Active spectroscopy and imaging techniques illuminate a target and capture reflected light to determine the materials present and generate images. In this way, active spectroscopy and imaging is used to identify and chemically analyse objects at a large distance from the target.
There are many active imaging techniques, including hyperspectral, multispectral, and Raman. A key application of active imaging in the security and defence sectors is terahertz (THz) imaging.
Hyperspectral and multispectral imaging are techniques that generate three-dimensional data sets containing the intensity of light reflected by an object as a function of location (x, y) and wavelength (λ). A “spectral image” therefore has a collection of many spectral values in each pixel.
Each pixel in a hyperspectral image has intensity values over a continuous spectral range. Essentially, each pixel behaves as a spectrometer by measuring the optical intensity over a continuous spectrum simultaneously.
A multispectral image is essentially a set of images that have been measured at discrete wavelengths and then superimposed. Each pixel in a multispectral imaging therefore has intensity values at multiple discrete wavelengths.
Hyperspectral and multispectral images allow the materials present in the target to be visually contrasted and identified through their characteristic spectral signature.
Raman imaging is a technique that generates images based on a target’s Raman spectrum. When incident on the sample, high-power excitation light will be Rayleigh scattered and Raman scattered.
The ratio of Raman-to-Rayleigh scattered light is at most one part in a million. As a result, the intense, Rayleigh scattered excitation light can saturate the detector. A Rayleigh edge filter is placed in front of the detector to block Rayleigh signals and allow high transmission of the weak Raman signals.
As Raman scattering is an inelastic process, energy is not conserved, and so the light is wavelength-shifted (“Raman-shifted”). The Raman shift is characteristic of the scattering material, and is continuously measured whilst the excitation source is scanned across the target. The measured Raman shifts are then converted to a false-colour image that differentiates the materials present in the target.
Terahertz (THz) imaging
Active imaging in the terahertz (THz) region of the electromagnetic spectrum is used to detect concealed objects or materials without causing damage. A target is illuminated with light in the frequency range 0.1-10 THz (i.e. millimetre-scale wavelengths). The light penetrates clothing and common packaging materials, such as cardboard and plastic, but not metals. THz photons also do not have sufficient energy to cause ionisation (i.e. THz light is “non-destructive”).
As a result, THz light can be used to safely screen people for hidden metallic objects, typically weapons. Other materials, such as explosives and biohazards, may exhibit unique spectral fingerprints in the THz range which can be used for identification and chemical analysis.