# Converting mcd/lux/m^2 to mcd/m^2 ?



## jmccormick (Dec 6, 2016)

I'm trying to take specifications for units listed as mcd/lux/m^2 and compare them against other numbers which are provided as mcd/m^2. Just so you know that this is a real problem, here is a source document where you can find the first unit: http://www.dot.ga.gov/PartnerSmart/Business/Source/special_provisions/shelf/sp652.pdf

So if I have a surface which is visible from 180 degrees and is rated as either 220 mcd/lux/m^2 or 0.30mcd/lux/m^2, how do I find what that number would be in a more conventional unit, like mcd/m^2?


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## Enderman (Dec 9, 2016)

Does mcd/lux/m^2 even make sense?
cd and lux are basically the exact same thing, only difference is that lux can be measured at any distance and "cd" is specifically at 1m.
If you have lux and cd and you divide them by eachother you get no units, which is meaningless...
Just like meters / meters = no units.


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## ssanasisredna (Dec 9, 2016)

jmccormick said:


> I'm trying to take specifications for units listed as mcd/lux/m^2 and compare them against other numbers which are provided as mcd/m^2. Just so you know that this is a real problem, here is a source document where you can find the first unit: http://www.dot.ga.gov/PartnerSmart/Business/Source/special_provisions/shelf/sp652.pdf
> 
> So if I have a surface which is visible from 180 degrees and is rated as either 220 mcd/lux/m^2 or 0.30mcd/lux/m^2, how do I find what that number would be in a more conventional unit, like mcd/m^2?



The first measure mcd/lux/m^2 is a measure of retroreflectivity, i.e. how much light will be reflected at a given illuminance. The second measure mcd/m^2 is a luminance value (also called nits when done as candela/m2). You cannot convert between the two as they are used to measure completely different things.

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meters / meter is not meaningless, it is unit less or in otherwise a ratio which has meaning.


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## Enderman (Dec 9, 2016)

ssanasisredna said:


> meters / meter is not meaningless, it is unit less or in otherwise a ratio which has meaning.



"meaningless" as in "it does not mean anything in the physical world"

10m is a distance

10 is... a number? a shoe size? a number of potatoes?
Get my point?


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## ssanasisredna (Dec 10, 2016)

Enderman said:


> "meaningless" as in "it does not mean anything in the physical world"
> 
> 10m is a distance
> 
> ...



I get your point it just is not a good one.

Ratios matter and they have a physical interpretation and they are dimensionless.

The ops question is a prime example.


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