Photon Unit Calculator

Calculation Theory

Photon Energy and Wavelength

The energy of light (or photon energy), E, is inversely proportional to the wavelength by the equation:

E = hc/l     (equation 1)

where h is Planck's constant (6.6261 x 10^-34 J*s) and c is the speed of light (2.9979 x 10⁸ m/s).  

Photon energy can be expressed using any unit of energy such as the electronvolt (eV) or the Joule (J). For short wavelength sources in the EUV to X-ray range, researchers often discuss photon energies in units of eV (or keV for hard X-rays) out of convenience.  The International System of Quantities (SI) have defined 1 eV to the exact value of 1.602176634×10⁻¹⁹ J.  Thus, equation 1 above can be rewritten for photon energy in eV and wavelength in nm as:

E (eV) = 1239.8 / l (nm)     (equation 2) 

Flux and Power

Power (P) is the rate of energy transfer with the SI unit of 1 Watt = 1 J/s.  The power of short wavelength light sources in the EUV to X-ray range are often expressed as photon flux (F) in the units of photons per second (ph/s).  Average power is related to flux and photon energy by:

 P = F × E     (equation 3)

 P = F × hc/l     (equation 4)

P (mW) = 1.989 × 10^-10 × F/l (nm)     (equation 5)

For any given flux and wavelength (in nm), equation 5 can be used to calculate the corresponding average power (in mW).