Online Spectral Imaging in Process Analytics - Introduction to Spectral Imaging

Spectral Imaging or Chemical Imaging is the determination of the chemical identity of species and the visualization of its distribution. Optical spectroscopy is able not only to detect the chemical substrate by its wavelength specific absorption but also to detect differences in the wavelength dependent scattering behavior of particulates. The laterally resolved spectroscopy produces a three-dimensional data cube with two local axes, x and y and a spectral axis λ. Figure 1 illustrates the essential differences of the different techniques used to measure spectral images. In the so-called Whiskbroom Imaging (“mapping”), defined object areas or the entire object is measured point-by-point. This type of imaging is very flexible in relation to the object and the grid size and generally requires only a single detector; such as a photomultiplier tube (PMT) or a diode array (DAD). A “staring” imager (“imaging”) takes two-dimensional images in a series, one after the other, at several wavelengths. A prerequisite for this technique is that the object must remain stationary during the measurement (“Stop Motion”), thus only at-line applications can be realized.