A method of determining the efficacy of a radiation assembly, the method comprising: measuring irradiance data from a radiation assembly focused at a target wavelength; multiplying irradiance values at a selected range of wavelengths between 280 nm and 320 nm with efficacy values of a vitamin D3 phototherapy action spectrum at the corresponding wavelengths to determine a weighted irradiance value at each wavelength, wherein the phototherapy action spectrum defines a wavelength having maximum vitamin D production per minimal erythemal dose at a predetermined energy level; summing the weighted irradiance values to determine a total weighted irradiance value; and dividing the total weighted irradiance value by a total of the irradiance values at the selected range of wavelengths to determine the efficiency of the radiation assembly; and characterised by forming the vitamin D3 phototherapy action spectrum at the predetermined energy level, wherein forming the vitamin D3 phototherapy action spectrum comprises: determining a percentage of photoproduct conversion for the predetermined energy level across a spectrum of wavelengths; and multiplying the photoproduct conversion at a plurality of wavelengths with a ratio of CIE previtamin D3 production to CIE erythema action spectrum at the corresponding wavelengths, wherein the vitamin D3 phototherapy action spectrum for the predetermined energy level corresponds to a curve associated with the multiplied values at each wavelength. measuring irradiance data from a radiation assembly focused at a target wavelength; multiplying irradiance values at a selected range of wavelengths between 280 nm and 320 nm with efficacy values of a vitamin D3 phototherapy action spectrum at the corresponding wavelengths to determine a weighted irradiance value at each wavelength, wherein the phototherapy action spectrum defines a wavelength having maximum vitamin D production per minimal erythemal dose at a predetermined energy level; summing the weighted irradiance values to determine a total weighted irradiance value; and dividing the total weighted irradiance value by a total of the irradiance values at the selected range of wavelengths to determine the efficiency of the radiation assembly; and characterised by forming the vitamin D3 phototherapy action spectrum at the predetermined energy level, wherein forming the vitamin D3 phototherapy action spectrum comprises: determining a percentage of photoproduct conversion for the predetermined energy level across a spectrum of wavelengths; and multiplying the photoproduct conversion at a plurality of wavelengths with a ratio of CIE previtamin D3 production to CIE erythema action spectrum at the corresponding wavelengths, wherein the vitamin D3 phototherapy action spectrum for the predetermined energy level corresponds to a curve associated with the multiplied values at each wavelength. determining a percentage of photoproduct conversion for the predetermined energy level across a spectrum of wavelengths; and multiplying the photoproduct conversion at a plurality of wavelengths with a ratio of CIE previtamin D3 production to CIE erythema action spectrum at the corresponding wavelengths, wherein the vitamin D3 phototherapy action spectrum for the predetermined energy level corresponds to a curve associated with the multiplied values at each wavelength.