Tuning D* with Modified Thermal Detectors

Abstract

We report on the fabrication and characterization of a modified thermopile detector that has a spectral detectivity, D*, primarily determined by the absorbance of a polymer film. This was done by coating the detector with a metal mirror, followed by the polymer film, so that the film absorbances are responsible for most thermal conversion. The detector is designed to tailor the spectral response of optical systems more specifically to analytes in order to improve precision in methods such as multivariate optical computing and simple photometry. Interference effects in the thin-film response are eliminated by the textured surface of the silicon thermopile, which makes the spectral response relatively simple. The maximum detectivity due to a 1-micrometer-thick film is found to be 20% of the detectivity of the original wide-band detector at 10 Hz modulation frequency. We estimate the thermal diffusion length in the polymer at 10 Hz to be 40 micrometers. We also suggest that the detectivity of the modified detector can be approximated as the product of the D* of the underlying thermal detector and the absorbance of the modifying film, provided the modulation frequency is low and interference effects are defeated.

Multimode Imaging in the Thermal Infrared for Chemical Contrast Enhancement. Part 3: Visualizing Blood on Fabrics

“…Previously, we explored varying the response of thermal detectors with addition of chemical films that act as filtering or sensitizing layers . Adding chemical films to a thermal imaging detector is not a trivial exercise; therefore, we have focused our initial efforts on the use of chemical filters.…”

“…Previously, we explored varying the response of thermal detectors with addition of chemical films that act as filtering or sensitizing layers. 8 Adding chemical films to a thermal imaging detector is not a trivial exercise; therefore, we have focused our initial efforts on the use of chemical filters. The concept embodied in both approaches, however, is "like detects like", e.g., a chemical filter masks the spectral signature from similar chemicals in the scene, providing an approximation of the power of hyperspectral imaging.…”

Multimode Imaging in the Thermal Infrared for Chemical Contrast Enhancement. Part 1: Methodology

“…Using the alternating current (AC) response generated by the modulated light source gives an advantage over the conventional direct current (DC) response by enhancing the chemical contrast in the image, allowing for the detection and identification of a stain of interest. While this simple instrumentation is not suited to hyperspectral analysis, we discuss a “like-detects-like” chemical filtering approach described in one of our recent reports for the use of a broadband thermal detector for chemical identification purposes. Lodder et al and Hieftje et al have successfully used this approach, called molecular factor computing, for vapor detection and identification. − …”