Laser footprint

An air-borne method of exploration for ore deposits has been realized with the help of luminescence Lidar. Lidar embodies a high power UV laser and a coaxial telescope detection system installed aboard a helicopter. The laser yields pulses having a peak power of 0.4- 0.5 mj, a time delay of 1 ps, gate width of 1 ms, repetition rate of 30-50 Hz, and a wavelength varying from 275 up to 400 nm. The laser footprint is about 400-500 cm and the distance between the closest footprints about 1 m. The air-borne survey is carried out from an altitude of 50-75 m. Sensitivity is 0.2- 0.5% by exposed area for most minerals (Seigel and Robbins 1982 Seigel and Robbins 1985). 

A laser footprint is the projection of the laser beam, and buffer zone, onto the ground or target area. The laser footprint may be part of the laser surface danger zone if the laser footprint lies within the nominal ocular hazard distance (NOHD) of the laser. 

Safety mnst be the first consideration of any process analytical installation. Electrical and weather enclosures and safe instrnment-process interfaces are expected for any process spectroscopy installation. The presence of a powerfnl laser, however, is nniqne to process Raman instruments and mnst be addressed due to its potential to injnre someone. Eye and skin injnries are the most common resnlts of improper laser exposure. Fortunately, being safe also is easy. Becanse so many people have seen pictnres of large industrial cutting lasers in operation, this is often what operations personnel erroneonsly first envision when a laser installation is discnssed. However, modem instmments nse small footprint, comparatively low power lasers, safely isolated in a variety of enclosures and armed with various interlocks to prevent accidental exposnre to the beam. 

While these measurements provide a wealth of information on the gross structure of surfactant concentration gradients, the point sampling (2.5 cm in diameter sampling footprint) nature of the measurement technique the 1 to 2 meter per second speed of the ship serving as the platform and the 20 Hz repetition rate (20 laser pulses per second) of the laser make it difficult... 

Salvetti P, Rosen JM, Reichel E. Subthreshold infrared footprinting with indocyanine green for localizing low-intensity infrared photocoagulation. Ophthalmic Surg Lasers Imaging 2003 34 44 48. 

Hydroxyl radical oxidative modification, one of the footprinting approaches, utilizes hydroxyl radicals to oxidize certain amino acid side chains followed by extent of modification measurement by MS. For example, fast photochemical oxidation of proteins (FPOP) [35] involves irradiation of protein sample solutions containing hydrogen peroxide by a KrF excimer laser and generation of hydroxyl radicals in the solution. FPOP occurs on the microsecond timescale for the labeling... 

A non-contact laser surface profilometer which allows for precise characterization of surface profile has been purchased from Rodenstock Precision Optics. Profiles of microscopic surface terrain over spans of up to 60mm are achievable at a scan rate of 30 rnm/min. ITie laser beam allows surface height resolution on the order of one micron. In initial tests with the system, both longitudinal and radial profile scanning were successfully performed on cylindrical tensile test specimens. Radial scanning is possible because the footprint of the focused solid state laser (0.8 pm) is small compared to the radius of curvature of the specimen. 

The mass spectrometer was equipped with a 20-Hz N2 laser. The laser irradiation energy was adjusted by a neutral density filter and the beam size was adjusted to 35 xm with an adjustable pinhole placed close to the laser entrance window. The actual laser beam size was obtained by increasing laser fluence till a burn mark was left behind to allow off-line measurements of the laser beam footprint (rrrNote 6). 

For these studies, the laser beam was incident typically at a fixed azimuth angle of 45° but the depth of focus was sufficiently long that the entire footprint could be considered to be uniform in the ablating surface. It was considered impractical to pivot the laser... 

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