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A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Optical Sensors".

Deadline for manuscript submissions: closed (25 September 2023) | Viewed by 27625

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Special Issue Editor

Dr. Carlos Angulo Barrios

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Department of Photonics and Bioenginering, CEMDATIC, ETSI Telecomunicación, Universidad Politécnica de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain
Interests: photonics; materials science; sensors; nanotechnology
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Special Issue Information

Dear Colleagues,

Optical sensors are a broad class of devices based on the use of light to detect variations in physical or chemical magnitudes. These sensors exploit the fact that different measurands of light (intensity, wavelength, phase, polarization) can be altered by the effect of the magnitude of interest on the sensitive part of the sensor. The multiple properties of light, high availability of optical components and systems, and abundance of optical responsive and sensitive materials have allowed optical sensors to be applied to a huge number of applications. In particular, key economic and societal sectors, such as healthcare, food and agriculture, manufacturing and construction, environmental monitoring, and defense and security, have benefited from the use of optical sensors, either as simple and cheap devices or integrated in rather sophisticated multisensor architectures.

This Special Issue focuses on low-cost optical sensors, that is, optical sensors based on simple configurations that can be implemented by using cost-effective materials and fabrication processes. Cost-saving and simplicity requirements may limit the performance of specific aspects of these devices as compared with expensive and complex optical sensors; however, this apparent drawback can be overcome by superior affordability and competitiveness in low- and moderate-demand applications. Both theoretical and experimental papers and comprehensive reviews are welcome. Topics include, but are not limited to, (bio)chemical, temperature, velocity, liquid level, pressure, displacement (position), vibration, force radiation, strain, acoustic field and electric field optical sensors based on cost-effective materials, components, and processing. We invite and encourage your participation in this Special Issue.

Dr. Carlos Angulo Barrios
Guest Editor

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Keywords

optical sensor
cost-effectiveness
affordability
sensor processing
(bio)chemical optical sensor
temperature optical sensor
velocity optical sensor
liquid level optical sensor
pressure optical sensor
displacement optical sensor
vibration optical sensor
force radiation optical sensor
strain optical sensor
acoustic field optical sensor
electric field optical sensor

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Published Papers (10 papers)

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Research

19 pages, 7689 KiB

Open AccessArticle

Evaluating the Performance of Video-Based Automated Passenger Counting Systems in Real-World Conditions: A Comparative Study

by Cristina Pronello and Ximena Rocio Garzón Ruiz

Sensors 2023, 23(18), 7719; https://doi.org/10.3390/s23187719 - 7 Sep 2023

Cited by 6 | Viewed by 3108

Abstract

Automatic passenger counting (APC) systems in public transport are useful in collecting information that can help improve the efficiency of transport networks. Focusing on video-based passenger counting, the aim of this study was to evaluate and compare an existing APC system, claimed by its manufacturer to be highly accurate (98%), with a newly developed low-cost APC system operating under the same real-world conditions. For this comparison, a low-cost APC system using a Raspberry Pi with a camera and a YOLOv5 object detection algorithm was developed, and an in-field experiment was performed in collaboration with the public transport companies operating in the cities of Turin and Asti in Italy. The experiment shows that the low-cost system was able to achieve an accuracy of 72.27% and 74.59%, respectively, for boarding and alighting, while the tested commercial APC system had an accuracy, respectively, of 53.11% and 55.29%. These findings suggest that current APC systems might not meet expectations under real-world conditions, while low-cost systems could potentially perform at the same level of accuracy or even better than very expensive commercial systems. Full article

(This article belongs to the Special Issue Low-Cost Optical Sensors)

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27 pages, 27960 KiB

Open AccessArticle

Image-Based Sensor for Liquid Level Monitoring during Bottling with Application to Craft and Home-Brewing

by Josip Musić, Ivo Stančić, Barbara Džaja and Vesna Pekić

Sensors 2023, 23(16), 7126; https://doi.org/10.3390/s23167126 - 11 Aug 2023

Cited by 1 | Viewed by 2865

Abstract

Although craft and home brewing have fueled the beer renaissance in the last decade, affordable, reliable, and simple sensing equipment for such breweries is limited. Thus, this manuscript is motivated by the improvement of the bottle-filling process in such settings with the objective of developing a liquid level sensor based on a novel application of the known optical phenomena of light refraction. Based on the different refraction indices of liquid and air (and critical angle based on Snell’s law), along with a novel LED light source positioning, a reliable liquid level sensor system was built with the aid of an embedded microcontroller. The used operating principle is general and can be used in applications other than the proposed one. The proposed method was extensively tested in a laboratory and limited production settings with a speed of 7 Hz using different liquids and container shapes. It was compared for accuracy to other sensing principles such as ultrasound, infrared, and time-of-flight. It demonstrated comparable or better performance with a height error ranging between −0.1534 mm in static conditions and 1.608 mm for realistic dynamic conditions and good repeatability on the production line with a 4.3 mm standard deviation of the mean. Full article

(This article belongs to the Special Issue Low-Cost Optical Sensors)

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16 pages, 4610 KiB

Open AccessArticle

An Edge-Filtered Optical Fiber Interrogator for Thermoplastic Polymer Analysis

by Vincent Backmann, Felix Dorner and Klaus Drechsler

Sensors 2023, 23(6), 3300; https://doi.org/10.3390/s23063300 - 21 Mar 2023

Cited by 1 | Viewed by 1799

Abstract

The present paper deals with the determination of thermodynamic quantities of thermoplastic polymers by using an optical fiber interrogator. Typically, laboratory methods such as differential scanning calorimetry (DSC) or thermomechanical analysis (TMA) are a reliable state-of-the-art option for thermal polymer analysis. The related laboratory commodities for such methods are of high cost and are impractical for field applications. In this work, an edge-filter-based optical fiber interrogator, which was originally developed to detect the reflection spectrum of fiber Bragg grating sensors, is utilized for the detection of the boundary reflection intensities of the cleaved end of a standard telecommunication optical fiber (SMF28e). By means of the Fresnel equations, the temperature-dependent refractive index of thermoplastic polymer materials is measured. Demonstrated with the amorphous thermoplastic polymers polyetherimide (PEI) and polyethersulfone (PES), an alternative to DSC and TMA is presented as the glass transition temperatures and coefficients of thermal expansion are derived. A DSC alternative in the semi-crystalline polymer analysis with the absence of a crystal structure is shown as the melting temperature and cooling-rate-dependent crystallization temperatures of polyether ether ketone (PEEK) are detected. The proposed method shows that thermal thermoplastic analysis can be performed with a flexible, low-cost and multipurpose device. Full article

(This article belongs to the Special Issue Low-Cost Optical Sensors)

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15 pages, 3325 KiB

Open AccessArticle

Development of a Frugal, In Situ Sensor Implementing a Ratiometric Method for Continuous Monitoring of Turbidity in Natural Waters

by Raul Sanchez, Michel Groc, Renaud Vuillemin, Mireille Pujo-Pay and Vincent Raimbault

Sensors 2023, 23(4), 1897; https://doi.org/10.3390/s23041897 - 8 Feb 2023

Cited by 4 | Viewed by 2982

Abstract

Turbidity is a commonly used indicator of water quality in continental and marine waters and is mostly caused by suspended and colloidal particles such as organic and inorganic particles. Many methods are available for the measurement of turbidity, ranging from the Secchi disk to infrared light-based benchtop or in situ turbidimeters as well as acoustic methods. The operational methodologies of the large majority of turbidity instruments involve the physics of light scattering and absorption by suspended particles when light is passed through a sample. As such, in the case of in situ monitoring in water bodies, the measurement of turbidity is highly influenced by external light and biofouling. Our motivation for this project is to propose an open-source, low-cost in situ turbidity sensor with a suitable sensitivity and operating range to operate in low-to-medium-turbidity natural waters. This prototype device combines two angular photodetectors and two infrared light sources with different positions, resulting in two different types of light detection, namely nephelometric (i.e., scattering) and attenuation light, according to the ISO 7027 method. The mechanical design involves 3D-printed parts by stereolithography, which are compatible with commercially available waterproof enclosures, thus ensuring easy integration for future users. An effort was made to rely on mostly off-the-shelf electronic components to encourage replication of the system, with the use of a highly integrated photometric front-end commonly used in portable photoplethysmography systems. The sensor was tested in laboratory conditions against a commercial benchtop turbidimeter with Formazin standards. The monitoring results were analyzed, obtaining a linear trendline from 0 to 50 Nephelometric Turbidity Unit (NTU) and an accuracy of +/−0.4 NTU in the 0 to 10 NTU range with a response time of less than 100 ms. Full article

(This article belongs to the Special Issue Low-Cost Optical Sensors)

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18 pages, 3084 KiB

Open AccessCommunication

Integrated Optical Deformation Measurement with TIR Prism Rods

by Alexander Wolf

Sensors 2023, 23(2), 943; https://doi.org/10.3390/s23020943 - 13 Jan 2023

Cited by 1 | Viewed by 1709

Abstract

In this paper, a novel optical measurement principle for deformation, especially torsion, is presented. A laser beam is guided via total internal reflection (TIR) in a prism rod. Every single reflection causes an increasing change in the beam path, which can be measured [...] Read more.

10 mm

and a length of

120 mm

, the system achieves torsion sensitivities between

350 µ m / °

and more than

7000 µ m / °

, depending on the actual torsion angle

φ

. A decency level of sensitivity is defined for comparison, which is exceeded by a factor of

~ 55

φ = 0

. The presented principle of TIR prism rods can be adapted to measure different load cases. Using two laser beams, bending and torsion can be distinguished and combined load cases analyzed. The resulting system can be integrated into machine elements, such as screws, to perform condition monitoring on mechanically loaded components. Full article

(This article belongs to the Special Issue Low-Cost Optical Sensors)

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13 pages, 2983 KiB

Open AccessArticle

Low-Cost Photoreactor to Monitor Wastewater Pollutant Decomposition

by Alberto Ruiz-Flores, Araceli García, Antonio Pineda, María Brox, Andrés Gersnoviez and Eduardo Cañete-Carmona

Sensors 2023, 23(2), 775; https://doi.org/10.3390/s23020775 - 10 Jan 2023

Cited by 2 | Viewed by 2751

Abstract

Actually, the quality of water is one of the most important indicators of the human environmental impact, the control of which is crucial to avoiding irreversible damage in the future. Nowadays, in parallel to the growth of the chemical industry, new chemical compounds have been developed, such as dyes and medicines. The increasing use of these products has led to the appearance of recalcitrant pollutants in industrial wastewater, and even in the drinking water circuit of our populations. The current work presents a photoreactor prototype that allows the performance of experiments for the decomposition of coloured pollutants using photocatalysis at the laboratory scale. The design of this device included the study of the photometric technique for light emission and the development of a software that allows monitoring the dye degradation process. Open-source hardware platforms, such as Arduino, were used for the monitoring system, which have the advantages of being low-cost platforms. A software application that manages the communication of the reactor with the computer and graphically displays the data read by the sensor was also developed. The results obtained demonstrated that this device can accelerate the photodegradation reaction in addition to monitoring the changes throughout the process. Full article

(This article belongs to the Special Issue Low-Cost Optical Sensors)

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12 pages, 5022 KiB

Open AccessArticle

A Self-Referenced Refractive Index Sensor Based on Gold Nanoislands

by Carlos Angulo Barrios, Teona Mirea and Miguel Huerga Represa

Sensors 2023, 23(1), 66; https://doi.org/10.3390/s23010066 - 21 Dec 2022

Cited by 5 | Viewed by 3576

Abstract

We report on a self-referenced refractive index optical sensor based on Au nanoislands. The device consists of a random distribution of Au nanoislands formed by dewetting on a planar SiO₂/metal Fabry–Pérot cavity. Experimental and theoretical studies of the reflectance of this configuration reveal that its spectral response results from a combination of two resonances: a localized surface plasmon resonance (LSPR) associated to the Au nanoislands and the lowest-order anti-symmetric resonance of the Fabry–Pérot cavity. When the device is immersed in different fluids, the LSPR contribution provides high sensitivity to refractive index variations of the fluid, whereas those refractive index changes have little impact on the Fabry–Pérot resonance wavelength, allowing its use as a reference signal. The self-referenced sensor exhibits a spectral sensitivity of 212 nm/RIU (RIU: refractive index unit), which is larger than those of similar structures, and an intensity sensitivity of 4.9 RIU⁻¹. The proposed chip-based architecture and the low cost and simplicity of the Au nanoisland synthesis procedure make the demonstrated sensor a promising self-referenced plasmonic sensor for compact biosensing optical platforms based on reflection mode operation. Full article

(This article belongs to the Special Issue Low-Cost Optical Sensors)

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18 pages, 5791 KiB

Open AccessArticle

Dust Particle Counter for Powder Bed Fusion Process

by Stanisław Karcz, Grzegorz Skrabalak, Andrzej Brudnik and Grzegorz Gajoch

Sensors 2022, 22(19), 7614; https://doi.org/10.3390/s22197614 - 8 Oct 2022

Viewed by 2018

Abstract

The paper presents a novel dust detector based on an innovative laser system that can be successfully used in applications where continuous dust monitoring is necessary. The measurements obtained with FeNi18Co9Mo5 (maraging MS1 steel) particles are compared with the particle fall times calculated using the Navier–Stokes equation. The measurement powder was subjected to sieve analysis and laser system detection. Based on the results obtained, a formula was developed to determine the dust concentration depending on the number and size of particles. With filtration applied, the detector measurement range was from 16 to 100 µm. The developed solution can be the basis for the development of a dedicated sensor for powder bed fusion processes. Full article

(This article belongs to the Special Issue Low-Cost Optical Sensors)

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10 pages, 2656 KiB

Open AccessArticle

Real Time Water-In-Oil Emulsion Size Measurement in Optofluidic Channels

by Juliana N. Schianti, Igor Y. Abe, Marco I. Alayo and Daniel O. Carvalho

Sensors 2022, 22(13), 4999; https://doi.org/10.3390/s22134999 - 2 Jul 2022

Cited by 2 | Viewed by 1990

Abstract

In this work, we investigated a platform for real-time emulsion droplet detection and size measurement in optofluidic platforms. An 8.2 µm core diameter input optical fiber and a multi-mode Gradient Refractive Index (GRIN) output fiber were integrated into an acrylic microfluidic channel platform consisting of three layers. Water-in-oil emulsions were investigated, since relevant applications have emerged in the recent past for these types of emulsions, such as drug encapsulation as well as droplet-based Polymerase Chain Reaction (PCR) amplification of DNA, among others. The main contribution of this work is in understanding the main physical phenomena (i.e., total internal reflection, refraction, and interference) behind the complex transmittance pattern obtained for these droplets. For this purpose, a frequency domain electromagnetic wave propagation modelling of the structure using the Finite Element Method (FEM) was used along with experimental measurements. Full article

(This article belongs to the Special Issue Low-Cost Optical Sensors)

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22 pages, 13354 KiB

Open AccessArticle

Scheimpflug Camera-Based Technique for Multi-Point Displacement Monitoring of Bridges

by Lei Xing, Wujiao Dai and Yunsheng Zhang

Sensors 2022, 22(11), 4093; https://doi.org/10.3390/s22114093 - 27 May 2022

Cited by 7 | Viewed by 3035

Abstract

Owing to the limited field of view (FOV) and depth of field (DOF) of a conventional camera, it is quite difficult to employ a single conventional camera to simultaneously measure high-precision displacements at many points on a bridge of dozens or hundreds of meters. Researchers have attempted to obtain a large FOV and wide DOF by a multi-camera system; however, with the growth of the camera number, the cost, complexity and instability of multi-camera systems will increase exponentially. This study proposes a multi-point displacement measurement method for bridges based on a low-cost Scheimpflug camera. The Scheimpflug camera, which meets the Scheimpflug condition, can enlarge the depth of field of the camera without reducing the lens aperture and magnification; thus, when the measurement points are aligned in the depth direction, all points can be clearly observed in a single field of view with a high-power zoom lens. To reduce the impact of camera motions, a motion compensation method applied to the Scheimpflug camera is proposed according to the characteristic that the image plane is not perpendicular to the lens axis in the Scheimpflug camera. Several tests were conducted for performance verification under diverse settings. The results showed that the motion errors in x and y directions were reduced by at least 62% and 92%, respectively, using the proposed method, and the measurements of the camera were highly consistent with LiDAR-based measurements. Full article

(This article belongs to the Special Issue Low-Cost Optical Sensors)

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