When most people buy a light bulb, they look for watts (W). Recall that watt is a unit of power, (i.e., the rate at which energy is consumed from the electricity supplier). It does not say anything about the light.
The most common measure of light output (or luminous flux) is the lumen. All lamps are rated in lumens, as shown in the figure below, and every bulb has 3 parameters listed on the package:
- Lamp lumen output or light output
- Power consumption in watts
- Life of the bulb in hours
Watch the 1:41 movie below to find out more about lumens.
Lumens; how is Lighting Measured?
Alright, next question is, how lighting is measured. Now, we have so much of light here, how do we measure the amount of light that we get? Usually, the light output is measured in luminous flux, or the amount of light falling on horizontal surface. All lamps, for example, this one (a standard bulb), is listed with how many lumens? Lumens is the output from a particular bulb. How much light do we get from this bulb? Lumens. Not watts. Watts is the second one that you see here. And that is how many watts or how much power are we drawing from the electrical wires? And the third one that is indicated on every package that you see is the number of hours. In other words, this is the life, expected life of this bulb. And you can see here, Lumens is the light output. Lumens is an indication of how much light we get from these bulbs. For example, as I said, 40 watt bulbs give you 500 lumens, and here we have a 60 bulb which gives you 865 lumens. It is not proportional, but as the wattage increases, you get more and more number of lumens.
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.
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?
Now watch a 1:15 movie that shows how a light meter is used to measure lumens per ft2. Remember, .
Click here for transcript of Light measurement video.
Using a Light Meter
I put this 60 watt bulb here. And this 60 watt bulb is producing 865 lumens of light. When I hold this bulb very close to this surface area here, where the light is being measured, all this 800 is falling on a small area. Obviously, if we try to measure the lumens that are falling on this small area, we'll see that the number of lumens per square foot is very, very high. As you can see roughly there, we have about 1200 lumens/square foot -- very highly concentrated. As I move this bulb away, the value goes down because the same number of lumens are being distributed over a larger area, as you can see here. Now, it is about 100 lumens. And if I go further up, it’s going to reduce.