Phenomenon Description: Under overcast field conditions or rapidly shifting light, spectral reflectance curves obtained from multiple measurements of the same target exhibit large fluctuations, with significant discrepancies particularly pronounced in the visible and near-infrared bands. Root Cause Analysis: Portable spectroradiometers typically utilize passive optical measurements, relying on sunlight and diffuse skylight. Cloud movement, changes in solar altitude angles, or shifting shadows from surrounding vegetation directly impact the incident spectral energy distribution, leading to biases in the calculated reflectance. Meanwhile, the time lapse between white panel calibration and target measurement amplifies this error. Solution: The "Dual-Frequency Calibration Method" is recommended: immediately re-measure the reference white panel after measuring every 5–10 targets to establish a time-series correction factor for illumination variations. When conditions permit, equip a fiber optic field-of-view limiter and prioritize operations during clear, stable weather windows (10:00–14:00). For critical plots, miniature spectral irradiance meters can be deployed to synchronously log ambient light variations for post-processing data normalization.

Phenomenon Description: When measuring the same vegetation type (e.g., wheat, pine), the spectral curves acquired in the early morning versus the afternoon can show a reflectance discrepancy of over 15% in the near-infrared band (750–1300 nm), and the stability across different measurement angles is poor. Root Cause Analysis: Variations in leaf internal structure and moisture content are the primary drivers. In the early morning, high leaf moisture content enhances absorption in the near-infrared band, reducing reflectance; at noon, leaf moisture evaporates, causing reflectance to rise. Additionally, the specular reflection effect of leaves leads to the inclusion of more soil background signals during non-vertical observations, weakening absorption features in water-sensitive bands (1450 nm, 1940 nm). Solution: Unify the measurement time window (10:00–14:00 recommended) and avoid early morning dew and strong winds. Maintain the angle between the probe and the target surface normal at ≤ 10°, with the probe positioned 0.3–0.5 meters from the target surface (depending on the field of view angle). For high-precision requirements, utilize a "leaf clip" accessory to fix the leaf angle, or apply envelope removal and continuum normalization to the spectral curves post-measurement to accentuate water absorption features.

Phenomenon Description: When measuring beneath forest canopies, on overcast days, or near dusk, the spectroradiometer exhibits noticeable jagged noise or even negative reflectance values in the shortwave infrared (SWIR) band, severely hindering the extraction of minerals or vegetation allelochemicals. Root Cause Analysis: Detector dark current drifts with variations in temperature and integration time. When the target signal is weak (low reflected energy), the proportion of dark current noise rises significantly, and internal electronic noise is not effectively deducted. Some portable devices do not record dark current in real time or rely solely on factory default values, causing calibration to fail. Solution: Enable the instrument's "real-time dark current deduction" mode (if available). Use an opaque lens cap to record the dark current curve before measurements, and re-acquire it every 20 minutes. Increase the integration time to bring the signal intensity to 70%–90% of the maximum value, and perform multiple scans (10–20 times) for averaging to minimize random noise. In extreme low-light environments, consider using an auxiliary halogen light source for illumination.

Phenomenon Description: When taking measurements in deserts or during early crop growth stages, the spectral curves simultaneously display mixed characteristics of both vegetation (chlorophyll absorption peak at 680 nm) and soil (iron oxide absorption or flat reflectance), making it difficult to directly extract vegetation physiological parameters. Root Cause Analysis: The field of view (FOV) of the spectroradiometer concurrently contains both the vegetation canopy and exposed soil. According to the linear mixing model, a mixed spectrum is the area-weighted sum of each endmember spectrum. High soil background brightness and flat spectral features tend to obscure the distinctive vegetation "red edge" and water absorption traits, underestimating indices like NDVI. Solution: Select a probe with a smaller field of view angle (such as 8° or 10°) and appropriately adjust the measurement height to ensure the target vegetation accounts for over 90% of the FOV area. During post-processing, spectral unmixing algorithms (such as constrained least squares) can be applied to separate vegetation and soil contributions. Another effective approach is to manually clear bare soil particles or depress non-target weeds before measuring, while capturing adjacent bare soil spectra for differential correction.

Phenomenon Description: Following prolonged field operations, the overall signal intensity of the spectroradiometer drops, with attenuation particularly pronounced in the shortwave infrared band, and replacing the white panel or increasing the integration time fails to restore it. Root Cause Analysis: Excessive bending of the optical fiber (radius < 10 cm) causes light leakage, decreasing coupling efficiency. Furthermore, field dust, sand, or plant sap adhering to the front end of the fiber optic probe or the optical lens induces dual effects of scattering and absorption. Transmittance loss caused by contamination on the fiber or lens face varies non-linearly across bands, leading to errors in reflectance calculation. Solution: Check before use to ensure the fiber optic bending radius is greater than 15 cm, avoiding trampling and sharp-angle folds. Before and after each field campaign, use a specialized fiber cleaning stick or high-purity anhydrous ethanol (≥ 95%) with dust-free cotton swabs to gently wipe the probe or lens face. If signals remain abnormal, perform a "fiber optic light loss test" to verify if the spectral profile is smooth.

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