Common Lighting Terminology

  • What are the definitions of the following lighting terms?
  • Do you use these terms in professional practice?
  • What other lighting terminology do you use on the job?
LUMINOUS FLUX • EFFICACY • FLUX • ILLUMINANCE • CCT
FOOTCANDLE • EFFICIENCY • LUMINOUS INTENSITY • CRI
Managing Energy Cost in Office Building
Data Trends - Office - EngeryStar

Critical Terminology

Luminous flux is analogous to the flow rate of water, represented by gallons per hour.
Luminous flux:
Rate of flow of visible light emitted from a light source over time, measured in lumens (lm).
Lux:
Equal to one lumen per square meter.
Foot-candle:
Foo-tcandle: equal to one lumen per square foot (1 foot-candle = 10 lux).
Illuminance:
The amount of luminous flux that covers a surface (measured in lux or foot-candles).
Luminous Intensity:
Luminous Intensity: the concentration of light emitted from a given source in a particular direction, measured in candela (cd) (1 cd = 1 lm per steradian).

Efficiency and Efficacy

A high-efficacy luminaire provides more light for less energy.
Efficiency:
The ratio between the useful output of energy and the input of energy.
Luminous Efficacy:
Luminous Efficacy compares the amount of light produced by a lamp (lumens), to the amount of power consumed to produce it (watts).
Efficiency is usually dimensionless—we compare the lumens exiting a fixture to the lumens produced by the light source.

Efficacy is normally used where input and output units differ. We compare the lumens produced to the number of watts consumed.
Which is more efficient?
Efficiency = miles/gallon (energy out, energy in)

Which has a higher efficacy?
Efficacy = fun/gallon

Correlated Color Temperature (CCT)
Correlated Color Temperature (CCT)
A specification of the color appearance of light emitted by a lamp, relating its color to the color of light from a source when heated to a particular temperature.
A light source’s CCT rating indicates the general warmth or coolness of the light it emits, in terms of color quality & appearance.

Color Rendering Index (CRI)
Color Rendering Index (CRI) Measures the color fidelity of a light source to render the colors of various objects as compared with an ideal light source.

A lamp’s CRI measurement (up to 100) indicates how accurately its light renders a given set of color samples.
A light source’s CCT rating indicates the general warmth or coolness of the light it emits, in terms of color quality & appearance.

Why is my energy bill higher since my LED installation?

Bills could be higher if…
  • No meter read or estimates of a meter-read.
  • Outside weather conditions changed.
  • New or upgraded equipment installed.
  • Operating hours changed as originally stated in the energy audit.
  • Additional lighting was added at the facility after the energy audit.
  • Additional air-conditioning or heat running the prior month.
  • Your Power Company/supplier placed you in peak demand because of high use of energy (cold or hot spell) and put you into paying peak costs per kWh.
There could be many reasons why this bill is higher but let’s start by explaining why it CANNOT be a higher kWh.
When we do an audit, we replace higher wattage bulbs and fixtures with high quality low wattage LED bulbs and fixtures. Since the LED is “same power on/same power off” there is no spike in power when turned on. They are replaced with bulbs and fixtures that are at least 55% LESS POWER then what had previously existed. For that reason, the LED lights could never be higher than what was there.
 
When we replace any light bulb or tube with high-quality, low wattage LEDs, we reduce the electricity consumption resulting in saved money. For example, we typically replace a 400-watt high-pressure sodium light with a 150-watt LED lamp, saving 250-watts and the LED light will produce a brighter, better light. Multiply those savings by all the bulbs and tubes that are replaced with LEDs in the facility. In other words, 250-watt saving is still a 250-watt saved regardless of whether 1 or 100 air conditioning units are running. (NOTE these savings from wattage reduction are independent of the AC units or other equipment in the building)

In conclusion, a LED cannot cause a higher energy bill. Once you install a LED that is few watts, it will ALWAYS be fewer watts.
Why is my energy bill higher since my LED Installation?  Handout Link

Light Label Facts