Differing formulae for power emission

[PatF]

I have run across two formulas for Planck's Law of Radiated Power Density.

According to http://hyperphysics.phy-astr.gsu.edu/hbase/bbrc.html#c1" it is

Bf(T)=((2*pi*h*c^2)/(lambda^5))(1/(e^(h*c/lambda*k*T)-1))

However, in one of your forums, the pi is missing as it is here.

http://en.wikipedia.org/wiki/Planck's_law_of_black_body_radiation

What happened to pi?

Just curious.

 

[tiny-tim]

pi? i ate it!

What happened to pi?

Hi PatF!

(have a pi: π )

In wikipedia, the units are "spectral radiance", which involves watts per square metre per unit solid angle

but in hyperphysics it's watts per cubic metre.