In quantitative thermography and radiometry, radiant flux is usually discussed at the level of the entire detector or optical system. However, when analyzing measurements pixel by pixel, one faces the question:

• What is the most rigorous way to define the flux incident on a single pixel?
• Should it be expressed through the projected footprint on the scene and the acceptance solid angle?
• Or through sensor-side parameters such as pixel pitch and f-number?
• Under what conditions are these two approaches equivalent?

I would be very interested to hear the community’s perspective on whether a consistent, pixel-level formulation is already well established, or if this is still an open point in radiometric theory.