  Measurements of the scalar dissipation rate are performed in an impulsively
started gas jet, using planar laser induced fluorescence. The measurements are
well resolved spatially. The deteriorating effect of experimental noise on this
experiment is treated with a Wiener filter, which is shown to be applicable to
this large-scale inhomogeneous flow. The accuracy of the scalar dissipation
rate is within $20\%$, as determined from an explicit calculation of the
filtering errors. The residual fields that remain after the filtering are
analysed in detail and their statistical properties show that these resemble
white noise to a good approximation. The level of corrections is minimal for
the scalar field but it is of the order of $40\%$ for the scalar dissipation
rate. An examination of the filtering operation using modeled spectra and the
measured spatial resolution shows that the Wiener filter produces errors in the
estimate of the scalar dissipation rate $\sim30\%$, for Taylor-scale Reynolds
number up to 1000. The implications of this modelling are discussed with
respect to common experimental situations and point out the relative merits of
improving the spatial resolution as compared to improvements in the signal to
noise ratio.
