast in lighting as well as the human eye can. Where we may be able to filter out the sunlight glaring off of the lobby marble floor, the camera may be blinded. Obviously, the best option is to locate and align the camera so that it does not suffer from this problem, however, in some applications, high contrast is inevitable. Select a camera that has offers a wide dynamic range and, as noted for megapixel cameras, if in doubt, test it out!
Infrared Cameras: As noted earlier, camera sensing elements are able to “see” further into the infrared (IR) band of the electromagnetic spectrum than our eyes can. Some cameras are designed to sense only infrared energy and produce monochrome images that show the heat emitted by hot bodies. Humans, animals and vehicles can be clearly differentiated from surrounding grass and foliage.
An alternative is to shine IR light on the surveillance area, and there are a number of cameras that include built-in IR-emitting LEDs. Standalone IR lighting fixtures can also be focused on the scene. Using this lighting technology, nighttime surveillance becomes covert since we cannot see the light. An added advantage is that light trespass is no longer a problem.
LED Lighting: In addition to the LED infrared lighting described above, cameras are also available with built-in, visible light LEDs. LEDs operate on low-voltage DC and very low power levels; also they can have extremely good color rendition, long service life, are shock resistant and are mercury-free. However, despite their high efficiency, LED lights require heat management since their output trails off at higher temperatures — usually a heat sink is needed — and capital costs remain high.
As with IR, separate LED light modules are available and some are now being installed for parking lot lighting. LED lighting technology is the object of high levels of research and development funding — efficiencies of production and operation are improving product performance and reducing capital and operating costs every few months. ?
David G. Aggleton, CPP, CSC, is president and principal consultant of Aggleton & Associates, Inc., located on New York. He has been practicing in the security system consulting and design field for over 30 years and has completed more than 500 design and implementation projects over the last 15 years. He can be reached at firstname.lastname@example.org.