We study the possibility of introducing programmable filters in the Fourier plane of an optical free-space wavelength demultiplexer to provide wavelength-band selection or programmable optical add and drop multiplexing. Various 4f configurations are modeled, and we com- pare the respective advantage of two main cases: the first where the dispersive element is superimposed in the Fourier plane with a program- mable transmittance, the second where the dispersive element is com- bined with the lens(es) and the Fourier plane is free to set the program- mable element. An analytical model of the beam propagation is proposed in the case of a Gaussian beam to make easier the choice of adjustable parameters and optical configurations. The potential use of the Fourier plane free configurations is assessed in terms of wavelength-band reso- lution, selectivity, and attenuation dynamic range. © 2004 Society of Photo-