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Multiphase flows involving liquid droplets in association with gas flow occur in many industrial and scientific applications.Recent work has demonstrated the feasibility of using optical techniques based on laser extinction to simultaneously measure vapor concentration and temperature and droplet size and loading.This work introduces the theoretical background for the optimal design of such laser extinction techniques,termed WMLE (wavelength-multiplexed laser extinction).This paper focuses on the development of WMLE and presents a systematic methodology to guide the selection of suitable wavelengths and optimize the performance of WMLE for specific applications.WMLE utilizing wavelengths from 0.5to 10 μm is illustrated for droplet size and vapor concentration measurements in an example of water spray,and is found to enable unique and sensitive Sauter mean diameter measurement in the range of ～1-15μm along with accurate vapor detection.A vapor detection strategy based on differential absorption is developed to extend accurate measurement to a significantly wider range of droplet loading and vapor concentration as compared to strategies based on direct fixed-wavelength absorption.Expected performance of the sensor is modeled for an evaporating spray.This work is expected to lay the groundwork for implementing optical sensors based on WMLE in a variety of research and industrial applications involving multi-phase flows.