Influence of the distance between a reflectance sensor and soil samples with different roughness on their spectra

The study assessed the influence of the distance between a reflectance sensor and soil samples with various roughness states (R1– the lowest, R2 – the medium, R3 – the highest roughness state) on their spectra level, under laboratory condition. Studied soil samples were illuminated at three light source zenith angles (θs equal to 20°, 40°, 60°) and observed by the sensor to the nadir, from various distances (Hs) from 10 to 54 cm. These dark (the Mollic Gleyic Fluvisol) and light (the Cutanic Stagnic Luvisol) soil materials with their minimum roughness were characterized by diffused reflectance spectra. The relative differences (RD) between the spectra level of soil samples with R1, R2, R3 roughness states and the diffused reflectance level of soil materials were calculated with 1 nm interval in range of 420–2,300 nm. Higher roughness state and higher θs, result in higher RD. Thus, for the dark and light soil samples with R3 roughness state and illuminated at θs= 60°,the RD are the highest reached 63 and 39% (Hs=54 cm) and reached 77 and 63% (Hs=10 cm), respectively. The spectra level of the soil samples in R1 and R3 roughness states, illuminated at θs=20° and soil samples with R1 roughness, illuminated at θs=60°, reached a stable level, at a specific Hs. It means, that a spectra does not significantly change with a further increase Hs. However, the soil samples in R3 roughness, illuminated at θs=60° have not reached the stability.