Reflectance characteristics of selected materials for reference targets for 10.6-microm laser radars

Backscattering properties of topographic targets in the visible, shortwave infrared, and mid-infrared spectral ranges for hard-target lidars

Hard-target lidars rely on the reflectivity and backscattering properties of topographic targets, which are rather difficult to evaluate, resulting in uncertainties when assessing the performance of the instrument. In this work, backscattering properties and hemispherical reflectance of topographic targets are measured in the visible, near-infrared, and mid-infrared spectral ranges. A laboratory setup mimicking a hard-target lidar is used to measure the backscattered signals at various angles of incidence, which are then fitted using a bidirectional reflectance distribution function Phong model. We show that these results are useful for optimizing active stand-off detection and hard-target lidars and for increasing their overall efficiency.

Development of a pulsed backscatter-absorption gas-imaging system and its application to the visualization of natural gas leaks

The design and evaluation of a backscatter-absorption gas-imaging sensor that operates in a pulsed mode is described. It is capable of video visualization of natural gas leaks. Its development was motivated by the need for a methane imaging system to operate at ranges and sensitivities useful to the natural gas industry. The imager employs pulsed laser illumination at a repetition rate of 30 Hz and an average power of ~150 mW to image gas at standoff ranges of as long as 100 m, using a backscatter target with a reflectivity of 0.016 sr(-1). This is a tenfold improvement over an earlier raster-scanned imager. Natural gas leaks as small as 1.6 x 10(-4) standard liters/s [equal to 0.02 standard cubic feet per hour (scfh)] were imaged at short ranges; leaks as low as 7.9 x 10(-4) standard liters/s (0.1 scfh) were observed at long ranges. Data are compared with model predictions, and potential extensions to a fieldable prototype are discussed. The optimization of a direct-injection focal-plane array for detecting short (nanosecond) laser pulses is described.

Lidar effective multiple-scattering coefficients in cirrus clouds

We delimit a regime, valid for most ground-based lidar probings of cirrus clouds, in which the field-of-view dependence of multiple scattering reaches a plateau. In this regime and assuming the phase function to be constant around pi, we formally demonstrate Platt's modification of the single-scattering lidar equation, with a parameter eta(P) accounting for the reduction of the effective scattering coefficient defined so that (1 - eta(P)) is the amount of energy scattered in the forward peak. Then, to cope with nonconstant backscattering functions, we discuss the introduction of an effective backscattering coefficient that is an average of the scattering probabilities around pi.

Backscatter absorption gas imaging: a new technique for gas visualization

This paper presents a new laser-based method of gas detection that permits real-time television images of gases to be produced. The principle of this technique [which is called backscatter absorption gas imaging (BAGI)] and the operation of two instruments used to implement it are described. These instruments use 5-W and 20-W CO(2) lasers to achieve gas imaging at ranges of approximately 30 and 125 m, respectively. Derivations of relevant BAGI signal equations that can be used to predict the performance of a gas imager are provided. The predictions of this model and the measured range performance of an extended-range gas imager are compared. Finally, the results of gas sensitivity measurements and imaging tests on flowing gases are presented. These can be used to generate a realistic estimate of the BAGI sensitivity expected in detecting leaks of many different vapors.