tl;dr: Multiply the candela by 4, then square root it to get the throw in meters. Or square the throw in meters then divide by 4 to get the candela.
Flashlight specs sometimes mention their throw in meters (m) or their intensity in candela (cd or kcd for thousand candela). These measurements are both about how bright a flashlight hotspot is, rather than the total brightness, which is measured in lumens.
- Lumens (lm) is how much light there is in total. More lumens means a brighter light and generally takes more power to produce.
- Lux is how much light there is in a square meter. You can think of Lux as how intense or concentrated the lumens are. If you have 100lm, you can make them more intense by pointing them all in the same direction with reflectors or lenses. Going further away from a light source makes it less intense and reduces the lux, so lux measurements by themselves aren’t very useful for comparing lights.
- Candela (cd) is how intense a light us at 1 meter. 1 lux at 1m is 1cd. At twice the distance, the area the light hits is 4 times the size, so you’d need a 4cd light to produce 1 lux at 2 meters.
- Throw (m) is an ANSI measurement that relates to the distance that a light generates 0.25 lux.
Types of flashlights
When buying a flashlight you generally want a certain amount of lumens (total light) and throw (for seeing things at a distance). With a headlamp or flashlight for use inside, throw over 300m isn’t especially useful unless you want to point at things. If you have a light with lots of throw indoors then you’ll only be able to see one small area at a time.
Mid range lights with 200m to 400m are useful indoors and out.
More “throwy” lights are useful outdoors, where 500m or more throw can come in handy for seeing things far away. Mega throwers can throw over 1000m (1km or 2/3 of a mile).
Understanding ANSI throw
The 0.25 lux when measuring throw is about the same brightness as a full moon.
0.25 lux from the moon is enough to hike fairly easily.
(Note: I was wrong with my explanation below. Thanks to Streamtronics for correcting)
because the light from the ground around you doesn’t have to go far to your eyes. But 0.25 lux on something that’s far away won’t be so bright. This means that a light with 100m ANSI throw won’t allow you to see 100m away clearly. If you shine the light at a wall 100m away then the light will be bright enough for someone standing next to the wall to see but for you to see the wall, the light has to bounce back another 100m to your eyes. A rough guide is that you can half the ANSI throw to get usable throw.
Converting between ANSI throw and candela
You can convert between throw (m) and candela (cd) quite easily. Multiply the candela by 4, then square root it to get the throw in meters. Or square the throw in meters then divide by 4 to get the candela. Watch out for the “k” in “kcd”, which means 1000.
As Streamtronics pointed out in the comments, technically you divide the candela by 0.25 (as throw is measured at 0.25 lux). Dividing by 0.25 is the same as multiplying by 4.
Here’s some examples from candela to meters:
- 500cd = 45m
- 1kcd = 63m
- 5kcd =141m
- 10kcd = 200m
- 20kcd = 282m
- 50kcd = 447m
- 100kcd = 632m
- 200kcd =894m
- 500kcd = 1,414m
And some examples from meters to candela:
- 50m = 625cd
- 100m = 2.5kcd
- 150m = 6kcd
- 200m = 10kcd
- 300m = 23kcd
- 500m = 63kcd
- 750m = 141kcd
- 1000m = 250kcd
- 2000m = 1000kcd (or 1Mcd)