A Versatile Optical Frontend for Multicolor Fluorescence Imaging with Miniaturized Lensless Sensors

Lensless imaging enables exceptionally compact fluorescence sensors, advancing applications in \textit{in vivo} imaging and low-cost, point-of-care diagnostics. These sensors require a filter to block the excitation light while passing fluorescent emissions. However, conventional thin-film interference filters are sensitive to angle of incidence (AOI), complicating their use in lensless systems. Here we thoroughly analyze and optimize a technique using a fiber optic plate (FOP) to absorb off-axis light that would bleed through the interference filter while improving image resolution. Through simulations, we show that the numerical aperture (NA) of the FOP drives inherent design tradeoffs: collection efficiency improves rapidly with a higher NA, but at the cost of resolution, increased device thickness, and fluorescence excitation efficiency. To illustrate this, we optimize two optical frontends with full-width at half maximums (FWHMs) of 8.3° and 45.7°. Implementing these designs, we show that angle-insensitivity requires filters on both sides of the FOP, due to scattering. In imaging experiments, the 520-$μ$m-thick high-NA design is 59$\times$ more sensitive to fluorescence while only degrading resolution by 3.2$\times$. Alternatively, the low-NA design is capable of three-color fluorescence imaging with 110-$μ$m resolution at a 1-mm working distance. Overall, we demonstrate a versatile optical frontend that is adaptable to a range of applications using different fluorophores, illumination configurations, and lensless imaging techniques.

💡 Research Summary

This paper addresses a fundamental limitation of lensless fluorescence imaging systems: the angle‑sensitive nature of conventional thin‑film interference filters. In lensless configurations, excitation light arrives over a wide range of incident angles, causing the filter’s passband to blue‑shift and allowing excitation leakage that can be orders of magnitude stronger than the fluorescence signal. To overcome this, the authors revisit a technique that incorporates a fiber‑optic plate (FOP) as an angular filter and collimator. The FOP consists of an array of glass fibers with a high‑index core and an absorptive black cladding. Light incident within the acceptance angle α (defined by the numerical aperture NA = sin α) undergoes total internal reflection and is transmitted; light at larger angles is absorbed in the cladding. By selecting the NA of the FOP, the system can trade off collection efficiency against spatial resolution.

The authors first characterize a commercial multi‑band interference filter (ZET488/647/780+800 lpm) and quantify its blue‑shift with angle using the standard thin‑film formula λ(θ)=λ₀·√