Quantitative thermal imaging using single-pixel Si APD and MEMS mirror

Abstract

Accurate quantitative temperature measurements are difficult to achieve using focal-plane array sensors. This is due to reflections inside the instrument and the difficulty of calibrating a matrix of pixels as identical radiation thermometers. Size-of-source effect (SSE), which is the dependence of an infrared temperature measurement on the area surrounding the target area, is a major contributor to this problem and cannot be reduced using glare stops. Measurements are affected by power received from outside the field-of-view (FOV), leading to increased measurement uncertainty. In this work, we present a micromechanical systems (MEMS) mirror based scanning thermal imaging camera with reduced measurement uncertainty compared to focal-plane array based systems. We demonstrate our flexible imaging approach using a Si avalanche photodiode (APD), which utilises high internal gain to enable the measurement of lower target temperatures with an effective wavelength of 1 µm and compare results with a Si photodiode. We compare measurements from our APD thermal imaging instrument against a commercial bolometer based focal-plane array camera. Our scanning approach results in a reduction in SSE related temperature error by 66 °C for the measurement of a spatially uniform 800 °C target when the target aperture diameter is increased from 10 to 20 mm. We also find that our APD instrument is capable of measuring target temperatures below 700 °C, over these near infrared wavelengths, with D* related measurement uncertainty of ± 0.5 °C.

A Review of Thermal Spectral Imaging Methods for Monitoring High-Temperature Molten Material Streams

“…Such pixel-to-pixel crosstalk can affect the spatial resolution of the detector and thus complicate the reconstruction of the desired image [ 41 ]. Another major contributor to this problem is size-of-source effect (SSE), which arises due to imperfections within the optical system, leading to reflections and scattering inside the instrument [ 42 ]. Measurements are affected by the power received from outside the field of view, leading to increased measurement uncertainty since each pixel receives power over a wider area of the target than the camera was designed for.…”

“…How increasing aperture size affects the temperature measurements of a commercial bolometer thermal imagers is shown in Figure 5 . To reduce uncertainty in the temperature measurement, instead of FPAs, Hobbs et al proposed a micromechanical systems (MEMS) mirror-based high-speed 2-dimensional raster scanning in combination with a single-pixel Si avalanche photodiode detector (APD) with an effective wavelength of 1 μm [ 42 ]. To demonstrate the benefits of this imaging approach for quantitative thermal imaging, comparison was made with a commercial bolometer-based FPA thermal imager.…”

“… SSE analysis for ( a ) Si APD and ( b ) bolometer camera imagers with increasing target aperture size at a furnace temperature of 800 °C. Reproduced from [ 42 ] under license CC BY 4.0. …”

A Review of Thermal Spectral Imaging Methods for Monitoring High-temperature Molten Material Streams

“…Such pixel-topixel crosstalk can affect the spatial resolution of the detector and thus complicate the reconstruction of the desired image [38]. Another major contributor to this problem is sizeof-source effect (SSE), which arises due to imperfections within the optical system leading to reflections and scattering inside the instrument [39]. Measurements are affected by the power received from outside the field-of-view, leading to increased measurement uncertainty since each pixel receives power over a wider area of the target than the camera was designed for.…”

“…How increasing aperture size affects the temperature measurements of a commercial bolometer thermal imagers is shown in Error! Reference source not found.. To reduce uncertainty in the temperature measurement, instead of FPAs, Hobbs et al proposed a micromechanical systems (MEMS) mirror-based high-speed 2-dimensional raster scanning in combination with a single-pixel Si avalanche photodiode detector (APD) with an effective wavelength of 1 µm [39]. To demonstrate the benefits of this imaging approach for quantitative thermal imaging, comparison was made with a commercial bolometer based FPA thermal imager.…”