Rescue Blankets-Transmission and Reflectivity of Electromagnetic Radiation

Abstract

Rescue blankets are medical devices made of a polyethylene terephthalate sheet coated with a thin aluminum layer. Blankets are used for protection against hypothermia in prehospital emergency medicine and outdoor sports, but totally different qualities are typical for these multi-functional tools. On the one hand, rescue sheets prevent hypothermia by reducing thermo-convection and diminishing heat loss from evaporation and thermal radiation. On the other hand, the sheets promote cooling by acting as a radiant barrier, by providing shade and even by increasing heat conduction when the sheet is in direct contact with the skin. As foils are watertight and windproof, they can function as vapor barriers and even as stopgap bivouac sacks. We evaluated three experimental studies, one on heat loss by rescue blankets according to surface color, one on transparency with ultraviolet radiation, high-energy visible light and visible light, and one on infrared radiation from rescue blankets. When evaluating the effects of different bands of the electromagnetic spectrum on rescue sheets, we focused on ultraviolet radiation (200–380 nm), high-energy visible light in the violet/blue band (380–450 nm), visible light (380–760 nm) and infrared radiation (7500–13,500 nm). Rescue sheets transmit between 1% and 8% of visible light and about 1% of ultraviolet B radiation (280–315 nm), providing sufficient transparency and adequate protection from snow blindness. Reflection of visible light increases detectability in search and rescue missions performed in good visibility conditions, while reflection of infrared radiation increases detectability in poor visibility conditions and provides protection against hypothermia.

Rescue blankets hamper thermal imaging in search and rescue missions

“…The capacity of fleeces, down jackets and all-weather jackets, gear frequently used by mountain rescue personnel during the winter season, to block IR transmission was assessed [11]. In addition, two different brands of rescue blanket commonly used by ground emergency medical services (EMS) of the Austrian Red Cross (ARC), Mountain Rescue Tyrol (MRT) and EMS helicopter in Austria were investigated [11]. The setting of the investigation was the snow-covered backcountry in the Stubai Valley, Tyrol, Austria.…”

“…Experiments were conducted after local sunset to exclude direct external solar influx. Parts of this research have previously been published in a review on electromagnetic radiation reflectivity of rescue blankets [11].…”

“…Thermal imaging was performed with an IR camera lifted to altitudes of 5 m and 50 m above ground with a gas-filled balloon (Search Systems, Craigavon, UK). We chose short distances between the object and the camera to minimize the effects of absorption and refraction by air and by water vapour in order to keep interference from atmospheric conditions low [11,12]. Furthermore, the two distances were within the recommended maximum viewing distances for recognition of a target having a length of 1 to 2 m. Temperature and humidity were measured simultaneously (testo, 175 H1, 79822 Titisee-Neustadt, Germany).…”

“…For thermography a radiometric thermal camera (DJI Zenmuse XT, Shenzhen, China) was used to detect longwave IR in the spectral range between 7500 and 13,500 nm (uncooled microbolometer, image resolution: 640 (H) × 512 (V), pixel pitch: 17 µm, lens model: 13 mm; digital zoom: 8×; sensitivity: < 50 mK at f/1.0; accuracy of ± 10 °C; operating temperature: −10 to 40 °C at a relative humidity between 5 and 95%). The camera was mounted on a Zenmuse XT gimbal at a controllable range of tilt: + 30 to −90°, pan: ± 320° and roll: ± 15° and was remotely controlled [11,15]. To diminish reflection we kept the view angle within 60° of straight-on and oblique measurements [11,12].…”

“…The camera was mounted on a Zenmuse XT gimbal at a controllable range of tilt: + 30 to −90°, pan: ± 320° and roll: ± 15° and was remotely controlled [11,15]. To diminish reflection we kept the view angle within 60° of straight-on and oblique measurements [11,12]. The camera was activated and calibrated half an hour prior to measurements to allow adjustment of the thermal imaging device to the low surrounding air temperatures.…”