# B Convert from lux to candela

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1. Apr 20, 2017

### mammou manel

hello
i'm working on a project that consist on reading the light of LEDs on PCB to determine the color code (R,G,B) and light intensity
but the sensor gives me values in LUX which is the illuminance and i need to convert them to Candela
I found this formula in wikipedia but i didn't understand how to apply it

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2. Apr 20, 2017

### Staff: Mentor

3. Apr 20, 2017

### mammou manel

hi, the problem is that i didnt know how to apply the formula
lx = cd / m²
is m² the square of the distance between the sensor and the The light source ?

4. Apr 20, 2017

### Staff: Mentor

Here's what the article said for meters:

where the m represents distance to the light source in meters.

5. Apr 20, 2017

### mammou manel

thats what i didn't understand!
how to determine the distance from the light source d2 in square meters.... is it the square of the distance between the sensor and the The light source in m

6. Apr 20, 2017

### sophiecentaur

That doesn't make sense so I am not surprised you are confused. It means d is in metres and that's what you enter into the formula. It is not usual to discuss the units of the internal parts of mathematical expressions. Of course, an equation must have balanced dimensions each sideband that's a good check to apply, sometimes.
"lux = candela / square meters" just means that lux is expressed in candela per square metre - that's not a formula and the article is written badly. The formula is written just above the attempt to 'explain' it.
The formula has a hidden constant (of 1) in there when the distance is in metres. If you look at the calculator on that site, it shows a constant in there when you work in feet (about 10 or about 1/10, depending which way you are going). The units here are just practical units for measuring and specifying illumination and I, too, find it a bit arbitrary and confusing.

7. Apr 20, 2017

### nasu

The article does not say what you quote (from mammou's post).
The actual quote is "divided by the square distance from the light source d2 in square meters (m2)". It is obvious that the square meters refer to the square distance. Maybe just a misreading of the sentence. A comma in the right place may help. :)

8. Apr 20, 2017

### mammou manel

so how can i convert from lux to candela? i'm really confused

9. Apr 21, 2017

### sophiecentaur

You just use the formula and forget the notion (or problem) of dividing by square metres.
Personally, I think the "square distance" should be replaced by "distance squared" and that avoids the confusion. The dimensions take care of themselves.