Some results from a survey of 43 patients who had a monofocal intra-ocular-lens (IOL) in one eye and a concentric bifocal intra-ocular-lens in the fellow eye are reported. Twenty patients with 6/9 or better post-operative visual acuity in both eyes, participated in the main part of the study. Optical transfer functions for the bifocal lens showed that, compared to an optimal single-vision correction, there is a 50% contrast degradation of the distance retinal image across all spatial frequencies above around 3 c/deg. For the patients in the main study, there was a close correspondence between practical measurement of contrast sensitivity and the theoretical predictions of the modulation transfer functions. Measuring contrast sensitivity proved an effective means of assessing misalignment of the bifocal IOLs.
International audienceAn analytical approach has been developed to take account of the influence of the lateral walls on a stationary isothermal gas flow through a rectangular microchannel. The study concerns pressure-gradient-driven flows in channels where the length is large compared to the critical smallest dimension, namely, the channel height. The calculation of the bulk velocity is based on the Stokes equation treatment and uses the property of the Laplace operator. This novel method remains very easy to use when the second order term with respect to the Knudsen number is taken into account in the wall boundary conditions. The method is notably of high practical interest when applied to rectangular-cross-section microchannels that connect upstream and downstream high capacity reservoirs. The mass flow rates measured along such systems are fitted to first or second order polynomial forms following the mean Knudsen number of the flow. The present calculation also leads to a completely explicit second order expression for the mass flow rate. Thus, the first and second order experimental and theoretical coefficients can be identified immediately, allowing direct evaluations of physical gas-wall interaction features, in particular, the first, second order slip and the accommodation coefficients. An example of the implementation of the proposed technique is given. The slip and accommodation coefficients are extracted from the measurements of the mass flow rate through microchannels of rectangular cross- section
International audienceAn experimental setup based on the constant volume technique is developed to measure the mass flow rate through microtubes under isothermal stationary flow conditions. Four different working gases (helium, nitrogen, argon, and carbon dioxide), and two surface materials (stainless steel and Sulfinert) are considered. The Knudsen number calculated for the experimental conditions varies from $10 À4 (hydrodynamic regime) to $ 5 (transitional regime). In the reduced range ð10 À4 À 0:1Þ corresponding to the hydrodynamic and slip regimes, an approach based on the analytical solution of the Stokes equation subjected to a first order velocity slip boundary condition is used. The velocity slip coefficient and the tangential momentum accommodation coefficient are extracted from the experimental data of the mass flow rate using their analytical expressions. The results are summarized in the tables representing the accommodation coefficients for the corresponding gas-surface material combinations. The influence of the molecular mass on the tangential momentum accommodation coefficient is discussed. In addition, an original technique is proposed to extract the accommodation coefficient in the whole experimentally available range ð10 À4 À 5Þ of the gas rarefaction. The accommodation coefficients obtained using this technique are close to those found in the slip regime
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