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According to the table at the bottom of the Wikipedia page for the candela, the dimension for candelas is J (joules). Why is this not W (watts)?

The luminous intensity for light of a particular wavelength λ is given by

$I_v(\lambda) = 683.002·\bar{y}(\lambda)·I_e(\lambda)$

where $I_v(λ)$ is the luminous intensity in candelas, $I_e(λ)$ is the radiant intensity in W/sr and is the standard luminosity function.

Since $\bar{y}(\lambda)$ seems to be unitless and between 0 and 1, and $I_e(\lambda)$ is expressed in W/sr, why is $I_v$ not also W/sr (or J/S since sr is dimensionless)?

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In the Wikipedia page,there is written(in Luminous energy) a reason why it is used.(see point 3) –  Curious Nov 30 '12 at 5:12
If I understand the Wikipedia page correctly the symbol J doesn't stand for joules but rather for joules per second per unit solid angle. This seems daft to me :-) –  John Rennie Nov 30 '12 at 11:22
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up vote 2 down vote accepted

I believe (as per John Rennie's answer) that $\mathrm{J}$ in this case does not stand for Joules.

The Wikipedia page for the International System of Units (SI) makes a distinction between "unit sybols" (the familiar $\mathrm{m}$ for metres, $\mathrm{J}$ for joules, etc.) and "dimension symbols". The difference is that there are dimension symbols only for the SI base units (metre, kilogram, second, Ampere, Kelvin, mole and candela) and not for any of the derived units like Newtons and Joules. Confusingly, the dimension symbol for candela is $\mathrm{J}$, even though this is also the unit symbol for Joules.

The symbols you mention on the Candela page are dimension symbols rather than unit symbols, so in this case the $\mathrm{J}$ stands for candelas rather than Joules.

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