- Brightness is a non technical term to describe the intensity of a light source.
- Light Flux is a technical term referring to the quantity of energy per second radiating from a light source that is incident on a surface of area 1 m2
Energy per second has units of Watts, so that the SI unit of flux is the watt per meter squared.
It is generally cold in the polar regions because the sun' s energy is distributed over a wider area resulting in a low flux compared to the equator.
The maximum flux of energy coming from the Sun incident on Earth is 1370 Watts per square meter. This would be the flux experienced at noon on the equator during an equinox. This flux is the same as the flux from a 100 Watt light bulb viewed from a distance of 7 cm.
Flux is also be referred to as the apparent brightness of a light source.
- The human eye responds to fluxes in a geometric rather than a linear way, for example if you show someone a 1Watt bulb and a 100 Watt bulb and ask them to select a bulb that is half way between the two in brightness , they will choose a 10 Watt bulb , a 50 Watt bulb appears to our senses to be much closer in brightness to a 100 Watt bulb than to a 1 Watt bulb.
This kind of relationship in which the flux (at a given distance) from a 1 W bulb might be assigned a brightness of 1, that from a 10 W bulb a brightness of 2, that from a 100 W bulb a brightness of 3 etc., is known as a logarithmic relationship. In mathematics , logarithms are defined as functions having the property that the logarithm of a product equals the sum of the logarithms
log(ab)=log(a)+log(b)
- Logarithms are useful because they do describe the relationship between light flux and brightness as perceived by the senses. Logarithms are also handy when attempting to show a large range of numbers in a graph.
- A historical measure of flux is Hipparchus's magnitude scale. This is a logarithmic scale in which a difference 5 units of magnitude corresponds to a flux factor of 100. The magnitude scale is still in use today because it is convenient to be able to express fluxes that can differ by huge factors in terms of small numbers, all stars that can be seen with the naked eye have magnitudes between 0 and 5. The zero of the magnitude scale was originally set to the apparent visual magnitude of Alpha Centauri. The scale is backwards, a star with a small magnitude is brighter than one with a larger magnitude. This is because it was originally thought that all stars were actually identical, that the apparent differences in brightness were entirely to do with how far away they were. from Earth. Magnitude was really then a measure of distance, the fainter the star, the farther away it was, therefore the greater its magnitude.
- Luminosity is a measure of the intrinsic brightness of a light source, eliminating any reference to how far the source is from the observer.
Luminosity is really the total power output of the source and as such is measured in Watts. The luminosity of the Sun is 3.86 X 1026 Watts.
Luminosity is related to flux by the inverse square law. The flux at a distance R equals the luminosity divided by the area of a spherical shell of radius R , which is 4pR2.
Stars vary enormously in their luminosities, the range runs from very faint stars, one ten thousandth as luminous as the Sun, to extremely brilliant stars 100,000 times more luminous than the Sun.
Luminosity can be measured using the magnitude scale by measuring the magnitude a star would have if placed 10 parsecs from an observer. This is called the absolute magnitude of the star. The enormous range in luminosities corresponds to a spread in absolute magnitudes of 25.
