[Enlarge image]Photograph of an angle-variable module illuminated by a halogen lamp. The drawn path demonstrates consecutively used light. A living firebug is shown for size comparison.
Thanks to the process of evolution, many insects have developed and optimized some sort of dielectric mirrors on their outer shells to efficiently reflect, absorb or scatter light.1 Particularly interesting is the angle-variable effect, where different colors appear when viewed from different angles. Inspired by this angle variability, we developed our most promising filter-based spectral module.
Filter-based spectroscopy has revolutionized spectral analysis, primarily due to its compactness and miniaturization potential. Unfortunately, most filter-based systems face a significant challenge in low detection efficiency because the majority of broadband incident light is reflected rather than transmitted through the filters used.
To address this efficiency challenge, we previously presented a folded-beam path architecture for several interference edge filters and a linear variable filter (LVF).2 The principle is based on multiple reflections with a fixed angle of incidence between the filters and a mirror placed parallel to them. By consecutively using collimated light, we demonstrated a substantial increase in detection efficiency compared with the conventional upright illumination principle. However, these spectral modules are constrained by a lack of design versatility. For example, if it is necessary to scale down the LVF-based module, the existing LVF must be replaced with one having a larger linear dispersion. Additionally, it is not possible to tailor the spectral output channels.
In recent work, we developed an efficient angle-variable module with no moving parts and a high degree of design freedom.3 It comprises two bandpass filters and inclined mirrors. The oblique light bundle, reflecting within the arrangement, generates a range of incidence angles on the filter for successive spectral components of the light. For our proof of concept, we used non-polarizing bandpass filters (VersaChrome) from Semrock, typically used for fluorescence microscopy, in a rotated assembly.4 By varying the angle of incidence between 0° and 60°, the transmitted center wavelength of these filters can be tuned by nearly 70 to 80 nm without splitting the s- and p-polarization. Our proposed static module was capable of detecting a wavelength range between 550 and 700 nm across 11 spectral channels. Using a line camera, we measured the transmission spectra of extra virgin olive oil and its fluorescence spectra excited by a 532-nm solid-state laser, showing reliable results in comparison with a commercial spectrometer.
Next, we will continue to develop modules with continuous spectral acquisition using tailored divergent or convergent light instead of strongly parallel light. Ultimately, we aim to apply a freeform mirror to manipulate efficiency, the number of channels and spectral resolution, using only one or several interference filters to create new handheld sensors.
Researchers
Aliaksei Kobylinskiy, Matthias Kraus and Lukas Werner, University of Applied Sciences Jena and University of Kassel, Germany
Hartmut Hillmer, University of Kassel, Germany
Robert Brunner, University of Applied Sciences Jena and Fraunhofer Institute for Applied Optics and Precision Engineering, Germany
References
1. M.A. Barry et al. Sci. Rep. 10, 12024 (2020).
2. A. Kobylinskiy et al. Appl. Opt. 61, 9996 (2022).
3. A. Kobylinskiy et al. Opt. Lett. 49, 638 (2024).
4. A.J. Bares et al. Optica 7, 1587 (2020).