## 3 Materials and Experiments  
Six NIR multi-wavelength light property channels of 850 nm, 940 nm, 1300 nm, 1400 nm, 1550 nm, and 1650 nm were examined via a human finger using the constructed reflectance NIR multi-wavelength light system (Figure 2). As can be seen in Figure 2a, incident light from a halogen lamp (Thorlabs, Newton, NJ, USA, OSL2 IR) travels through the collimated lens L1 and forms parallel light. Then, the parallel incident light is tuned by switching the monolithic multi-wavelength light electric filter wheel, forming narrowband multi-wavelength light. From the schematic diagram of the monolithic multi-wavelength light electric filter wheel in Figure 2b, 6 NIR filters of 850 nm (Thorlabs, FBH850-10), 940 nm (Thorlabs, FBH940-10), 1300 nm (Thorlabs, FBH1300-12), 1400 nm (Thorlabs, FBH1400-12), 1550 nm (Thorlabs, FBH1550-12), and 1650 nm (Thorlabs, FBH1650-12) are embedded into the monolithic electric wheel (Thorlabs, FW102C). Channel filters of 850 nm and 940 nm have a half of the maximum (FWHM) of 10 nm, while the other four filters have the larger value of 12 nm. This narrowband monolithic multi-wavelength light filter is used to extract highly accurate narrowband NIR light, avoiding broadband light absorption interference from other biomaterials. This light is further focused on the human finger by lens L2, and the reflected light is collected by lens L3. Finally, the intensity I of reflected light is recorded by the power meter (Thorlabs, PM16-122). In accordance with biomedical ethics protocols, all study participants were fully informed of the experimental procedures and potential risks associated with the non-invasive glucose-monitoring methodology. Informed consent was obtained prior to initiating human experiments. The experimental protocol comprised the following sequence: Participants' fingertips were disinfected using sterile alcohol swabs and allowed to air-dry until complete ethanol evaporation. The human fingertip was then placed on the test location of our custom NIR multi-wavelength light imaging system, as shown in Figure 2a, in order  
Figure 1: Workflow diagram of the HMML-BP network of our highly accurate narrowband multi-wavelength light NIR sensing system. to measure diffuse reflectance spectroscopy. Each experiment lasted for 6 s. To establish ground-truth reference values, capillary blood samples were concurrently collected via aseptic lancet puncture and analyzed using a clinically validated glucometer (Johnson, New Brunswick, NJ, USA, OneTouch VerioVue). When the electric filter wheel selects different wavelengths, the power meter automatically feeds back the optical power in real time to achieve circuit integration. The tissue exposure time is about 0.5 s for each wavelength, while the multiplexing time of each wavelength is approximately 500 ms. Through this standardized protocol, we acquired 130 paired datasets comprising multi-wavelength light NIR signatures, as well as corresponding blood glucose concentrations, enabling the comprehensive characterization of optical fingertip properties under varying glycemic conditions. Since human blood glucose sensing is complex, many critical factors (motion artifacts, stray light, sweat, temperature, and so on) have been analyzed: (1) Motion artifacts: To improve stability, our glucose system uses the highly accurate electric wheel, all optical information was tested after achieving system stability, and the experiments were conducted immediately for about 6 s to maintain this stability. (2) Stray light: To minimize the interference of stray light, the whole experimental system was enclosed in a black box, with only a small hole opened at the finger acquisition site to allow placement. The incident and reflection paths were separated by a black baffle. Furthermore, diffuse light from frosted glass was obtained after each human measurement, serving as the reference light for removing system fluctuations.