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. 2023 Mar 6;23(5):2857.
doi: 10.3390/s23052857.

A Dew-Condensation Sensor Exploiting Local Variations in the Relative Refractive Index on the Dew-Friendly Surface of a Waveguide

Subin Hwa  1 , Eun-Seon Sim  1 , Jun-Hee Na  2 , Ik-Hoon Jang  3 , Jin-Hyuk Kwon  4 , Min-Hoi Kim  1   4
Affiliations

A Dew-Condensation Sensor Exploiting Local Variations in the Relative Refractive Index on the Dew-Friendly Surface of a Waveguide

Subin Hwa et al. Sensors (Basel). .

Abstract

We propose a sensor technology for detecting dew condensation, which exploits a variation in the relative refractive index on the dew-friendly surface of an optical waveguide. The dew-condensation sensor is composed of a laser, waveguide, medium (i.e., filling material for the waveguide), and photodiode. The formation of dewdrops on the waveguide surface causes local increases in the relative refractive index accompanied by the transmission of the incident light rays, hence reducing the light intensity inside the waveguide. In particular, the dew-friendly surface of the waveguide is obtained by filling the interior of the waveguide with liquid H2O, i.e., water. A geometric design for the sensor was first carried out considering the curvature of the waveguide and the incident angles of the light rays. Moreover, the optical suitability of waveguide media with various absolute refractive indices, i.e., water, air, oil, and glass, were evaluated through simulation tests. In actual experiments, the sensor with the water-filled waveguide displayed a wider gap between the measured photocurrent levels under conditions with and without dew, than those with the air- and glass-filled waveguides, as a result of the relatively high specific heat of the water. The sensor with the water-filled waveguide exhibited excellent accuracy and repeatability as well.

Keywords: dew-condensation sensor; dew-friendly surface; optical waveguide; refractive index; specific heat; water.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
(a) Schematic diagram of the sensor, (b) key denotations for the constituent media and surfaces of the sensor as well as those for the absolute refractive indices of the media, and (c) the path difference between the incident light rays, with and without dew.
Figure 2
Figure 2
The geometry of the waveguide with curvature and the design strategy in which the condition of the total reflection at the external interface is considered.
Figure 3
Figure 3
Optical simulation results (the light wavelength: 650 nm). Optical paths obtained with various internal media for the waveguide, i.e., (a) glass, (b) oil, (c) water, and (d) air.
Figure 4
Figure 4
In the optical simulation results: (a) variations in the intensity according to the simulated dew condensation for the various internal media, and (b) the regions on the waveguide surface where the incident light rays are transmitted under conditions with dew for the various internal media.
Figure 5
Figure 5
Responses of the actual sensors with the water-, glass-, and air-filled waveguides to dew condensation (the laser light wavelength: 650 nm).
Figure 6
Figure 6
The photos of the (a) air-filled waveguide, (b) glass-filled waveguide, and (c) water-filled waveguide before the dew formation (‘initial’), after the dew formation (‘with dew’), and after the dew evaporation (‘without dew’) in sequence.
Figure 7
Figure 7
Responses of the five actual sensors with the water-filled waveguides to dew condensation.
See this image and copyright information in PMC

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