# Using "h" or "hbar" in the calculation for generation rate

#1

Hi Ahsan,

According to the whitepaper and In this link: https://kb.lumerical.com/en/solar_cells_methodology.html

You mentioned that the denominator for g is hbar instead of h, this would mean that you are off by a factor of pi.
I saw that this is also the case in the script of solar charge generation rate of FDTD Lumerical.
Thanks.

generation rate
#2

Hi Gede,

The value of “g” is found by dividing the absorbed power at each frequency (wavelength) with the power of a photon at that frequency. This gives us the number of photons absorbed at that frequency. Now the power of a photon at a certain frequency is given by,

E_ph = h * f

This can be re-written as

E_ph = h / (2pi) * (2pi) * f = hbar * (2pif) = hbar * w

This is how we ended up with an “hbar” rather than an “h” in the denominator because we are using the angular frequency (w) rather than frequency (f).