Why energy matters？ RGB, Camera.

For "Radiance" or "radiance pixel values" why its unit is "µW/(cm2*sr*nm)"? it is a "energy" unit，i think it because only the "energy" can cause the sensor to action, to record the different level pixel values ,right?

Absolutely right! The unit of “Radiance” or “radiance pixel values” is indeed expressed in energy terms. Let’s break it down:

1. Radiance:

   • Radiance represents the amount of electromagnetic energy (light) emitted, reflected, or transmitted by a surface per unit area, solid angle, and wavelength.
   • It’s a fundamental quantity in remote sensing, as it characterizes the energy received by the sensor.
   • The unit for radiance is typically micro-Watts per square centimeter per steradian per nanometer (µW/(cm²·sr·nm)).
   • Here’s what each component means:
     • µW: Micro-Watts (energy per second).
     • cm²: Square centimeters (area).
     • sr: Steradians (solid angle).
     • nm: Nanometers (wavelength).

2. Why Energy Matters:

   • You’re spot on! The sensor responds to energy (radiation) across different wavelengths.
   • By measuring the energy, the sensor records varying pixel values (DN) corresponding to different surface properties.
   • These pixel values allow us to distinguish features like vegetation, water bodies, and urban areas.

In summary, radiance provides essential information about the energy emitted or reflected by Earth’s surface, enabling us to analyze and interpret remote sensing data.