Evaluation of the Influence of Geometric Parameters on the Accuracy of a Visible Near Infrared (VNIR) Spectroscopy-Based Nitrogen Phosphorus Potassium (NPK) Soil Sensor

Optimal sensor geometry is crucial for minimizing external light interference, as optical sensing relies on light transmission, which is susceptible to ambient light disruption. This study investigates the influence of geometric parameters on the accuracy of optical sensors used for detecting soil macronutrients. The geometric configuration—specifically the alignment of the photodiode and LEDs was determined using the law of reflection, focusing on parameters such as path length (x), angles of incidence (θi) and reflection (θr), and component distances (d1, d2, and D). Experimental analysis revealed that the optimal values for x, D, and θr were 2 cm, 7 cm, and 60˚, respectively. At these settings, the sensor achieved minimal error, with RMSE values of 2.1, 0.1, and 1.6 for Nitrogen, Phosphorus, and Potassium concentration measurements, respectively.