Lumens and the Brightness of an LED Flashlight (Torch)
Flashlights of Yesterday and Today
Do you remember the flashlights of days gone by? If you wanted a lot of bright light, the flashlight weighed several pounds and even then you might have that big dark spot in the center of the beam. Either that or the area the beam covered was so small as to be completely impractical for some uses.
Flashlight technology has changed with the development of LED bulbs. Flashlights can now weigh mere ounces and deliver a lot of light, either in a concentrated beam, in a wide field or both.
A Little History of the Flashlight
In the late 1880s the first dry cell battery was invented making flashlight technology possible. In 1897, David Misell invented a flashlight made of a paper tube, brass end caps and a bullseye bulb. It operated on three, size D dry cell batteries. These were passed out to officers in the NYC police department with positive feedback. Early flashlights produced 8 to 10 lumens per watt.
Before LED technology, incandescent bulbs were used. These glass bulbs were under a vacuum or contained argon, krypton or xenon gas inside and had a tungsten filament to provide the light.
LED technology changed flashlights considerably. Since these bulbs are more efficient, fewer batteries are needed to produce the light desired. These flashlights have shrunk in size and grown in total output of light. LED lights produce about 100 lumens per watt or ten times that of incandescent bulbs.
Lumens and Related Terms
Lumen-Maximum amount of light delivered at the source
Candlepower (cd)-An obsolete, unregulated measurement.
Candela (cd)-Replaces Candlepower in terminology
Lumens as a Measurement
A quick check on Amazon revealed flashlights being advertised according to the number of lumens. They began on the low end with 19 lumens, then 28, 55, 600 and 2000 lumens. But what does this tell us about the flashlight. Does it tell us how bright the light will be?
One flashlight on Amazon was referred to as a 300 lumens, ultra bright flashlight. But do lumens measure brightness? In the comments on one flashlight for sale, a man said that the 2,000 lumens light he bought wasn't as bright as his 750 lumens light. He implied the company producing the light was not being truthful.
One way of looking at this is that both were speaking the truth. The 2,000 lumen light was not as bright as the man's 750 lumen light. But the one for sale truly was a a 2,000 lumen light. What was the problem? The problem has to do with the definition of the word lumen and the misunderstanding people have of this measurement of light.
Lumens is Total Output of Light
What is a lumen? Lumens are the total output of light from the source. It is described as being the total amount, or volume of light being delivered from a light source such as a flashlight. Each lumen is one candela (cd) or candlepower. A 2,000 lumen flashlight will have a total light output of 2,000 candlepower.
Think of your garden hose. Open the faucet feeding into the hose to maximum. Let's assume the water is flowing out of the faucet at 5 gallons per minute. That is the total output of water. Compare that to our 2,000 lumen flashlight. At its maximum battery strength, the total output is 2,000 candlepower. In this comparison, the light from the flashlight is just like water from a hose.
When we put a nozzle on the hose, the power of the water stream can be increased. We can set it as a narrow, powerful stream for washing mud off tires, or as a wide pattern good for sprinkling flowers. The power between the two is very different, but don't forget that the water from the faucet is still open to 5 gallons per minute.
But does the flashlight come with a nozzle, like the hose? Yes it does. It's called a lens. The lens focuses the light. It can be concentrated into a narrow, bright beam or opened up into a broad, relatively dimmer beam. But there is always the same total output, the same 2,000 lumens.
So when you purchase a flashlight, and the number of lumens is touted as a measurement of the flashlight's brightness, you know that is not necessarily true. Brightness depends on the kind of lens that is already on the flashlight. Is it a narrowly focused lens or a broadly focused lens? Some flashlights come with a movable lens so that the concentration of light can be changed from narrow to broad, but it does not change the total output, the number of lumens.
LED Flashlight With Multiple Intensity Settings
Radiance and Related Words
Radiance-The measurement of the brightness of light on the object (Target)
Output vs Radiance
So there are actually two things to measure on a flashlight, aren't there? One is the total output of light, which is called lumens. The other measurement is of the power of the light, called radiance. It is a measurement of the effect of the light on its target.
Suppose a flashlight is pointed at a person's eye. Radiance is the power, strength or intensity with which the light hits the eye. Again, compare it to the water hose. A narrowly focused stream will destroy flowers and a narrowly focused beam of light is strong enough to harm a human eye. This strength, or power of the light is its radiance.
When we purchase flashlights, what we are really interested in is the radiance of the light, how bright it is. This has more to do with the lens of the light than it does the lumens or total output of the flashlight.
Now, common sense tells us that if we open the water valve fully, there is more water for the nozzle to work with and focus. Less water gives the nozzle less water to focus, so there is a relationship between output and and power, but the nozzle, or in the case of a flashlight a lens, is required to produce the power.
Output vs Radiance
Measuring Output vs Radiance
One final way of understanding the light from a flashlight is to consider the source of the light and the object, or target of the light.
Lumens, the measurement of output, is related to the source of the light.
Radiance, the measurement of power, is measured at the object, or target, of the light.
The next time you pick up a flashlight at the hardware store, and you look at the information on the package, don't fall for the advertising gimmick that touts lumens as brightness. You might or might not be disappointed, depending on the lens attached to the light. If the lens is set to focus into a narrow beam, the radiance will be high. If the lens gives out a broad beam, radiance will be low.
One way of getting around this problem is to purchase a flashlight with movable lens. These lights allow the front of the light to slide backward and forward, changing the focus and the concentration of the light. The narrow light will be the highest radiance. The broad beam will be the lowest radiance.
Advertising on Flashlight Packaging
Lumens and Advertising
Why don't companies which sell flashlights do a more thorough job of explaining these concepts on the packaging? Probably they are attempting to avoid confusing the consumer. What they are doing is combining the output and intensity measurements and simply referring to them as lumens.This can be seen on the package label above.
"Peak Intensity" is advertised to be 6,000 cd. The cd refers to candlepower or lumens, but the word intensity refers to radiance. The company selling this product feels that it is easier to combine the definitions than it is to explain them. The way I interpret the advertising for this product is to assume that Peak Intensity is radiance and High Beam intensity is output or lumens.
This particular flashlight, according to the packaging, has a "High beam," meaning it evidently also has a low beam. This is a good way to go in my mind, because you aren't stuck with a single intensity setting.