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Overview of Light units - Convert from/to Lux, Lumens, Nits, Candela

I knocked up this page as a quick reference for the SI base light units - Lux, Lumen, Nit, and Candela. Instead of long explanations, everything is summed up in a single table and diagram. Learn 10 times faster! If you like, send us a quick email if you found it useful or think it could be improved somehow.

Light typesVariables to take
into account
DescriptionSI unitSymbol Quantity Light source
Intensity
Light source
size
Light source
distance
from object
Size of
object
area
Time
span
a: Intensity of light irrespective of the distance.Candela/m2, Nit,
Lambert, Stilb
(cd/m2) Luminance,
Brightness
matters - - - -
b: Amount of light given off by a light source.Candela,
Candlepower
(cd) Luminous intensity matters matters - - -
c: Amount of light at a distance.Lumen/m2, Lux,
Footcandle
(lm/m2, lx, fc) Illuminance,
Illumination
matters matters matters - -
d: Amount of light at a distance on an area.Lumen (lm) Luminous flux|power matters matters matters matters -
e: As with d, but over time of 1 second.Talbot (T) Luminous energy matters matters mattersmatters matters

So for c, we say it matters how big and bright the size of the light source is, and it matters how
far away the object is from the light source, but the size of the object area isn't a consideration at all.



Click me for bigger size.
In other words...

  • You need to know the light source's intensity before you can find out the nits.
  • You need to know the light source's intensity and size before you can find out the candela.
  • You need to know the light source's intensity and size, and also the object/observer's distance from the light source, before you can find out the lux.
  • You need to know the light source intensity and size, and also the object/observer's distance from the light source, and the observer's size before you can find out the lumen.

    FAQ:

    Q: Why do some sites and sources use lumens to measure a light's overall power instead of candela?
    The obvious unit type to use for lamps, torches and suchlike is candela or candlepower, so why do some sites insist on using lumens? The answer to that is they shouldn't really, but they assume that the area that is lit by the light is completely surrounding the light. So technically, it's still a legitimate, if somewhat unnecessary unit to replace candela/candlepower. With this 360 degree spotlight assumption for lumens, you can convert from candela to lumens by using: 1 candela = 12.566 lumens (4*PI). However, be careful as Google suggests that there is only 1 lumen to 1 candela! This is because this time, they're assuming that the surface to be lit is 1 metre away, and is a 1 metre square. Of course, if we take surface distance and area into account, you'll find that lumens (and lux incidentally) can increase/decrease even as the candela remains constant! This is one more good reason everyone who wants to measure a light source's overall brightness, should be using candela instead of lumens, to remove this confusion once and for all.

    Q: How should I talk about lux and lumens?
    Nits and candela only need one thing (the light source), but whenever you measure lux or lumens, there needs to be two things - a light source, and an 'observer'. These are either implied or explicity specified. Often, both are just implied, and in which case, they can then usually be determined by the context of the wording. In some cases though, they should both be explicitly specified, as confusion can otherwise set in.
    As an example, I could say the lux of the office I work in is high. However, does this mean the bulb's light against the room, or the room's light against the person's eyes? After all, the room's light can be classed as a 'light source' just as much as the light bulb itself.

    Q: I want a bright laptop screen so I can work in the sun!
    Then simply find out the nits of the screen. As an extreme example, a 1 kilometre wide laptop screen could give out tons of candela, but still be a relatively dull screen to look at, thanks to the low light intensity at any single point on the screen.

    Q: How should I measure the brightness of the moon?
    Do you mean the intensity, or how much it lights up Earth? Use nits in the former, or lux in the latter (the moon provides 1 lux by the way).

    Q: I want to measure the brightness of a really cool flashlight I saw on eBay the other day.
    Use candela/candlepower (though you way want to check the angle of beam too!)

    Q: I want to measure the brightness of my office, so my employees can work to a comfortable degree.
    Well, for a rough measurement, use lux, by finding the light bulb's size+brightness, and its average distance from the room's surfaces (a massive office will have less lux than a smaller one, given the same light bulb). Alternatively, for more accuracy, you could find out directly the total amount of lux coming from all surfaces in the sight range of a person.

    Q: I want to measure how much overall light is being received by my solar panel to see how efficient it is.
    There's rarely a time to use lumen, but this is one such occasion.

    Q: That star in the night sky - it's so bright! Why?
    Because it has a high number of nits (incidentally the candela is incredibly high, because the star is so large).

    Q: .....But despite that, the night is still so dark. Why?
    The lux back on Earth will be miniscule from even all stars (0.00005 lux), because they are so far away.

    Q: I want to measure the light intensity of a laser, to see how dangerous it is to the human eye.
    You better measure in nits. Use candela instead if you want a few lawsuits.

    Q: I write on paper outside at night, because it's too hot indoors. I want to get a standard light bulb powerful enough, but cheap enough, so that I can see what I write.
    Well, you want a certain amount of nits coming from the paper - just enough to make the black ink comfortably distinguishable from the white paper. In the situation as a whole, and assuming spherical output from the light bulb, you'll want to know the lux (combines info about light bulb's overall brightness and the distance you'll work from it).

    Q: I work for a big company that wants to light up niagara falls for the public to see. What am I looking for?
    Candela, lots and lots of candela (also known as candlepower). You can concentrate the light to a smaller angle, so in theory, the light can be any distance from the falls and still maintain the same illuminance.

    Q: .....No, you misunderstand, the lights are inside the water, not lighting it up. I want the falls to appear at a certain brightness, say... twice as bright as the moon.
    In that case, measure the light/s nits then.



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    Offsite links

  • [light] Measurement Geometries - Excellent guide, also giving information on a 'steradian'.
  • Wikipedia information on Candela and Lumen.
  • The Unit of Luminous Intensity: Candela - Take a look at the bottom of the page for a massive amount of every-day light sources.





    All pictures and text on this page are copyright 2005 onwards Daniel White.
    If you wish to duplicate any of the information from this page, please contact me for permission.