Human detection is indispensable for surveillance, tracking and missing person investigations. Although spatial structure analysis is widely used for human body detection in computer vision fields, the detection is not feasible in case only the parts of human body are observed. Pixel-base detection based on spectral processing is a promising alternative which overcomes the complications in spatial information-based detection. Since the distribution of human skin is anomalous in spectral domain, it has been shown that the human skin is detectable based on the normalized difference human index (NDHI), which is a normalized difference in the reflectance measured at 1070nm and 1550nm. Optical passive sensing of close outdoor target is less reliable because the radiance level is affected by the shading and fluctuation of background light. In addition, illuminance at 1550nm is low due to water absorption, so that acquisition of reflectance at 1550nm is difficult under severe illumination conditions, e.g. cloudy weather and low altitude sun. In this paper, a portable human skin detector based on active infrared illumination is developed. In order to acquire accurate reflection information based on low power LED illuminations under various background light conditions, orthogonal rock-in amplifier is used. The system is composed of an illumination-sensing, filtering/detecting, calculating and indicating components. In the illumination-sensing part, targets are illuminated with a train of pulses of 1070 and 1550nm LEDs and, then, the reflected light is sensed by an InGaAs photo diode. The observed signals sampled by synchronized reference signals are demodulated by synchronous detector to eliminate the disturbance based on background light and signal noise. The outputs of the filtering/detecting component are voltages corresponding to the reflectance at 1070 and 1550nm, R(λ1070) and R(λ1550). In the calculating components, R(λ1070) - R(λ1550) and R(λ1070) + R(λ1550) calculated by operational amplifiers are transmitted to an analog divider to calculate NDHI value, defined as (R(λ1070) - R(λ1550))/(R(λ1070) + R(λ1550)). The indicating component displays voltage which is linearly correlated with the NDHI value. Buzzer sound and LED light are emitted in case the NDHI value exceeds a threshold. Multiple targets including human skin and background objects, e.g. leaves, wood, soil, cotton, polyester, are observed under the condition of clear sky, i.e. 80,000lux and dark room. It is confirmed that the voltage outputs of the detector are stable under various background illumination conditions. The correlation between NDHI values and the voltage output of the detector, i.e. R2=0.9991, guaranteed high reliability of the proposed detector.
Ključne reči :
Tematska oblast:
SIMPOZIJUM B: Biomaterijali i nanomedicina
Datum:
31.05.2012.
Br. otvaranja:
415
Contemporary Materials - 2012 - Savremeni materijali
