Ani Sobral Torres¹, Eduardo Landulfo¹, Demetrius Venable², David N. Whiteman³

¹Institudo de Pesquisas Energéticas e Nucleares, Universidade de São Paulo
²Howard University
³NASA - Goddard Space Flight Center

In this work we show the results of an evaluation of a Water Vapor Raman Lidar system total efficiency for the determination of a calibration factor within the context of an independent Lidar calibration methodology . This calibration factor is usually obtained by the measurement of the mixing ratio profile with an accompanying radiosonde or other sensor which makes the lidar accuracy dependent on the measurement of a second instrument. An independent calibration of the system is possible by measuring or calculating the relevant system parameters based on the decomposition of the instrumental function such as the optics transmission and detection efficiency. The latter could be determined by using diffuse daylight, however changes in the atmosphere profile and optical properties, can lead to more systematic uncertainties. Bearing that in mind we used in this study a calibrated light source instead. The lidar system transmission efficiency E(lambda) at the laser and raman produced wavelengths, includes factors such as reflectivity of the telescope, the transmission of conditioning optics, the transmission of any filters, and the quantum efficiency of the detectors. In addition the efficiency ratio E(lambdaN)/E(lambdaH) was determined with accuracy through the use of a quasi-blackbody calibration lamp. We have evaluated the system efficiency with a calibrated tungsten lamp with two sets of interference filters (wide band and narrow band) installed at the Howard University Raman Lidar facility.

We present results and analysis of the transmission of these interference filters, the fitting of the lamp spectral irradiance with a 4th order polynomial function, and the number of counts measured by the Lidar system in a mapping of the telescope area, involved on the determination of the optical transmission efficiency, taking into account the error propagation of the parameters involved.