says lumens are a unit of luminosity, but how if lumens are candela times steradians? But it would have a luminous flux of zero, since the human eye can't detect radio photons. From a radiometric perspective, it has a radiant flux of exactly the same amount. An astronomer would say it has a luminosity of (say) $X$ watts. Imagine a radio source that doesn't emit any visible light. (See the linked Wikipedia article for more details.) It is analogous to luminosity/radiant flux, but not the same thing, because it involves weighting the energy of the light by how well or poorly the human eye can sense it. Radiant and Luminous Intensity Definition: The radiant (luminous) intensityis the power per unit solid angle from a point. It is a term from "photometry", which is the measurement of light *as perceived by the human eye" (I put it in scare quotes because in astronomy, the word "photometry" usually means measurement of light without any reference to human perceptions, and is sometimes casually used to refer to imaging or analysis of images). In radiometry, the usual term for this is radiant flux. In astronomy, luminosity is exactly as you've defined it. The radiometric units are the same, except they refer to the true emitted power, with no wavelength-weighting. through a 4-day span under field conditions showed that the 3 readings of long-wave flux density never differed by >10, and usually differed by <2-3, although. One can imagine this as the human-useful amount of light emitted by a source. This quantity is dimensionally equivalent to power, and is therefore measured in joules per second, or watts $(\mathrm)$. Radiance: radiant flux radiated per unit area, per unit solid angle, per wavenumber light with wavenumber between and + d Isotopes are forms of an element whose nuclei have the same atomic number, the number of protons in the nucleus,but different atomic masses because they contain different numbers of neutrons. Luminosity refers to the total energy released by an object per unit time. Photometric units can often seem complex at first sight.
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