We’ve all been there. The power goes out and you grab your phone and flip open the flashlight mode. Ah, the wonders of modern technology!
But then you see it. Tiny at first, but now that you’ve noticed it, it’s tugging at you, nagging to be noticed again and ever-growing in the forefront of your consciousness. It’s your battery percentage and it’s at 7%. You know that the next round of Candy Crush Saga can’t wait but yet you need the light to find the bathroom.
Now that you’ve made it to the bathroom (good deal), and you’re taking care of business, everything goes dark.
Well, it was already dark, but now it’s nothing but blackness. The kind of soul-crushing blackness that can make a person rethink who they are, and how they ended up where they did in life. In this particular instance, it’s on a toilet in the middle of a blackout with a dead phone, because while level 437 may have been moments away from conquering, the tiny LED in the back of your phone was doing what was asked of it. That little LED was holding your world together, keeping the soul-crushing darkness from reaching you, and also sucking down those last few milliamp hours your battery had left to give.
It is at this moment, as you sit on your throne in absolute darkness, contemplating how you’re even going to know whether your done wiping that you make a decision. Never again, never again will you make such a grave mistake. You will invest in a dedicated flashlight. But not some AAA rinky-dink flashlight. You want a serious flashlight. You want to be able to melt the space station should the need arise.
When you get online you see all kinds of numbers that sound like they just might back such claims as “world’s brightest flashlight.” You see numbers like 1,300 lumens (lm) and 102,000 candela (cd) but have no idea how those numbers relate or if they can even be compared. Now you’re lost and cringing as you think back to your recent dealings with the darkness.
I’ll let you in on a secret (that’s not very secret). Lumens and candela are indeed related, but caution should be used when evaluating using both numbers.
Candela is an SI (Système Internationale, translated: International System or as we know it, Metric System) unit. Furthermore, it is a base unit. This means that we can build other units measurement off of this unit of measurement.
To get an idea of what a candela is without getting all kinds of involved with the science of light let’s look at a candle.
A typical single-wick candle will emit about one candela. Place the candle so the flame is at the center of a hemisphere (blocking off light from half of the room) the candle still emits one candela in the unobscured directions.
Lumens are derived from candela (remember: candela is a base unit). Lumens are defined as candela-steradians or:
1 lm = 1 cd ⋅ sr
That is to say, a one candela light source that emits light in a cone that is equal to one steradian yields one lumen. A steradian is a unit of solid angle. There are 4π steradians in a full sphere.
Note: 4π ≈ 12.57
Given what we know from above. If we have a light source, let’s use our candle, which emits 1 cd, in all directions, we can calculate that the candle is emitting 12.57 lm.
1 cd ⋅ 4π sr = 12.57 lm
The same candle that gets blocked by our hemisphere still emits 1 cd but now only measures 6.28 lm.
1 cd ⋅ 2π sr = 6.28 lm
Because the hemisphere blocks half of the available light (the candle can only throw light in all directions not blocked by the hemisphere) we see a lumen measurement that is exactly half that of the unblocked candle output.
How many lumens a light source produces depends on how much of the measuring sphere is being illuminated (and at what intensity).
Conversely, if we place a light modifier around the candle so that all light that is emitted exits with a solid angle of 0.25 sr we can then calculate the candela output or max beam intensity.
12.57 lm / 0.25 sr = 50.28 cd
The same candle flame that puts out 1 candela can be made to appear as bright as 50 cd with a light modifier by taking the light that is emitted in every direction and focusing it in a cone of only 0.25 sr. In flashlight terms, this is often called a reflector.
That 1,300 lm flashlight and the 102,000 cd flashlight compared above are actually the same flashlight. I’ve included a calculator below for conversion. (Hint: the apex angle, or spread of the beam, is 7.3°) Go ahead and try the given examples.
I think of candela as how bright a beam is while lumens are a measure of how concentrated that beam is (when I have both numbers to compare). High candela, low relative lumens means small spot but super bright. High lumens, low relative candela means large spot but won’t reach, or throw a beam, as far.
At this point, you’ve seen first-hand how these numbers can influence each other have a rough basis to start making decisions on most lighting related purchase to ensure that you will never be caught with your pants down (or in the dark (or in the dark with your pants down)) again.
For more info on how this calculator works and the math behind it check out the technical description (coming soon).