Where does luminosity collide?

The total amount of energy a star or galaxy pours out as radiation each second: its intrinsic power output, set by the object itself and independent of how far away it happens to be. Prša and colleagues' 2016 Nominal Values for Selected Solar and Planetary Quantities fixed the nominal solar luminosity used as a standard unit, roughly 3.8 times ten to the twenty-sixth watts. The referent is a power radiated by a source, and astronomers keep it strictly apart from apparent brightness, which falls off with distance. It is not a collider's collision-rate figure of merit and not the brightness value of a pixel.[1]

Unit

A source's total radiant power, in watts or in solar luminosities

Recovery means

Not applicable

Measured by

Inferred from a star's distance and apparent brightness, or from its radius and surface temperature

A measure of a particle collider's performance: the rate at which it produces collisions, scaled per unit area, so that multiplying it by a reaction's probability gives the number of events expected. Summed over a running period it becomes the integrated luminosity, reported in inverse femtobarns, which fixes how much data an experiment collects. The ATLAS Collaboration's 2023 paper on Luminosity determination in pp collisions details how this quantity is calibrated and integrated over a run. The referent describes a machine's capacity to deliver collisions, not the power radiated by a star and not the brightness of an image.[2]

Unit

Instantaneous collisions per unit area per unit time; integrated luminosity in inverse femtobarns

Recovery means

Not applicable

Measured by

Computed from beam intensities and sizes, calibrated with van der Meer beam-separation scans, then integrated over a run

The perceived or measured brightness of a light or a picture element: in photometry the luminance of a surface, and in image editing the luminosity channel or luminosity blend that isolates lightness from colour. Poynton's 1998 Rehabilitation of Gamma is careful to separate true luminance, a weighted combination of the colour primaries defined by human vision, from the gamma-corrected quantity often loosely called by the same name. The referent is how bright something looks or reads out, whether as candela per square metre or as a per-pixel value. It has nothing to do with a star's radiated power or a collider's collision rate.[3]

Unit

The brightness of a light or a pixel, as photometric luminance or an image luminosity value

Recovery means

Not applicable

Measured by

Measured photometrically in candela per square metre, or computed per pixel as a weighted sum of the colour channels