Category: Optical and atmosphere
BRDF
BRDF describes how reflectance changes with viewing and illumination geometry, affecting apparent brightness across scenes.
Also known as: BRDF normalization, directional reflectance
Expanded definition
BRDF stands for bidirectional reflectance distribution function. Many surfaces are anisotropic, meaning they reflect light differently depending on sun angle and sensor view angle.
BRDF effects can cause brightness changes that look like real surface change, especially in time series and mosaics. Some products apply BRDF normalization to reduce these geometry-driven variations.
BRDF is often most visible in vegetation canopies, rough surfaces, and terrain. If you see systematic “flicker” across dates with no plausible ground change, geometry and BRDF are common causes.
Related terms
Radiometry
Radiometry is the measurement and calibration of electromagnetic energy recorded by a sensor.
Harmonization
Harmonization reduces differences between scenes or sensors so values are more comparable across time.
Optical Imagery
Optical imagery measures reflected sunlight, providing rich spectral information but being sensitive to clouds and illumination.