Kramers' law is a formula for the spectral distribution of X-rays produced by an electron hitting a solid target. The formula concerns only bremsstrahlung radiation, not the element specific characteristic radiation. It is named after its discoverer, the Dutch physicist Hendrik Anthony Kramers.
The formula for Kramers' law is usually given as the distribution of intensity (photon count)  against the wavelength  of the emitted radiation:
The constant K is proportional to the atomic number of the target element, and  is the minimum wavelength given by the Duane–Hunt law. The maximum intensity is  at . 
The intensity described above is a particle flux and not an energy flux as can be seen by the fact that the integral over values from  to  is infinite. However, the
integral of the energy flux is finite.
To obtain a simple expression for the energy flux, first change variables from  (the wavelength) to  (the angular frequency) using  and also using . Now  is that quantity which is integrated over  from 0 to  to get the total number (still infinite) of photons, where :
The energy flux, which we will call  (but which may also be referred to as the "intensity" in conflict with the above name of ) is obtained by multiplying the above  by the energy :
for 
for .
It is a linear function that is zero at the maximum energy .