# A Non-Contact Phosphor Thermometry Technique for Determining the Optical Absorptivity of Materials

## TL;DR

This paper introduces a non-contact, low-cost method using phosphor thermometry to measure how much light materials absorb at different wavelengths.

## Contribution

A novel non-contact phosphor thermometry technique for measuring material optical absorptivity is developed and validated.

## Key findings

- The technique successfully measured absorptivity values of two paints with high accuracy and low variability.
- The method demonstrated the ability to measure absorptivity at specific wavelengths, such as 600nm and 1550nm.
- The system provided resolvable and incremental absorptivity values for paint mixtures with unknown properties.

## Abstract

This work presents a bespoke, non-contact, and low-cost Phosphor Thermometry (PT) technique for the measurement of material absorptivity. The approach circumvents the challenges associated with traditional and intrusive calorimetric techniques, which require secure contact with the sample substrate. A thermographic phosphor (TP), Manganese-activated Magnesium Fluorogermanate (MFG), was used as a two-colour thermometer utilising the peak intensity ratio technique, enabling an empirical temperature measurement of a given Material Under Test (MUT). The system was calibrated to temperature across a dynamic range of 20°C to 140°C and subsequently assessed in terms of noise and relative sensitivity. A mathematical model describing the thermal behaviour of the samples was subsequently developed and used to infer the absorptivity value of the MUTs. Two paints, Black 3.0® and Avian-B500®, with known but contrasting absorptivities, were analysed, resulting in measured absorptivity values of 0.9385 and 0.0651 within a range of 0.0081 and 0.0127 for the two paints, respectively. Subsequent mixtures of both paints, with inherent unknown absorptivities, provided resolvable and incremental steps between the two extremities. Further measurements at specific narrow-band wavelengths of 600nm and 1550nm of Black 3.0® were performed, yielding median absorptivity values of 0.9598 and 0.9172 within a range of 0.0168 and 0.0396, respectively, therefore demonstrating the technique for the measurement of material absorptivity at discrete wavelengths. The potential of a non-contact calorimetric PT technique could provide a scalable, non-intrusive, and low-cost solution for measuring the wavelength-dependent absorptivity values of materials that are used across engineering and research fields.