Automated Low-Cost LED-Based Sun Photometer for City Scale Distributed Measurements

Abstract

We propose a monochromatic low-cost automatic sun photometer (LoCo-ASP) to perform distributed aerosol optical depth (AOD) measurements at the city scale. This kind of network could fill the gap between current automatic ground instruments—with good temporal resolution and accuracy, but few devices per city and satellite products—with global coverage, but lower temporal resolution and accuracy-. As a first approach, we consider a single equivalent wavelength around 408 nm. The cost of materials for the instrument is around 220 dollars. Moreover, we propose a calibration transfer for a pattern instrument, and estimate the uncertainties for several units and due to the internal differences and the calibration process. We achieve a max MAE of 0.026 for 38 sensors at 408 nm compared with AERONET Cimel; a mean standard deviation of 0.0062 among our entire sensor for measurement and a calibration uncertainty of 0.01. Finally, we perform city-scale measurements to show the dynamics of AOD. Our instrument can measure unsupervised, with an expected error for AOD between 0.02 and 0.03.

Photocapacitance discharge of LEDs applied in remote sensing detectors

“…This circuit is composed by a current source (IP) that corresponds to the photocurrent generated by the luminous flux detected. The photocapacitance (CJ) that is the inherent capacitance of a photodetector, its value is proportional to intensity of light detected and determined by bias voltage the applied [17]. Direct sensor to microcontroller interface is recommended for instrumentation systems based on capacitive sensors, due its low power consumption, and low measurement errors can be obtained with this configuration [27,28].…”

“…This configuration exhibits its highest sensitivity within a band-width near to the emission spectrum [13][14][15]. Based on this principle remote sensing systems were developed and applied in sun photometers [16][17][18], and sensors for NDVI measurement [19,20]. In these applications an electrical current proportional to detected light intensity is generated by photovoltaic effect, that requires a transimpedance amplifier for signal conditioning, and the analog to digital conversion is performed with the voltage obtained for subsequent data processing [19].…”

“…In this paper we evaluated the effectiveness of photocapacitance discharge effect of LEDs on the measurement of NDVI, through the monitoring of microalgae suspensions with different biomass concentrations. Results were compared with those obtained using photovoltaic mode, that is the configuration previously reported for remote sensing applications [16,17].…”