When shopping for a light source, most people look first at watts (W). However, watts measure power consumption—the rate at which electricity is used—not the amount of light produced. A higher wattage means higher energy use, not necessarily brighter light.
Terminology: "Bulb" vs. "Lamp"
In everyday language, we call the entire light source a "light bulb." Technically, however:
- Bulb: Refers only to the glass envelope that surrounds the light-producing component.
- Lamp: The correct industry term for the complete, replaceable light source (including the base, filament or arc tube, and glass).
Lighting professionals use "lamp" to describe the whole unit. In this course, we may use "lamp" and "light bulb" interchangeably for clarity, but it is helpful to know the technical distinction.
Measuring Light: The Lumen
The standard unit for light output (also called luminous flux) is the lumen (lm). Unlike watts, lumens tell you how much visible light a lamp produces—the higher the lumen rating, the brighter the light.
When you look at a lamp's packaging, you will often find three key specifications
- Luminous Flux (lumens)- or how much visible light the lamp produces. Eg. 800 lumen
- Power consumption (Watts) which tells us how much electricity the lamp uses. Eg. 9 Watts
- Rated Life (hours or years)- which tells us the expected operational life span. Eg. 10,000 hours or ~9 years
Text description of the light bulb packaging image.
The image is a side-by-side comparison of two packages of GE LED light bulbs. The left package, set against a blue background, features the "refresh" LED brand, labeled as "Energetic Daylight." It displays four bulbs with silver bases, highlighting features like "Enhanced Color Contrast & Boldness" and "13 Year Life." Additional information includes "Daylight," wattage equivalent, "Dimmable" indicator, "Brightness 800 lumens," and "Estimated Energy Cost $1.02 per year."
The right package, in an orange background, showcases the "relax" LED brand, described as "comfortable soft white light." Similarly, it displays four bulbs and highlights "Enhanced Color Contrast," "40w replacement," "5.5w energy use," and "13 Year Life." Other details include "Soft White," "Dimmable," "Brightness 450 lumens," and "Estimated Energy Cost $0.66 per year."
Watch this video below to find out more about lumens. (1:44)
Transcript: Energy 101: Lumens. (1:44)
Today you’ll see more light bulb options in stores. These bulbs will give you the light you want while saving you energy—and money.
Here’s something to consider. In the past, we bought light bulbs based on how much energy—or how many watts—they use. But today’s energy-saving light bulbs use up to eighty percent less power to give you the same amount of light. So wouldn’t it make more sense to buy light bulbs based on how much light they provide? With lumens, you can do just that.
Lumens are a measure of brightness. So if you know how many lumens you want, you’ll buy just the right bulb for any spot in your home.
Instead of buying an inefficient sixty-watt bulb, look for an efficient replacement that gives you about eight hundred lumens of light. If you’re replacing a one hundred-watt bulb, look for an energy-saving bulb that gives you about sixteen hundred lumens.
Just think: the more lumens, the brighter the light.
To help you shop for the light you want, you’ll find an easy-to-read label on light bulb packages. So you’ll have a simple way to see the bulb’s brightness, how much the bulb will cost to operate for a year, and other qualities like light color—from warm yellowish to cool bluish.
With more energy-saving choices appearing on store shelves, including compact fluorescents, or CFLs, light emitting diodes, or LEDs, and energy-saving incandescents, you’ll have more options that save you money.
So when shopping for a new bulb, look for lumens—or how bright the bulb is. Remember, lumens is the new way to shop for light.
Footcandles
A footcandle (fc) is the Standard unit of measure for illumination on a surface. It is a lumen of light distributed over a 1-square-foot (0.09-square-meter) area.
The average footcandle level on a square surface is equal to the amount of lumens striking the surface, divided by the area of the surface.
Example
A 40 watt bulb produces about 505 lumens and has a life of about 1,000 hours. When this bulb is used to light a room of 10 x 10 feet, these 505 lumens are distributed over 100 square feet of floor area. What is the illumination?
Lighting Efficacy measures how effectively a light source converts electrical energy (watts) into visible light (lumens).