Optical and Raman Spectroscopy for Glucose Sensing

Introduction to Optical Glucose Sensing

Optical glucose sensing measures glucose with light instead of the electrochemical reaction that traditional sensors rely on. One optical design is already FDA-cleared: the Eversense 365 implant, which reads a fluorescent chemical tag under the skin [1]. The needle-free designs are another story. They remain research projects, and the FDA warns that no smartwatch or smart ring is authorized to measure glucose on its own [7]. We sort the field into two areas: Implantable Fluorescence and Non-Invasive Spectroscopy.

Implantable Fluorescence-Based Glucose Sensing

Senseonics' Eversense system is the working example of implantable fluorescence [1]. Its sensor, a capsule about 3.5 mm by 18.3 mm, carries a fluorescent indicator molecule that binds glucose reversibly. More glucose means brighter fluorescence, and the sensor's own optics measure that change [1]. The binding is fully reversible, and the Eversense 365 is cleared for up to 1 year in adults 18 and older [1]. Its predecessor, the Eversense E3, was approved in February 2022 for up to 180 days, about 6 months, and has since been superseded by Eversense 365 [8]. The trade-off is placement. A health care provider inserts the sensor under the skin, so getting one means a minor procedure rather than a self-applied patch [1].

You may also read that implanted fluorescent sensors avoid compression artifacts, the false readings that can happen when body weight presses on a sensor site. We could not find an FDA document or published study that tests this, so we treat it as unproven. Our article on Implantable Fluorescence Sensors (Eversense) covers the system in more depth.

Non-Invasive Spectroscopy for [Glucose Monitoring](/)

Non-invasive spectroscopy is still in the research and development phase. Raman Spectroscopy shines a laser into tissue and reads the tiny share of light that scatters back at shifted wavelengths, a pattern that works like a molecular "fingerprint" for glucose. That signal is weak. In exchange, it is chemically specific and picks up little interference from water in the tissue [2].

RSP Systems, a Danish company, built a table-top confocal Raman spectrometer that focuses on a "critical depth" in the skin at the base of the thumb: the interstitial fluid below the outer skin layer and above the fat [2]. In a company-funded study, 35 people with diabetes used it at home for 60 days, and its calibration held for more than 10 days at a time [2]. Here's what surprised us: the authors reported accuracy comparable to the invasive CGMs available then, from a desktop instrument rather than a wearable [2].

NIR/Silicon Photonics takes a different route. Near-infrared (NIR) spectroscopy measures how much light glucose absorbs, and silicon photonics tries to shrink the spectrometer onto a chip small enough for a watch. Rockley Photonics was the best-known company here. In 2021 it said its sensor module was designed to let wearables track measurements including glucose trends [3]. Rockley filed for Chapter 11 bankruptcy protection in early 2023 and emerged that June [6]. As of October 2026, the FDA's 2024 warning is still posted: it has not authorized, cleared or approved any smartwatch or smart ring that measures or estimates glucose on its own [7].

Challenges in Optical Glucose Sensing

Two problems come up again and again in the reviews we read. Water Interference is the first. Infrared methods are skewed by water's strong light absorption, and NIR readings shift with the water, fat and protein in tissue [4]. Glucose leaves only a small spectral signature hidden in a highly variable background, so these devices need a very high signal-to-noise ratio plus multivariate calibration, statistical models that pull the glucose signal out of everything else in the spectrum [5]. Raman sidesteps some of the water problem, though its signal is weaker to begin with [2].

Skin Variation is the second, and it is the main reason universal calibration remains a hurdle for non-invasive technologies. NIR measurements are sensitive to tissue thickness, skin and room temperature, skin tone and melanin, and ambient light [4]. One simulation study found that the best NIR wavelengths and detector spacing changed with melanin concentration [4]. So a model tuned on one person's skin may not carry over to someone else's.

Conclusion

Both branches use light, but they sit at very different stages. Implanted fluorescence is a cleared product with a sensor rated for up to a year [1]. Needle-free Raman and NIR devices still have to show that a calibration can hold across different skin and shifting tissue water [4][5], and the best-known silicon photonics company in the field went through bankruptcy in 2023 [6]. Until a non-invasive device earns authorization, the FDA tells consumers to use only FDA-authorized devices to measure glucose [7].