Understanding Candela2019-03-06T15:22:08-05:00

Understanding Candela

by David Barnett, P.E.

While considered by consumers much less frequently than Lumen ratings, Candela output is an important consideration for selecting the best tactical flashlight.  Although Lumens are often mistakenly believed to a measure of “brightness”, the Candela is actually the unit of measure of brightness (luminous intensity).  (For more information on Lumens, please check out this earlier blog.)

Lumens and Candela are somewhat analogous to Current and Voltage and should never be confused.  Lumens measure the total amount of light output while Candela measures the intensity or brightness of the light at any given point.  Since intense light will travel farther than weaker intensity light, Candela output can be used to estimate the distance at which a light source can provide illumination.  The intensity, or brightness, of light is known to diminish according to the Inverse Square Law.   Simply put, this means that if you shine a flashlight on an object 30 meters away and then shine it on an object 10 meters away (1/3 the distance), the nearer object will be nine times (9X) brighter.

The FL1 Standard for rating flashlights uses this Inverse Square Law, along with measured peak Candela output to calculate a hypothetical beam distance specification.  Because the Standard’s mandated calculation is based on a very dim object intensity of only 0.25 lux (roughly the illumination provided by a full moon), users of tactical flashlights should consider the useful beam distance to be approximately half of the official rating (i.e. if a flashlight has a rated beam distance of 100 meters, it may be suitable for tactical applications within 50 meters or so).  The actual equation for converting Candela to FL1-rated beam distance is:

d = sqrt [4*Candela]

Candela measurements are taken at a particular point in space and specify brightness only at that point.  The FL1 Standard considers only the brightest point of a flashlights beam.  While this specification may be a bright spot (pun intended) for manufacturers wishing to manipulate ratings for marketing purposes, it is a serious detriment to consumers shopping for the best flashlight for their particular application.  A flashlight with a high Candela rating may have a bright spot in its beam that is extremely narrow.  Such a light will project light a long distance but only cover a very limited area.  This type light is of limited value for tactical applications where the need for situational awareness and large search areas demand much broader illumination.  Additionally, when used for close-range work, such a light will prove highly distracting and harsh as the small intense “hot spot” causes glare and splash-back reflections from illuminated objects.  (Anyone who has tried to use a light like this to read will understand how counter-productive a concentrated light source can be.)

For tactical applications, desired lighting conditions are achieved with a flashlight providing the largest area of uniform illumination possible at the maximum mission range.  While FL1 ratings do not provide adequate information to identify a flashlight meeting these criteria, the ratings can be used to disqualify certain models.  Here is a good way to begin your search; first define the maximum range for your mission.  For example, if using a flashlight for home defense, the maximum range would be the longest line-of-sight corridor in your home, say 20 meters.  As mentioned above, the useful beam distance is approximately half of the official rating so look for a flashlight with a rated beam distance of around 40 meters (more is NOT better).   Eliminate from consideration models with a beam distance rating significantly higher than this as the tight concentration of light will work against the goal of uniform illumination and hinder situational awareness and threat identification.

While everyone wants to be seen as the “best and brightest”, when it comes to finding the right flashlight for your mission, the best is likely not be the brightest.