A method for converting single mode Gaussian beams into beams with uniform irradiance profiles is described. This technique has application to laser cutting and welding, laser ablation, semiconductor mask fabrication, and other tasks. Currently, designs for rectangular and circular flat top profiles have been investigated. Experimental results are presented for an element that converts a single mode Gaussian beam into a square, flat top spot. The design is based on a Fourier transform relation between the input and output beam functions and can be implemented as a diffractive or refractive element. The form of the element reduces to a common equation that is scaled for the particular geometry involved. This scale factor contains the product of the widths of the input and output beams, the focal length of the system, and the wavelength. It is a dimensionless quantity that uniquely determines the quality of the target spot, regardless of wavelength or system geometry. A designer can thus start from a desired target quality and lay out the required optical system to achieve that quality, in contrast to an iterative approach.