# Application of Near-Infrared Spectroscopy for Monitoring and/or Control of Composting Processes
## 4 Near-infrared Spectroscopy
However, due to  
Figure 4: Chemometric methods for NIR spectra analysis (adapted from [PERSON] et al. [46]). the appearance of multicollinearity problems, nonlinearity of the data can occur. Multicollinearity includes a number of predictors that is greater than the number of samples, which makes the covariance matrix singular and noninvertible; subsequently, finding a unique solution is impossible from least squares [54]. The alternative is to use nonlinear models, such as artificial neural networks (ANNs). The application of ANNs started in the 1980s, along with development of computational techniques based on self-organizing properties and parallel information systems [55]. Due to their capability to solve real problems, ANNs have been extensively used in areas such as engineering [56; 57], biology [57], and agriculture [58]. ANNs mimic the data processing of the human brain. The relationship between the independent vs. dependent variables (inputs vs. outputs) is nonlinear, and this relationship is reflected by the neuronal linking network matrix of the network. Therefore, it is crucial in the development of ANNs to ensure consistent and significant experimental records for (i) training, (ii) testing, and (ii) validating the strength of the model [58]. A wide range of NIRS applications led to the development and constant improvement of NIRS devices' performance, which is confirmed by the increase in the number of published studies on this topic (Figure 1). In general, any NIR instrument includes (i) a source of radiation, (ii) a device for the interaction of the sample and radiation source, (iii) a wavelength selector, (iv) a detector, (v) a device for data collection, processing, and storage, as well as (vi) a device for controlling the instrument [59; 60]. The radiation source is a quartz bulb containing traces of iodine with a tungsten filament, with power in the range of 25-100 W and operating temperatures from 2000 to 3000 K, respectively. A queue of light-emitting diodes (LEDs) is used for vigorous NIR radiation sources in small instruments. The sample-radiation interaction device promotes interaction between monochromatic or polychromatic NIR radiation and the sample, obtaining spectral information about a particular sample as a function of its position with respect to the wavelength selector [59]. The frequency (wavelength) domain contains the spectral information utilized for qualitative and quantitative reasons. The key feature of the selection device for NIR wavelength separation is its resolution. It denotes the capacity to distinguish between two relatively near wavelengths. This feature is determined by the bandwidth of the peaks in the samples under consideration. The detector is a device used to measure the intensity of radiation. The optical and electrical characteristics of semiconductors are determined by their energy bandgap. A disadvantage of PbS and PbSe detectors is the nonlinearity of their response. However, they exhibit good detectivity over the entire NIR range, even beyond the typical NIR spectral range. The instrument controller, user interface, and data acquisition/storage device is a microcomputer or microprocessor with the capability to control the main functions on the spectrophotometer and to acquire and process the detector intensities to produce the final spectral information (i.e., absorbance intensities as wavelength functions). The instrument software should have the capability to store spectral data in several formats compatible with the chemometric software package to be used [59]. In contrast, with regard to the spectrophotometers with a wide wavelength range and high spectral resolution, the development of so-called dedicated instruments has expanded recently (Figure 1). These dedicated instruments are portable; their size has been reduced, together with a reduction in wavelength ranges to address specific applications (tailored for customer requirements).