  When measuring diameters of partially resolved sources often a technique
called gaussian deconvolution is used. This technique yields a gaussian
diameter which subsequently has to be multiplied with a conversion factor to
obtain the true angular diameter of the source. This conversion factor is a
function of the FWHM of the beam or point spread function and also depends on
the intrinsic surface brightness distribution of the source.
  In this paper conversion factors are presented for a number of simple
geometries: a circular constant surface brightness disk and a spherical
constant emissivity shell, using a range of values for the inner radius. Also
more realistic geometries are studied, based on a spherically symmetric
photo-ionization model of a planetary nebula. This enables a study of optical
depth effects, a comparison between images in various emission lines and the
use of power law density distributions. It is found that the conversion factor
depends quite critically on the intrinsic surface brightness distribution,
which is usually unknown. The uncertainty is particularly large if extended
regions of low surface brightness are present in the nebula. In such cases the
use of gaussian or second moment deconvolution is not recommended.
  As an alternative, a new algorithm is presented which allows the
determination of the intrinsic FWHM of the source using only the observed
surface brightness distribution and the FWHM of the beam. Tests show that this
implicit deconvolution method works well in realistic conditions, even when the
signal-to-noise is low, provided that the beam size is less than roughly 2/3 of
the observed FWHM and the beam profile can be approximated by a gaussian.
