An indirect skin emissivity measurement in the infrared thermal range through reflection of a CO2 laser beam
C. Villaseñor–Mora, F.J. Sánchez–Marin, and S. Calixto–Carrera
Centro de Investigaciones en óptica, Loma del Bosque 115, Lomas del Campestre, León, Gto., México 37150, Phone: 52 477 4414200, Fax: 52 477 4414209
Recibido el 7 de mayo de 2009
Aceptado el 25 de septiembre de 2009
Abstract
An indirect procedure to measure human skin emissivity is proposed. This procedure uses a 10.6 µm CO2 laser, to project a controlled energy on the skin, and a power meter to measure the projected, reflected, emitted and background energies. To eliminate the effects of background radiation, two power measurements are taken: one of the skin and background emission and another that includes the skin emission itself, the background radiation, as well as the reflection of the laser beam by the skin. Those two measurements are subtracted to obtain the reflected energy and, with this, the corresponding reflectivity of the skin. With such subtraction, background and other sources of noise are eliminated, and using the Kirchhoff law the emissivity is calculated. The emissivity values obtained with this procedure were corroborated using a theoretical blackbody. Both methods give practically the same values, which validates our procedure. In addition, our values are in accordance with those previously reported by other researchers, but our procedure is simpler, faster and innocuous. An additional contribution of this work is the analysis of the way the skin reflects the infrared radiation, in the mid range. It was found that the reflection of the skin is more specular than Lambertian, for the wavelength that was used in this work.
Keywords: Skin emissivity; skin reflection; skin reflectivity; emissivity without blackbody; Lambertian surfaces.
Resumen
En el presente trabajo se propone un procedimiento indirecto para medir la emisividad de la piel humana. Este procedimiento usa un láser de CO2 de 10.6 µm para proyectar una energía controlada sobre la piel, y un radiómetro para medir las energías proyectadas, reflejadas, emitidas y de fondo. Para eliminar la influencia de la radiación de fondo, se toman dos lecturas con el radiómetro: una que incluye la energía de la piel y la emisión de fondo y otra inmediatamente después que incluye la energía de la piel, la radiación de fondo, y la reflexión de la piel. Se obtiene la diferencia de esas dos mediciones para obtener la energía reflejada y, con esto, la reflectividad correspondiente de la piel. Al obtener la diferencia, la radiación de fondo y otras fuentes de ruido se eliminan, y usando la ley de Kirchhoff la emisividad es calculada. Los valores de emisividad obtenidos con este procedimiento fueron corroborados con el método directo usando un cuerpo negro teórico. Ambos métodos arrojan prácticamente los mismos valores, lo cual valida nuestro procedimiento. Adicionalmente nuestros resultados están acordes con aquellos obtenidos por otros investigadores, pero nuestro procedimiento es más simple, más rápido e inocuo. Una aportación adicional de este trabajo, es el análisis de la forma en que la piel refleja la radiación infrarroja en el rango medio. Se encontró que la reflexión de piel en esta longitud de onda es mas especular que Lambertiana.
Descriptores: Emisividad de la piel; energía de la piel; reflectividad de la piel; emisividad sin cuerpo negro; superficies Lambertianas.
PACS: 78.20.Ci; 06.20.Jr; 92C50
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