Printing apparatus for radiation thermal transfer of colorant from a donor to a receiver, including a flash tube for emitting high intensity radiation; a polarizer for receiving high intensity radiation from the flash tube and polarizing such radiation; and liquid crystal cells disposed to receive polarized radiation from the polarizer. Electrodes modulate the liquid crystal cells so that they change the polarization of the radiation passing through them. A second polarizer receives radiation from the liquid crystal cells and is arranged to pass different intensities of radiation depending on their polarization. The colorant donor and the receiver are positioned in colorant transfer relationship with the second polarizer at a colorant transfer position so that radiation which passes through the second polarizer illuminates the colorant donor so that colorant is transferred to the receiver.