An In-Situ Study of Laser-Induced Plume Radiance and Thermal Effects of Laser Irradiated Polycarbonate and Poly(Methyl M
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AN IN-SITU STUDY OF LASER-INDUCED PLUME RADIANCE AND THERMAL EFFECTS OF LASER IRRADIATED POLYCARBONATE AND POLY(METHYL METHACRYLATE) SAMPLES Pearul W. Yip U.S. Army Materials Technology Laboratory, Watertown, MA 02172-0001 ABSTRACT Threats of lasers to human eyes and electro-optics on the modem battlefield has been significantly increased due to the rapid growth of laser technology in military applications. Currently polycarbonate (PC) and poly [methyl methacrylate] (PMMA) are being used as transparencies for aircraft and goggle materials for aircrews and combat soldiers [Ref 1,2]. A carefully monitored laser test was conducted on these two particular polymers using a mediumpower, continuous-wave CO2 laser to determine the brightness of the laser-induced plume radiance and how much luminance was transmitted to the back of the sample. Plume luminance for photopic (daytime) and scotopic (nighttime) visual conditions, as well as the recovery times for human eyes exposed to these levels of luminance were calculated. Our test results demonstrated that the plume induced by the CW CO 2 laser irradiation can be an ocular hazard to soldiers operating behind these conventional transparent materials. INTRODUCTION Flash effects on the human eyes caused by intense white light sources such as those produced by arc welding, and solar eclipse have been well documented [Ref 3]. Research on bright flashes generated by a laser-induced plume (an envelope of electronically excited gases of molecular and molecular fragments, ejected from the surface of the irradiated sample ) is limited. Despite the increasing prevalence of lasers on the modern battlefield, research into the ocular hazard associated with lasers has been centered on the effects of direct exposure to laser radiation [Ref 4]. Since the region hazardous to the retina is between the spectral range of 400-14(00 nanometers [Ref 51, which covers the visible and the near-infrared spectral regions, we believed that it is also important to examine the ocular hazard associated with the bright white light of the laser-induced plumes. This investigation was designed to determine three major properties as a function of laser intensity : ) the plume radiance or the flash brightness of PMMA and PC resulting from the continuous-wave (CW) CO 2 laser irradiation; 2) to determine the burn-through times or the laser hardness of the polymers; 3) to determine the char-formation time, defined as the time at which a sudden drop of light transmission occurred due to the rough, highly scattering char surface. EXPERIMENTAL All samples were approximately 0.25-inch thick. They were cut into one-inch diameter disks. The samples were placed in the path of a medium-power, CW CO2 laser beam, and irradiated until bum through. The laser intensity levels used were 1, 3, 5 and 9 kW/cm 2 , and the beam diameter was one centimeter. An optical multichannel analyzer (OMA) was used to measure the plume radiance from both the front and rear aspects of the samples during laser irradiation in order to determine the
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