Radiance

The amount of light or energy traveling in a particular direction, arriving at the sensor from a specific patch of ground or atmosphere. It is the fundamental thing an optical instrument actually records.

Why it matters

Almost every satellite product starts as radiance — it gets calibrated and corrected for the atmosphere to produce reflectance, temperatures, and the maps people use. Getting radiance right is the foundation of trustworthy data.

Where you’ll meet it

• MODIS and VIIRS deliver calibrated radiance products that feed nearly all their downstream land, ocean, and atmosphere products.
• Landsat instruments record radiance in each band before conversion to surface reflectance.
• PACE’s OCI instrument measures radiance across many wavelengths to study ocean color and aerosols.

In plain terms

It’s like the exact brightness your eye catches from one spot on a scene — before your brain adjusts for the lighting and figures out the true color.