[quote author=Fu-Kwun Hwang link=topic=427.msg3829#msg3829 date=1237373633]
The above equation is a normalized function, i.e., integration will give you 1.
So you can multiple it by any number.
[/quote]
ok i understand normalized function http://mathworld.wolfram.com/NormalDistribution.html
ok i also understand the area under this normalized function is 1

but i am  puzzled by the missing r*r.

equation on http://en.wikipedia.org/wiki/Planck%27s_law [img]http://upload.wikimedia.org/math/1/0/c/10c96a8f2af7d9631a0f4dde5b817b62.png[/img]

equation in your XML : fT = "h4/((Math.exp(cst/(r*T))-1)*(r*r*r))"

notice your XML : fT has only r*r*r but the equation on wikipedia has r*r*r*r*r

unless when normalized the r*r is absorbed? then it make sense to me now! I don't really know why now but at least i can accept the logic. (it that correct ?)......hahaha.

[quote author=Fu-Kwun Hwang link=topic=427.msg3829#msg3829 date=1237373633]
If you change the y-axis to log-scale, you will know how big/small it is.
[/quote]
log scale i understand. i try to experiment more :)

Maybe i lack the prior knowledge, sorry! ;D
i don't remember encountering this equation [img]http://upload.wikimedia.org/math/1/0/c/10c96a8f2af7d9631a0f4dde5b817b62.png[/img]for black body radiation, only vaguely remember
Stefan–Boltzmann law Main article: Stefan–Boltzmann law

This law states that amount of thermal radiations emitted per second per unit area of the surface of a black body is directly proportional to the fourth power of its absolute temperature. The total energy radiated per unit area per unit time j^{\star} (in watts per square meter) by a black body is related to its temperature T (in kelvins) and the Stefan–Boltzmann constant ? as follows:

[img]http://upload.wikimedia.org/math/6/5/b/65b752b1d64dee1b8dc63df6b6228c24.png[/img] where sigma=5.67 x 10-8Wm-2K-4
http://en.wikipedia.org/wiki/Black_body