ASABE Technical Library - Abstract

Click on “Download PDF” for the PDF version or on the title for the HTML version.
If you are not an ASABE member or if your employer has not arranged for access to the full-text, view access and subscription options

Development of an FMCW-Based Six-Band Handheld Active Optical Sensor for Reflectance-Based Spectroscopy

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  2026 ASABE Annual International Meeting  2600044.(doi:10.13031/aim.202600044)
Authors:   Albert Ansah Benson, Paul Armstrong, Niaqian Zhang
Keywords:   frequency-modulated (FMCW), Low Drop Out Regulator (LDO), Metal Oxide Field Effect Transistor (MOSFET), Silicon Photo Multiplier (SIPM), transimpedance amplifier (TIA), Analog to Digital Convertor (ADC), Voltage Controlled Oscillator (VCO), High pass filt

Abstract. Optical sensing has been widely explored for estimating nitrogen content in plant leaves by leveraging their wavelength-dependent spectral characteristics (Hasituya, Li, Elsayed, Hu, & Schmidhalter, 2020; Schlemmer et al., 2013). This is typically achieved using hyperspectral or multispectral systems. However, hyperspectral sensors are often large, complex, and expensive (Hagen & Kudenov, 2013), making them more suitable for satellite or aerial remote sensing. For localized measurements, multispectral sensors are more common, yet most are passive or, when active, limited to only one or two spectral bands. Existing active sensors also remain susceptible to ambient illumination, which degrades measurement stability and reduces sensitivity. This work presents the design and development of a handheld optical sensing device for assessing nitrogen concentration in maize leaves using amplitude-based detection from a frequency modulated wave. The system achieves ambient-light-immune measurements by employing frequency modulation of the LED output at a carrier frequency of 1 MHz with a deviation of 100 kHz, combined with high-pass filtering to reject low-frequency ambient light interference (Kahn & Barry, 1997). Unlike conventional nitrogen meters that rely on passive sensing or only a few active wavelengths, the proposed device uses an active, collimated LED array spanning six spectral bands (450 nm, 515 nm, 520 nm, 620 nm, 660 nm, 750 nm), enabling enhanced spectral resolution and improved characterization of leaf reflectance. A precision optical assembly incorporating plano-convex lenses minimizes beam divergence and ensures uniform illumination, while a silicon photomultiplier (SiPM) provides high-gain detection (Masuda et al., 2023; Piatek, 2024). Preliminary experiments using Arbor Scientific colour filters placed over a calibrated Spectralon 99% diffuse reflectance target demonstrated successful spectral discrimination under both dark room and ambient illumination conditions up to a distance of 8 cm. Statistical analysis at the optimal measurement height of 8 cm yielded standard deviations ranging from 0.018 to 0.103, variance values between 0.0003 and 0.0106, and coefficients of variation predominantly below 25%, confirming repeatable measurements within the 3.5 cm to 8 cm sensing range. This work establishes a foundation for compact, low-cost, and robust 6 band multispectral nitrogen assessment tools that can support nitrogen decision-making in precision agriculture.

(Download PDF)    (Export to EndNotes)