We investigate in this paper the wireless interrogation of chipless and passive mechanical sensors using a range-Doppler imaging algorithm. As an example, a Crookes radiometer illuminated by a light of controlled illuminance is interrogated by using a 24GHz FM-CW radar. A remote detection algorithm is proposed to estimate the reading range and Doppler harmonics embedded in the electromagnetic signal backscattered by the radiometer. From these harmonics the accurate estimation of illuminance is achieved for long reading ranges of at least 3.5m in a cluttered environment.