Novel concept for light-direction sensors based on the wave nature of light

Sun sensors are designed to detect and measure both the intensity and direction of light. They play a key role in fields such as renewable energy, optics, and aerospace, where the ability to detect the angle of incidence of light has multiple applications. For example, in energy systems, the sun sensors are used in solar trackers to optimise the orientation of solar panels. In optics, they help align light beams within precision systems, and in the aerospace sector, light-angle sensors are used for attitude determination and control [1][2].

Traditionally, conventional sun sensors work by relying on geometric principles, using the ray approximation of light to detect its direction [3]. In such systems, angular sensitivity is reached by projecting light through different occlusive elements onto photodetectors. The most basic approach employs a planar photodetector whose signal varies with the cosine of the incidence angle, providing only coarse resolution. More advanced designs include four-quadrant detectors or CMOS technology with internal walls that cast angle-dependent shadows [4][5], while digital alternatives use arrays of photodetectors that are exposed through aligned apertures or slits to determine direction [6][7]. Other methods include waveguides that react to both direction and wavelength, resonant filters based on diffraction, or microstructures that selectively block or transmit light [8].

 

Despite their wide use, conventional light-angle sensors face multiple limitations. Several sun sensors require bulky, precisely aligned optical components. Geometric method-based sun sensors present a trade-off between angular resolution and field of view, while digital sensors frequently underuse the active area. Resonant filter-based systems are usually sensitive to certain wavelengths, which restricts their applicability to broadband light sources. Consequently, current designs often reveal trade-offs between accuracy, reliability, compactness, and cost. In this view, this work presents the analysis and development of a novel light-angle sensor that addresses the limitations of traditional sun sensors by exploiting the wave nature of light [9]. The angular dependence of the transmittance spectrum through an interferent optical layer is the basis of the proposed concept. This effect can be attained through engineered structures such as diffraction braggs, gratings, metasurfaces, or rugate filters [10-13]. This angularly modulated spectral response enables compact and high-resolution sensing without the need for external optics.

Figure 1: Representation of the measurement principle: a) angle dependence of transmission band in an interference filter, b) Transmission spectra sampled through different sensing elements featuring different coloured filters (angle-independent), and c) Representation of a device sensible to both, elevation and azimuth angles. The device would use an array of elements combining different coloured and interference filters.

 

Figure 1 provides a conceptual illustration of the sensor, which consists of two main components: the interference filter and coloured absorptive filters. When source light interacts with the interferent layer, a specific transmittance pattern is generated, which varies as the angle of incidence changes. This modulated light then passes through the coloured filters, each of which transmits a specific wavelength band to a corresponding photodetector. Depending on the received transmittance distribution, the photodetectors generate different signal levels and, by comparing these signals, the system can accurately determine the direction of the incoming light. Therefore, the interferent filter is the primary sensing component, as it translates the angle of incidence into a unique spectral pattern, while coloured filters isolate specific wavelength regions, allowing each photodetector to respond differently depending on the incidence angle, converting angular information into measurable spectral variations. Figure 2 illustrates this principle through a simulated unpolarised transmittance spectrum of a TiO₂-Al₂O₃ bragg-based structure, which clearly shows angle-dependent spectral features, and two commercial colour-filtered photodetectors. In particular, Figure 2a shows the transmittance as a function of wavelength and incidence angle, overlaid with the spectral response curves of red and blue filters. Based on the interaction of the generated transmittance spectrum and the sensitivity of each filter, Figure 2b displays the resulting electrical signals of the photodetectors. Finally, Figure 2c exhibits sensing response of the proposed sun sensor. In particular, this figure shows the relative change between both signals across the angular range, which reveals a monotonic response. The obtained behaviour confirms that the chosen combination of the bragg-based structure and coloured filters enables accurate, continuous, and high-resolution light-angle detection, validating the viability of the proposed sensor concept.

 

Figure 2: Interferential light-angle sensor response: a) Real coloured-filters response vs. Unpolarized transmittance spectrum of a particular interference filter structure, b) Response of photodetectors to the interaction between coloured filters and the incidence-angle-dependent spectrum of the interferent element, and c) Relative change revealed by the complete sensor.

In conclusion, this work presents a first insight into a new light-direction sensing approach based on wave optics. By combining an angle-sensitive interferent filter with colour-selective detection, the proposed sensor offers a promising balance of compactness, accuracy, robustness, cost-effectiveness, integration potential, and angle estimation without the need for bulky optics or complex alignment. Simulations confirm the feasibility of the concept and its potential for integration into next-generation light-direction detectors. Future work will focus on experimental validation, refinement of the filter design, and consideration of fabrication constraints for practical implementation.

 

Acknowledgements

This work has been supported by the ESA-OSIP through CN-4000145474 (Activity ID EISI_S_I-2024-01142) and through the project TED2021-131552B-C22 funded by European Union "NextGenerationEU".

 

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