# A 1064 nm Dispersive Raman Spectral Imaging System for Food Safety and Quality Evaluation
## 3 Application to Authenticate Turmeric Powder
### Subsurface Detection of Turmeric Powder and Metanil Yellow  
Spatially offset Raman spectroscopic (SORS) technique retrieves Raman information of the inner content of the sample from diffusely scattering media [36]. The SORS technique collects scattering signal from a series of offset distance from the laser excitation spot [70]. It uses a laser focus unit at a fixed 45\({}^{\circ}\) incident angle to deliver laser light onto the sample surface. An optical probe is fixed perpendicular to the sample to collect Raman scattering signal. Figure 8 shows the SORS technique to acquire Raman information of the inner content. First, Raman spectrum of sample at the no-offset position is measured by overlapping the focal point of laser and the optical probe. Then, the sample and the laser focus unit is moved at an incremental step away from the probe creating an increased offset distance between the probe and the laser focus point on the sample. As the offset distance is increased, the laser penetrates deeper through the thick top surface layer. At an increased offset distance,  
Figure 6: Single-band binary detection images of turmeric-metanil yellow mixture samples. Figure 7: Relationship between actual metanil yellow concentration and percentage of its detected pixels in 1147 cm\({}^{-1}\) binary image. the signal contribution from the deep subsurface layer increases, gradually outweighing the signal contribution of the top surface layer, enabling the detection of subsurface layer material. The SORS signal can be used to retrieve Raman information of multiple layers from the same sample. The SORS method has been applied for diagnosis of breast cancer by measuring calcification composition through varying tissue thickness [71]. It has been applied for evaluation of internal maturity of tomatoes by evaluating the carotenoids change in them [72]. Although, the SORS method can be used for detection and analysis of subsurface layer material, it cannot be used to measure the depth of the sample. Empty gelatin capsules of size 000 (length 26.1 mm, single-wall thickness 0.11 mm) were obtained from Capsuline (Pompano Beach, FL, USA) to prepare capsule-turmeric samples. A one-layer capsule sample was prepared by packing turmeric tightly in a capsule and capping it. A two-layer capsule sample was prepared by inserting the turmeric packed capsule inside an empty capsule and capping the outer capsule only. Similarly, three-, four-, and five-layer capsule samples were prepared by inserting the turmeric packed capsule inside empty capsules and capping the outermost capsule. The wall thickness of the capsule layers ranged from 0.11 mm (one-layer capsule) to 0.55 mm (five-layer capsule). Each capsule sample was held immobile in the positioning platform. The 1064 nm laser module focused a laser spot on the capsule body near the edge of its cap. First, Raman spectrum of capsule-turmeric sample was obtained. Then, the positioning table moved the sample and the laser focus unit in incremental steps in the right direction away from the Raman probe, creating an increased offset distance for each subsequent spectral acquisition. A total of 31 spectra along the longitudinal flat surface body of each capsule sample proceeding away from the capped end were collected using 20 s exposure time, 400 mW laser power, and a step size of 0.1 mm from no offset to 3 mm offset range. The spatially offset Raman spectra of samples were subtracted with the empty capsule spectrum to eliminate the influence of capsule signal in the SORS data. Figure 9 shows the spatially offset Raman spectra of the one-layer capsule-turmeric sample. The 1599 cm\({}^{-1}\) and 1631 cm\({}^{-1}\) are turmeric peaks. The pattern of increasing 1599 cm\({}^{-1}\) and 1631 cm\({}^{-1}\) peak intensity from no offset up to 1.0 mm offset shows that the signal contribution of the subsurface material (turmeric) gradually increases as the offset distance increases. After 1.0 mm offset, the signal contribution from turmeric layer gradually decreased.