Lighting metrics are used to understand and predict how a lighting system will operate. They deal with quantity of light (light output and light levels), quality of light (brightness and color), and fixture efficiency (electrical efficiency and how much light leaves the fixture).
QUANTITY OF LIGHT
Luminous Flux (Light Output). This is the quantity of light that leaves the lamp, measured in lumens (lm). Lamps are rated in both initial and mean lumens.
Initial lumens indicate how much light is produced once the lamp has stabilized; for fluorescent and high-intensity discharge (HID) lamps, this is typically 100 hours.
Mean lumens indicate the average light output over the lamp's rated life, which reflects the gradual deterioration of performance due to the rigors of continued operation; for fluorescent lamps, this is usually determined at 40% of rated life.
A number of factors affect a lamp's light output over time, including lamp lumen depreciation, the lamp's interaction with the ballast, supply voltage variations, dirt or dust on the lamp, and the ambient temperature in the fixture.
To avoid confusion, note that "lumen output" is a term also used to describe a fixture's light output, not just a lamp's. Even more factors can affect light output in this case, including the distribution characteristics of the fixture, fixture surface depreciation, and dirt and dust buildup.
Illuminance (Light Level). This is the amount of light measured on the workplane in the lighted space. The workplane an imaginary horizontal, tilted or vertical line where the most important tasks in the space are performed. Measured in footcandles (fc) (or lux in metric), light levels are either calculated or, in existing spaces, measured with a light meter. A footcandle is actually one lumen of light density per square foot; one lux is one lumen per square meter. Like lumens, footcandles can be produced as either initial or maintained quantities.
Initial footcandles indicates a light level after new lamps are installed.
Maintained footcandles indicates a light level after light loss factors are considered over a period of time. Light loss factors include those affecting light output (see above) and also room surface reflectances, room size/proportions, dirt and dust buildup. While light output may describe either the output of a light source or fixture, maintained footcandles always takes into account the efficiency of the fixture in transmitting light to the workplane.
The human eye is a sophisticated piece of machinery; it is able to adjust to a wide range of light levels, including about 10,000 footcandles on a sunny day to about 0.01 footcandles under full moonlight. However, optimum ranges of light levels have been established for various tasks so that those tasks are performed most efficiently (reading a magazine, for example, would be difficult under moonlight, while 10,000 footcandles would be excessive).
For more information, see Lighting Design Basics and Light Loss Factors.
QUALITY OF LIGHT
Luminance (Photometric Brightness). The light that we actually see, brightness can be measured as the light leaving a lamp, or the light reflecting from an object's surface. If not controlled, brightness can produce levels of glare that either impair or prevent a desired task being performed. Glare can be described as direct or reflected glare, which can then result in discomfort or disability.
Direct glare comes straight from the light source.
Reflected glare shows up on the task itself, such as a computer screen.
Discomfort glare does not prevent seeing makes it uncomfortable.
Disability glare prevents vision. A popular example is holding a glossy magazine at a certain angle; a veiling reflection results, impairing our reading of the page.
Color. The color quality of a lamp is revealed as its color temperature rating and Color Rendering Index (CRI) rating. For a detailed description of these metrics, see Color Metrics
FIXTURE EFFICIENCY
There are two ways to look at a light fixture's (luminaire's) efficiency; one indicates how well the lighting system transforms electrical input into useful light output, and the other indicates how well the fixture itself transmits light from the lamp(s) to the workplane.
Electrical Efficiency. Lighting systems require electrical input to work. This input is measured in watts (W), a measure of required electric power. A lighting system's rated input wattage, therefore, is the amount of power required for it to work at any given instant of time.
Lamp manufacturers publish nominal wattage ratings for their lamps; when fluorescent and HID lamps are operated as a system with a ballast, however, a new rated wattage will result, published by the ballast manufacturer. Ballast manufacturers publish up to three input wattage ratings. The ANSI number is the result of a standardized ANSI test of that given ballast manufacturer's ballast operating a given compatible lamp type (often called the "bench test" because the lamps and ballasts are operated bare on a bench). The next one or two are the manufacturer's ratings for tests in actual open and/or enclosed fixtures.
While the manufacturer's ratings can be considered more realistic (because the testing takes place closer to actual field conditions), the ANSI number should be used when comparing different ballasts because it reflects the results of a common, standardized test procedure.
Therefore, one way to compare the electrical efficiency of lamp-ballast systems is to determine a common light output level, then compare the input wattage for various systems.
A more popular way of achieving a comparison of the relative efficiencies of lighting systems is to use efficacy, expressed in lumens per watt (LPW or lm/W). To determine a system's efficacy, divide its lumen output by its rated input wattage.
When lighting professionals apply the results of efficiency to actual system operation (usually to determine the operating cost savings of a retrofit, they need to determine the amount of energy the lighting system consumes, not just its input wattage. To calculate the energy use of a lighting system, multiply input wattage (W) x time (hours of operation during a year).
Example for Lighting System:
| Input Wattage | 100W | |
| Lumen Output | 10,000 lm | |
| Efficacy | 100 LPW | 10,000 lm ÷ 100W |
| Hours of Operation | 3,120 h | 5 days/week x 12 hours/day x 52 weeks/year |
| Energy Use | 312,000 Wh | 100W x 3,120 hrs/year |
| Energy Use | 312kWh | 312,000 watt-hours (Wh) ÷ 1,000 = 312 kilowatt-hours (kWh) |
| Utility Charge/kWh | $0.075 | |
| Energy Cost/Year | $23.40 | 312kWh x $0.075/kWh |
