Remote sensing

With a suitable optical system TCSPC is able to reeord fluoreseenee deeay functions or diffusely reflected laser signals over remarkable distanees. The setup shown in Fig. 5.136 records the fluorescence of ehlorophyll in plants over a distance of several hundred meters. 

A diode laser sends a beam of 100 ps pulses to the target. The repetition rate of the laser pulses is 50 MHz, the average power 0.5 mW. A 20-cm (8-ineh) telescope (Meade LX90 EMC) is used to collect the photons from the target. The fluorescence and the reflected light are separated by a dichroie mirror and a 700 15 nm bandpass filter and detected simultaneously by two individual detectors. Consequently, detector 1 detects the diffusely reflected laser, deteetor 2 the fluorescence of the leaves. 

In spite of the low laser power, the chlorophyll fluorescence ean be detected over a distance of several hundred meters, with count rates of the order of 1,000 photons per second. At a count rate this low the background signal is an important 

The telescope and the laser were pointed into a forest approximately 300 m away. Curve 1 is the laser scattered at the target, curve 2 is the detected fluorescence. 

It is almost impossible to hit only one leaf over so great a distance. Therefore the signals from several leaves at different distances are detected. Moreover, the signal intensity fluetuates considerably on a scale of seconds due to the motion of the leaves in the wind. However, because the reflectance and the fluorescence signals are deteeted simultaneously, the reflectance can be used as an approximation of the instrument response function. This approach is not absolutely correct because the reflected photons can also come from nonfluorescent target components, but it delivers decay times with reasonable accuracy. The result of a doubleexponential fit is shown in Fig. 5.138. 

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The SPATE 9000, used in the experimental activity, is a remote sensing system able to detect... 

For remote sensing, spectroscopy at THz frequencies holds the key to our ability to remotely sense enviromnents as diverse as primaeval galaxies, star and planet-fonuing molecular cloud cores, comets and planetary atmospheres. 

Rather different circumstances are encountered when considering THz remote sensing of extraterrestrial sources. The major source of THz opacity in the Earth s atmosphere is water vapour, and from either high, dry mountain sites or from space there are windows in which the background becomes very small. Incoherent instruments which detect the faint emission from astronomical sources can therefore be considerably more sensitive than their laboratory... 

Technology developments are revolutionizing the spectroscopic capabilities at THz frequencies. While no one teclmique is ideal for all applications, both CW and pulsed spectrometers operating at or near the fiindamental limits imposed by quantum mechanics are now within reach. Compact, all-solid-state implementations will soon allow such spectrometers to move out of the laboratory and into a wealth of field and remote-sensing applications. From the study of the rotational motions of light molecules to the large-amplitude vibrations of... 

Waters J W 1993 Miorowave limb sounding Atmospheric Remote Sensing by Microwave Radiometry ed M A Janssen (New York Wiley) pp 383-496... 

Janssen M A (ed) 1993 Atmospheric Remote Sensing by Microwave Radiometry (New York Wiiey) The most oompiete guide to miorowave and THz atmospherio sensing. 

The sample cells for molecular fluorescence are similar to those for optical molecular absorption. Remote sensing with fiber-optic probes (see Figure 10.30) also can be adapted for use with either a fluorometer or spectrofluorometer. An analyte that is fluorescent can be monitored directly. For analytes that are not fluorescent, a suitable fluorescent probe molecule can be incorporated into the tip of the fiber-optic probe. The analyte s reaction with the probe molecule leads to an increase or decrease in fluorescence. 

The thermographic sensor is used as a remote sensing radiometer when a reference target is imaged. It is usually necessary to correct for emissivity and atmospheric transmission to determine surface temperature with a reasonable degree of accuracy. 

Laser sources that emit in the mid-ir region of the spectmm (2—5 -lm) are useful for detection of trace gases because many molecules have strong absorption bands in that region. Other appHcations include remote sensing and laser radar. Semiconductor lead—salt (IV—VI) lasers that operate CW at a temperature of 200 K and emission wavelength of 4 p.m are commercially available however, they have relatively low output powers (<1 mW) (120). 

Hereia optical spectroscopy for laboratory analysis, giving some attention to remote sensing usiag either active laser-based systems (13—16) or passive (radiometric) techniques (17—20), is emphasized. 

R. M. Measures, Laser Remote Sensing,]ohxi Wiley Sons, Inc., New York, 1984. 

H. S. Chen, Space Remote Sensing Systems An Introduction Academic Press, Inc., Orlando, Fla., 1985. 

R. E. Huffman, Atmospheric Ultraviolet Remote Sensing, Academic Press, Inc., Boston, Mass., 1992. 

H. Tannenbaum, "Laser AppHcations in Remote Sensing," Proc. Soc. Photo-Opt. Instrum. Eng. 49, (1975). 

None of the foregoing methods will tell the frequency or duration of exposure of any receptor to irritant or odorous gases when each such exposure may exceed the irritation or odor response threshold for only minutes or seconds. The only way that such an exposure can be measured instrumentally is by an essentially continuous monitoring instrument, the record from which will yield not only this kind of information but also all the information required to assess hourly, daUy, monthly, and annual phenomena. Continuous monitoring techniques may be used at a particular location or involve remote sensing techniques. 

Alfodi, T. T., Satellite remote sensing for smoke plume definition, in Proceedings of the 4th Joint Conference on Sensing of Environmental Pollutants. American Chemical Society, Washington, DC, 1978, pp. 258-261. 

Browell, E. V.. Lidar remote sensing of tropospheric pollutants and trace gases, in Proceed-... 

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