Diffuse optics (DO) encompasses a range of photonics tools based on the study of random photon migration in highly scattering media—biological tissues in particular. Due to its unique features, DO is emerging as a powerful means for clinical or homecare diagnostics.¹ The basic physics of DO is related to the detection of temporal or spatial alteration in photon distribution re-emitted on the tissue surface.² Due to the low power (typically few mW) of injected near-infrared light (600- to 1100-nm range), DO is inherently noninvasive. The photon temporal (or spatial) distribution carries information on the absorption—ultimately related to tissue chemical composition, such as water, lipid, collagen content, oxy- and deoxy-hemoglobin concentration, cytochrome c-oxidase³—and scattering properties—linked to tissue microstructure. Further, DO is one of the few noninvasive modalities capable of providing functional information, e.g., brain or muscle activation.⁴ It can be operated noncontact through remote light illumination and collection.⁵ It explores the tissue well below the skin at depths of up to 2 to 4 cm. It can provide quantitative operator-independent assessment of the tissue status, such as hemoglobin oxygenation level in the brain. Finally, it is highly scalable, sharing the same technology from large clinical tomographic systems down to wearable devices or homecare appliances.⁶ For all these aspects, DO is attracting more and more interest in many fields, such as monitoring vital signs like brain oxygenation in critical care or during interventions,⁷ tumor diagnostics as for breast cancer,⁸ investigating the impact of lifestyle and nutrition on our body,⁹ or other fields such as neuroscience and psychology,¹⁰ sports, and leisure.¹¹ Even further, the operator independence, the depth sensitivity, and the scalability in addition to noninvasiveness make this option attractive for telemedicine and homecare of patients remotely.

The possibility to probe human tissue in depth and non-invasively by visible and near-infrared light^1,2 pushed scientists and technologists to develop physics, methodologies, and instrumentation to study photon propagation in highly scattering media. In the 600 to 1100 nm wavelength range, human tissues are diffusive media characterized by a scattering coefficient much higher than the absorption. Furthermore, visible and near-infrared diffuse spectroscopy (NIRS) experienced a huge improvement as a suitable technique for detecting the structural or functional information inside the tissues by studying the light-tissue interaction process.

There is a growing number of applications at both a clinical and consumer level where near infrared spectroscopy (NIRS) is used to noninvasively monitor tissue hemodynamics in skeletal muscle or cerebral cortex. By exploiting the capability of near infrared light to penetrate deeply into biological tissues and the specific spectral features of oxygenated and deoxygenated hemoglobin, it is in fact possible, from light attenuation measurements, to noninvasively estimate the molar concentration of these chromophores in the tissue under investigation, and then retrieve tissue oxygen saturation (${\mathrm{S}}_{\mathrm{t}}{\mathrm{O}}_2$). This parameter can be used as a biomarker for physio-pathological conditions in the clinics¹ or as an indicator of skeletal muscle fatigue during exercise and sports activities.² Due to the diffusive nature of biological tissues, ${\mathrm{S}}_{\mathrm{t}}{\mathrm{O}}_2$ estimate requires either discrimination of absorption and scattering contribution, as provided by the time domain (TD) NIRS or frequency domain (FD) NIRS,³ or the adoption of specific measurement strategy, like in the space-resolved reflectance (SRS) approach.⁴

A high power setup for multichannel time-domain (TD) functional near infrared spectroscopy (fNIRS) measurements with high efficiency detection system was developed. It was fully characterized based on international performance assessment protocols for diffuse optics instruments, showing an improvement of the signal-to-noise ratio (SNR) with respect to previous analogue devices, and allowing acquisition of signals with sampling rate up to 20 Hz and source-detector distance up to 5 cm. A resting-state measurement on the motor cortex of a healthy volunteer was performed with an acquisition rate of 20 Hz at a 4 cm source-detector distance. The power spectrum for the cortical oxy- and deoxyhemoglobin is also provided.

Time-resolved reflectance spectroscopy (TRS), a nondestructive technique, can help the industry to provide high-quality fruit to encourage pear consumption. The absorption coefficient measured by TRS at 670 nm (μ_a670) represents a maturity index for pear fruit, with less mature pears high μ_a670 and more mature low μ_a670. The aim of this work was to study the quality characteristics, the sensory profiles and the ethylene production of ‘Abate Fetel’ pears sorted at harvest in different TRS maturity classes and stored in different atmospheres. At harvest, 540 pears were measured by TRS for μ_a670, ranked by μ_a670 in three maturity classes (less-LeM, medium-MeM and more-MoM mature) and randomized in nine samples according to 1-MCP treatment (treated, control), storage time (4–6 months) and atmosphere (air-NA; CA: 8–12 kPa O₂, 1 kPa CO₂). Fruits were examined at harvest and after 7 days of poststorage shelf life for skin color, firmness, soluble solids, acidity and ethylene production and were submitted to sensory analysis. At harvest and after storage, MoM pears were less green and showed a higher SSC content than LeM ones. After storage, MoM pears produced less ethylene and were perceived to be firmer (especially in 1-MCP-treated pears), more astringent and less juicy (when stored for 6 months) than LeM ones.

Mango fruit is a rich source of bioactive compounds such as carotenoids, phenolics, and ascorbic acid. This research aimed at predicting the content of these bioactive compounds in ‘Tommy Atkins’ mangoes using optical properties, i.e., the absorption coefficients related to chlorophylls (µ_a630, µ_a650, µ_a670, µ_a690) and carotenoids (µ_a540), and the scattering parameters (Mie’s A and b), measured during the shelf-life period at 20 °C by time-resolved reflectance spectroscopy. The µ_a540 and Mie’s b increased during shelf-life, while µ_a670 and Mie’s A decreased. Ascorbic acid (AA) and the total antioxidant capacity decreased during shelf-life, while the total carotenoids increased, and the total phenols (TPC) did not significantly change. The major constituent of the nonsaponified extracts, (all-E)-β-carotene, increased during the shelf-life period. A similar trend was observed for the total (all-E)-violaxanthin esters, the total (9Z)-violaxanthin esters and the total neoxanthin esters. Carotenoids are responsible for the yellow-orange color of mangoes: (all-E)-β-carotene was mainly related to a* and h° pulp color while the total (all-E)-violaxanthin esters were mainly linked to b*, C*, and the yellowness index. Using multiple regression analysis, good prediction models were achieved for the total carotenoids (R²_adj = 83.1%), the total xanthophylls (R²_adj = 78%), (all-E)-β-carotene (R²_adj = 77%) and the total (all-E)-violaxanthin esters (R²_adj = 74%), while less satisfactory predictions were obtained for AA and TPC.