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.