# lux, candela - proper terminology



## Dr.Jones (Jul 2, 2011)

This is a plea for using the proper units (and physical quantities) to describe the performance of a flashlight.

Often one can read about "lux" or "[email protected]" to describe 'spot brightness' or throw.

*lux* is the unit of *illuminance E*, that is the luminous flux F (lumens) per area A falling onto a surface. It's more a unit for a whole lighting setup onto a target than for a light source itself. 

With a single light source like a flashlight, the illuminance E drops with increasing distance d, described by the inverse square law: E is proportional to 1/d² (actually that's an approximation which gets more accurate with increasing distance). If one now multiplies that illuminance E by the squared distance, E*d² gives a constant value, which is independent of the distance and only depends on the flashlight itself. This is another physical quantity, the luminous intensity I, or, measured at the spot, the spot *luminous intensity I*, or peak luminous intensity I. It's unit is *candela* (*cd*; candlepower (cp) is an obsolete unit equivalent to candela). 
I = E * d²

Of course, one lux in 1m distance gives one candela, which is why "10000 [email protected]" is somewhat the same as 10000 cd (or better 10 kcd), however the latter is the proper unit. ANSI/NEMA FL1 uses it, too.

Most of you already know that measuring the spot intensity in only 1m distance is not a good idea, as the beam often has not settled to it's 'final beam pattern' in that short distance. Even ANSI/NEMA FL1 demands a minimum distance of 2m; for throwers that is still much too near (I suggest a distance where the spot size is at least 10 times as big as the reflector/lens diameter). Doing so and calculating that back to "[email protected]" sometimes leads to strange units like "31736 lux @1m (calculated)" - just write 31.7 kcd instead.

I just would like to suggest 
* not using lux when you mean candela
* using the "k" for big numbers
* not denoting more than three significant digits; typical lux-meters have an accuracy of 5%, a number like 31736 seems to imply a dubious precision of less than 0.1%

While being at it, *throw* is usually defined as the distance at which the illuminance on a (perpendicular) target falls below a certain level; if you follow ANSI/NEMA, that level is 0.25 lx. Throw can be calculated from the spot intensity using the above formula rearranged to 
d = sqrt ( I / E0 )
with E0 = 0.25 lx for NEMA throw. (Some consider that value of 0.25 lx quite low, resulting in relatively high throw values; practical throw values might be lower, depending on the situation.)

You might find this a bit nitpicking; but I think proper terminology is a good thing, it was developed for a reason.


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## mvyrmnd (Jul 2, 2011)

Nicely written. This deserves to be a sticky.


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## rmteo (Jul 12, 2011)

Dr.Jones said:


> ....I think proper terminology is a good thing.



It is the *ONLY* thing - it is what engineering is about. :devil:


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## bbawkon (Jul 12, 2011)

While we're speaking of units...

Amps (A)
MilliAmps (mA)
Volts (V)
MilliVolts (mV)
AmpHours (Ah)
MilliAmpHours (mAh)
WattHours (Wh)
MilliWattHours (mWh)

Volts * Amps = Watts
Volts * Amps * Hours = WattHours
Amps * Hours = AmpHours


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## Norm (Jul 12, 2011)

Moved to General Light Discussion - Norm


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## ringzero (Jul 15, 2011)

Thanks Dr. Jones, for elucidating proper use of the units.

Improper usage of units - and just plain ignorance of basic optics - seems to have gotten worse on CPF in the last several years.

Regarding Lux versus Candela: the distinction is subtle to an average layman, who may not be familiar with Solid Angles and Steradians. I'm happy if CPF posters use either unit to specify Brightness or Peak Beam Intensity.

If CPF members could learn to consistently associate the concepts of Brightness, Throw, Intensity with Lux and/or Candela, then I'd be a lot happier. It gets old trying to explain the same basic concepts over and over.

ANSI output and brightness numbers, in Lumens and Candelas, are great as they are.

ANSI runtime numbers are time to 10 percent of initial output. This is better than nothing but 50 percent would probably more useful. At least the 10 percent runtime numbers are consistent across brands, which is an improvement. 

Regarding ANSI throw numbers: I read them then just divide by two which gets me back to a Throw number for 1 lumen/meter-squared. I think this is a more realistic minimal usable level of illumination for real world use.

All in all, those ANSI specs are a huge step forward for the portable lighting industry IMO.

Thanks again.

.


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## scs (Mar 3, 2015)

I was on FoxFury's website and came across one of their blog entries comparing the lux of their BT lights to Streamlight lights.

http://www.foxfury.com/blog/?p=1036

Their lux measurements don't seem to add up. If I'm not mistaken, Lux (@ distance "D") * D^2 = constant (the cd rating of the light). FF's readings don't yield a constant, but increases with distance.

Where's my mistake?

Thanks.


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## Dr.Jones (Mar 15, 2015)

I don't see one on your side...
Some of those values are fine: twice the distance, a fourth of the illuminance (with some % error), but with some of those lights the values seem odd. However the resolution is quite bad in those measurements, so at the longer distances there are quite some bigger errors to be expected; and at lower distances the beam might not have assumed it's 'final' beam pattern, so those may easily be off, too.


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