Using reflection interference contrast microscopy, we studied the thermal fluctuations of giant vesicles that weakly adhere to flat solid substrates. The absolute membrane-substrate separation distance was imaged and the average contact contour, including the contact area, the contact rounding, and the asymptotic contact angle, was determined. The static fluctuations in the flat, adhering part of the vesicle were analyzed. The spectrum of mean square amplitudes yielded the lateral membrane tension and the second derivative of the interaction potential. The vertical roughness and lateral correlation length were measured from the spatial autocorrelation of the undulations. The roughness was shown to obey the behavior predicted by functional renormalization in the observed tension regime of

10^{- 6}

10^{- 4}

m^{2}

. Moreover, the measured separation distances can be explained within the framework of undulation and van der Waals forces and confirmed the model of tension-induced adhesion. However, the adhesion energies as well as the measured separation distances exhibit a weaker dependence on the membrane tension than predicted.

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