Transillumination with non-ionizing radiation followed by the observation of transmitted and diffused light is the simplest, and probably the oldest method to obtain qualitative information on the internal structure of tissues or body sections. Although scattering precludes formation of high-definition image (unless complex techniques are employed), low resolution pictures complemented by information on the functional condition of the living sample can be extracted. In this context, we have investigated a portable optoelectronic instrumental configuration for efficient transillumination and image detection, even in ambient day-light, of in vivo samples with thickness up to 5 cm, sufficient for visualizing macroscopic structures. Tissue illumination is obtained with an extended source consisting in a matrix of 36 near infrared Vertical Cavity Surface Emitting Lasers (VCSELs) that is powered by a custom designed low-voltage current driver. In addition to the successful acquisition of morphological images of the hand dorsal vein pattern, functional detection of physiological parameters (breath and hearth rate) is achieved non-invasively by means of a monochrome camera, with a Complementary Metal Oxide Semiconductor (CMOS) sensor, turned into a wavelength selective image detector using narrow-band optical filtering.

Analysis and fine distinction of turbid fluids is challenging since standard techniques, usually applied to transparent liquids, fail to work properly due to scattering losses. The phenomenon of speckle pattern, which arises when such liquids are illuminated by coherent light, can be successfully applied to investigate the properties of these media. In this work, we exploit a simple instrumental configuration, combining a laser diode and a PC-interfaced, digital CMOS camera, for acquiring images and video of speckle pattern generated by illuminating turbid fluid samples that were obtained by dilution of opaque suspensions, such as commercial vegetable milk. By processing the acquired image sequences, we have been able to extract several statistical parameters that can be correlated with the concentration of the scattering elements of each sample, allowing an easy differentiation of the various fluids.

In this work we present a smart micro-opto-fluidic platform based on rectangular-section glass micro-capillaries with integrated reflectors. The micro-opto-fluidic setup was exploited for absorption spectroscopy measurements in the near infrared range from 1.0 μm to 1.7 μm. The external flat sides of the micro-capillaries are coated with thin Aluminum layers to create multipath micro-devices in which light can bounce multiple times. Broadband light provided by a Tungsten lamp is coupled into a multimode optical fiber and shone onto the flat surface of the micro-capillary. After travelling inside the micro-channel filled with the sample, radiation is directed towards an optical spectrum analyzer. The line-shape of the output light spectrum depends on the absorption features of the sample and, in particular, on the concentration of water in alcoholic solutions. Experimental results are in good agreement with the prediction given by the developed theoretical model that describes light transmission through the fluidic micro-channel. Moreover, a responsivity parameter, defined as the output power ratio at two wavelengths, is retrieved: water sensitivity is greatly enhanced by the multipath effect since it is proved to be directly proportional to the light bounces in the micro-channel.

A multiwavelength instrumental configuration, combining short-wave infra-red (SWIR) LEDs (with emission bands centered at $\lambda = 1.3\,\,\mu \text{m}$ , $\lambda = 1.45\,\,\mu \text{m}$ , and $\lambda = 2.15\,\,\mu \text{m}$ ) and InGaAs amplified photodiodes has been applied for contactless fluidic sensing by measuring light intensity transmittance across a channel microslide. The main goal is the proof of the principle of urea detection in water-based solutions, exploiting urea absorption band around $\lambda = 2.15\,\,\mu \text{m}$ . The functionality of the system has been tested on urea–water solutions with urea concentrations up to 0.2 g/ml (20 wt%), an interesting range for biomedical applications. Transmittance in the spectral range around $\lambda = 1.3\,\,\mu \text{m}$ is mainly affected by the fluid refractive index (real part). Around $\lambda = 2.15\,\,\mu \text{m}$ , the transmittance is conditioned by the absorption increment due to an increasing fraction of urea in the solution. Since even water (solvent of all tested solutions) absorbs around $\lambda = 2.15\,\,\mu \text{m}$ , its contribution can be accounted and compensated for by measuring the transmittance in the wavelength range around $\lambda = 1.45\,\,\mu \text{m}$ where water only exhibits a strong absorption band. By introducing $R_{1.45}(C)$ and $R_{2.15}(C)$ as the ratios between the output signal in presence of the solution and the output signal collected on the empty channel, at $\lambda = 1.45\,\,\mu \text{m}$ and $\lambda = 2.15\,\,\mu \text{m}$ , respectively, we have demonstrated that their ratio $\upsilon (C)\,\,= R_{1.45}(C)/R_{2.15}(C)$ is highly specific for urea detection with a sensitivity $S \sim 7$ (g/ml) ⁻¹ and limit of detection (LoD) $\approx ~0.006$ (g/ml).

Detection of air bubbles in fluidic channels plays a fundamental role in all that medical equipment where liquids flow inside patients’ blood vessels or bodies. In this work, we propose a multi-parameter sensing system for simultaneous recognition of the fluid, on the basis of its refractive index and of the air bubble transit. The selected optofluidic platform has been designed and studied to be integrated into automatic pumps for the administration of commercial liquid. The sensor includes a laser beam that crosses twice a plastic cuvette, provided with a back mirror, and a position-sensitive detector. The identification of fluids is carried out by measuring the displacement of the output beam on the detector active surface and the detection of single air bubbles can be performed with the same instrumental scheme, exploiting a specific signal analysis. When a bubble, traveling along the cuvette, crosses the readout light beam, radiation is strongly scattered and a characteristic fingerprint shape of the photo-detected signals versus time is clearly observed. Experimental testing proves that air bubbles can be successfully detected and counted. Their traveling speed can be estimated while simultaneously monitoring the refractive index of the fluid.

Ethylene glycol (EG) and isopropanol (ISO) are among the major toxic alcohols that pose a risk to human health. However, it is important to distinguish them, since EG is more prone to cause renal failure, and can thus be more dangerous when ingested than ISO. Analysis of alcohols such as isopropanol and ethylene glycol generally can be performed with a complex chromatographic method. Here, we present an optical method based on absorption spectroscopy, performed remotely on EG-ISO mixtures filling a microchannel. Mixtures of ethylene glycol in isopropanol at different volume concentrations were analyzed in a contactless manner in a rectangular-section glass micro-capillary provided with integrated reflectors. Fiber-coupled broadband light in the wavelength range 1.3–1.7 µm crossed the microchannel multiple times before being directed towards an optical spectrum analyzer. The induced zig-zag path increased the fluid–light interaction length and enhanced the effect of optical absorption. A sophisticated theoretical model was developed and the results of our simulations were in very good agreement with the results of the experimental spectral measurements. Moreover, from the acquired data, we retrieved a responsivity parameter, defined as power ratio at two wavelengths, that is linearly related to the EG concentration in the alcoholic mixtures.