We have identified seven (field) elliptical galaxies acting as strong gravitational lenses and have used them to measure cosmological parameters. To find the most likely value for Omega_m (= Omega_matter) and Lambda, we have used the combined probabilities of these lens systems having the observed critical radii (or image deflection) for the measured or estimated values of lens redshifts, source redshifts, and lens magnitudes. Our measurement gives Lambda=0.64 (+0.15 -0.26) if Omega_m + Lambda = 1, and the Omega_m = 1 model is excluded at the 97 % confidence level. We also find, at the 68 % (Omega = 0) -- 82 % (Omega = 0.3) confidence level, that an open universe is less likely than a flat universe with non-zero Lambda. Except for the possibility of strong perturbations due to cluster potentials and the systematic overestimate of the lens magnitudes, other possible systematic errors do not seem to influence our results strongly: correction of possible systematic errors seems to increase the significance of the result in favor of a non-zero Lambda model.
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