We present the technique and experimental laboratory setup for measuring and analysis of diffuse reflectance spectra obtained by tunable infrared quantum cascade laser with an average power of 15 mW. Using causality relations for real and imaginary part of reflectivity we can calculate extinction coefficient. We use the dumped harmonic oscillator model to calculate synthetic spectra and test Kramers – Kronig relations for spectra calculations. To improve the accuracy of calculated extinction spectra we apply extrapolation of experimental spectra and phase correction. Using experimental setup and numerical methods of spectra analysis we could identify the powder of 30 μg of Acetylsalicylic acid and 40 μg of L-Tyrosine.
A real-time automated system for remote substance identification on various surfaces without preliminary sample preparation is presented. In practice, it can be used, for example, as an alerting system to signal the presence of some contaminants. The main components of the system are diffuse reflectance spectra acquisition module, data processing module, and identification module. Development of each module was based on the choice of appropriate devices and algorithms, either existing or newly designed. The experimental setup consists of a quantum cascade laser emitting in the spectral range of 5.3 to 12.8 μm with a HgCdTe photodetector. To achieve better selectivity of substance recognition, identification algorithms were based on the absorption and transmission spectra calculated from the recorded diffuse reflectance spectra. Spectra conversion algorithms employed Kramers–Kronig relations, phase spectra extrapolation, and phase correction. The system was supplied with the recognition database composed of certain commercially available substances. The experiments showed that the usage of transmittance spectra significantly improved the sensitivity of the identification method; the remote identification limit of 30 μg acetylsalicylic acid has been experimentally confirmed. For similar substances, such limit was estimated as 10 μg / cm2 at a distance of 1 m.