J. Meléndez

PbSe photodetector arrays for IR sensors

Abstract PbSe Linear arrays have been fabricated to be used as multiple IR gas sensors at room temperature. PbSe layers were obtained by thermal evaporation in vacuum on SiO 2 /Si wafers. Samples became photoconductive after a sensitization process that includes thermal treatments in an iodine-enriched atmosphere. Both morphological features and chemical composition of the layers were studied along the different steps of the sensitization process with a Scanning Electron Microscope. Photoelectrical measurements were also carried out. By comparison between microscopic and photoelectrical measurements, some relations between changes in the morphology of the PbSe layers and changes in their photoconductive behavior were established. The dispersion observed in the photoconductive characteristics of the array elements is associated to the appearance of inhomogeneities during the sensitization process. Whereas the presence of iodine in the sensitization process accelerates the recrystallization rate of PbSe, the presence of oxygen could be responsible for the photoconductivity.

Spectrally selective gas cell for electrooptical infrared compact multigas sensor

Abstract Progress in a new electrooptical compact gas sensor system is reported. The sensor measures pollutant concentrations in the atmosphere by comparing the absorption of infrared light in a gas cell at specific wavelengths. This task is performed by a non-dispersive spectral analyzer that operates at discrete wavelengths and has no moving parts (spectral retina). Several gases can be detected with the same detection principle, by simply selecting different signal channels. The spectral analyzer is made by coupling an array of integrated narrow band-pass optical filters to an array of PbSe detectors. Simultaneous detection of CO and SO 2 with no cross-talk is demonstrated. A specific spectrally selective gas cell has been designed and fabricated. The new design greatly simplifies the fabrication process of the spectral analyzer and expands the range of operation towards longer wavelengths, useful for more sensitive detection of SO 2 and NO 2 .

Thermal infrared emission–transmission measurements in flames from a cylindrical forest fuel burner

We describe emission–transmission measurements performed at different heights in a flame from a cylindrical forest fuel burner, using a camera operating in the thermal infrared (7.5–13 µm). The forest fuel burner was made of a cylindrical wire mesh basket filled with a forest fuel (Pinus pinaster needles), which was ignited at the base of the basket. Three diameters of basket were used (20, 28 and 40 cm). Heat release rates, as calculated from weighing of the basket and heat of combustion of the fuel, ranged between 50 and 170 kW and flame heights ranged between 1 and 2 m. The emission–transmission device allows the determination of the transmittance of the flame and of a radiometric temperature. We show that radiation was dominated by soot in the spectral range of the camera, but that radiation from gaseous products of the combustion was not negligible. Using the Mie theory in its Rayleigh limit, we deduced some average volume fractions of soot from the measurements, which peaked at 6.8 × 10−6 in the persistent region of the flame. Then the total extinction coefficient and the total emissivity of the flame due to soot were calculated according to a standard method. Measured transmittance, soot volume fraction, total extinction coefficient and total emissivity were found to scale with the normalised height of measurement Z, defined as the ratio of the height of measurement to the height of the flame (0.25 < Z < 1.6).

Reduction of false alarm rate in automatic forest fire infrared surveillance systems

Abstract One of the main problems related to infrared remote sensing of forest fires by automatic systems concerns the rejection of false alarms. The study of the infrared spectral radiance emitted by a biomass fire has been used to define spectral algorithms that permit to separate fires from other sources considered as false alarms. The different behaviour of the medium (3–5 μm) and thermal (8–12 μm) infrared spectral regions has been the basis for the definition of a new “figure” of merit, the so-called Fire Index (FI). An experimental burn has been used to test the ability of FI to identify clearly fires. A second algorithm based on an intraband analysis, the Mid-IR Fire Index (MFI), has been proposed to give complementary information to FI. Evolution of FI and MFI values with distance for different IR sources has been calculated. Finally, an experimental validation of both algorithms has been performed by using two infrared cameras working in the medium and thermal infrared spectral windows.

Measurement of forest fire parameters with multi-spectral imaging in the medium infrared

Infrared (IR) imaging is a standard technique for the detection of forest fires but its use to measure the physical parameters of fires faces difficulties due to their spatial and spectral complexity. Multi-spectral images in the Medium IR make possible to classify the scene in different fire regions, thus enabling a region-specific processing and providing values for the instantaneous radiated power and the total radiated energy. Comparison with energy release determined from fuel consumption results in an estimation of a radiative energy fraction of 14 = 3 %. It is the first time that this parameter is measured by IR imaging in the field.

Electrooptical infrared compact gas sensor

Abstract The concept, design and present stage of development of a new type of electrooptical gas sensor system, with application to the simultaneous detection of several atmospheric pollutant gases, is described. The sensor system is based on non-dispersive infrared differential optical absorption spectroscopy. Spectral analysis is performed by a sensor device with no moving parts, consisting of an array of integrated bandpass multilayer filters coupled to an array of PbSe detectors. The principle of detection has been demonstrated for concentrations in the range of a few ppm.

Hyperspectral quantitative imaging of gas sources in the mid-infrared

An imaging Fourier transform spectrometer operating in the medium infrared (1800-5000  cm(-1)) has been used to image two gas sources: a controlled CO2 leak at room temperature and the exhaust of a combustion engine. Spectra have been acquired at a resolution of 0.5  cm(-1) using an extended blackbody as the background. By fitting them with theoretical spectra generated with parameters from the High-Resolution Transmission Molecular Absorption database, quantitative maps of temperature and gas column density (concentration·path product) for the gas plumes have been obtained. Spectra are related to gas plume parameters by means of a radiometric model that takes into account not only gas absorption, but also its emission and the atmospheric absorption, as well as the instrument lineshape function. Measurements for the gas leak show very good agreement between retrieved and nominal values of temperature and CO2 column density. This result has direct application to obtain quantitative imaging of exhaust emissions from automobiles and other mobile sources, as shown here with measurements of exhaust gases in a diesel engine.

The Design of an Infrared Sensor for the Measurement of Martian Surface Temperature and Gas Concentration

Design considerations for a temperature sensor to be launched onboard the space mission MetNet precursor to Mars are presented. An IR multispectral sensor has been proposed to measure Martian surface temperature to an accuracy of ±1 K without knowing ground emissivity. Two methods for temperature retrieval have been assessed, both relying on comparative measurements at two wavelengths and/or two temperatures. Spectral calculations of surface emission and atmospheric absorption-emission have been made with a simple Martian atmosphere model, and signal to noise ratios have been estimated at transparent wavelengths for several temperatures. Temperature accuracy has been derived for the two methods, showing that bi-color pyrometry has in principle the ability to provide the desired temperature accuracy for reasonable values of sensor detectivity, field of view and spectral filter width, for T≫200 K. This sensor can be applied also to estimate CO2 concentrations in the Martian atmosphere, by comparing signals measured at CO2 absorption and transparent bands.

Bispectral infrared forest fire detection and analysis using classification techniques

Infrared cameras are well established as a useful tool for fire detection, but their use for quantitative forest fire measurements faces difficulties, due to the complex spatial and spectral structure of fires. In this work it is shown that some of these difficulties can be overcome by applying classification techniques, a standard tool for the analysis of satellite multispectral images, to bi-spectral images of fires. Images were acquired by two cameras that operate in the medium infrared (MIR) and thermal infrared (TIR) bands. They provide simultaneous and co-registered images, calibrated in brightness temperatures. The MIR-TIR scatterplot of these images can be used to classify the scene into different fire regions (background, ashes, and several ember and flame regions). It is shown that classification makes possible to obtain quantitative measurements of physical fire parameters like rate of spread, embers temperature, and radiated power in the MIR and TIR bands. An estimation of total radiated power and heat release per unit area is also made and compared with values derived from heat of combustion and fuel consumption.

Forest fire studies by medium infrared and thermal infrared thermography

The new generation of dedicated satellites for remote sensing of forest fires now in advanced development demands validation measurements from airborne platforms. A digital image acquisition system in the medium infrared (MIR) and thermal infrared (TIR) bands has been set up specifically for imaging of fires. The system provides simultaneous, co- registered and radiometrically calibrated MIR and TIR images. This makes it possible to use image processing techniques based on pixel-by-pixel comparison of MIR and TIR brightness temperatures. Analysis of a laboratory flame and a wood bonfire shows that the MIR-TIR scatterplot can be used to classify the scene into different fire regions (cold background, hot nonflaming soil, hot flaming soil and pure flame). This technique has been applied also to observations of forest fires realized from a helicopter at distance of more than 1 Km, revealing that several fire regions can be demarcated, including a fire front in which flame emission makes a large contribution.