[Catieliecutty]

Luminance is an interesting property. It is defined as the luminous intensity of light travelling in a particular direction, divided by the area.

Two things interest me about it.

1/ it is an intrinsic rather than extrinsic property. If a flat screen TV has a luminance of 80 candelas per square metre, then two flat screen TVs stuck together have a luminance of 80 candelas per square metre.

2/ It is invariant under conservative optics. In particular, there is no way to _increase_ the luminance via passive optics. No matter what kind of prisms, lenses or mirrors you use, you can't increase the luminance from a particular source. The luminance of the sun is 1.5 gigacandelas per square metre. If you go to Mercury and use a lens a kilometre wide to concentrate the sun's rays down to a small point on Mercury's surface, the luminance at that point will be 1.5 gigacandelas per square metre, the same as it is under normal conditions on the surface of Pluto. (You can of course _reduce_ this luminance through absorption etc.)

[Tij84ye]

Luminance is an interesting property. It is defined as the luminous intensity of light travelling in a particular direction, divided by the area.

Two things interest me about it.

1/ it is an intrinsic rather than extrinsic property. If a flat screen TV has a luminance of 80 candelas per square metre, then two flat screen TVs stuck together have a luminance of 80 candelas per square metre.

2/ It is invariant under conservative optics. In particular, there is no way to _increase_ the luminance via passive optics. No matter what kind of prisms, lenses or mirrors you use, you can't increase the luminance from a particular source. The luminance of the sun is 1.5 gigacandelas per square metre. If you go to Mercury and use a lens a kilometre wide to concentrate the sun's rays down to a small point on Mercury's surface, the luminance at that point will be 1.5 gigacandelas per square metre, the same as it is under normal conditions on the surface of Pluto. (You can of course _reduce_ this luminance through absorption etc.)

So luminance isn't a measure of energy flux? Can you clarify how it is different from an energy flux?

[bestbyV]

http://en.wikipedia.org/wiki/Bolometer

[orison]

Devil's advocate mode.

> It is defined as the luminous intensity of light travelling in a particular direction, divided by the area. ... the same as it is under normal conditions on the surface of Pluto.

It'd be zero then, wouldn't it, because light never travels exactly in a particular prespecified direction, the photons would be a squillionth of a degree off.

[DextExexy]

Can you clarify how it is different from an energy flux?
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Energy flux is the rate of energy passing through a particular area. It has the units watts per square metre.

[www.forumsovetov.ru]

Can you clarify how it is different from an energy flux?

Energy flux is the rate of energy passing through a particular area. It has the units watts per square metre. Energy flux doesn't specify direction. Luminance does. To put it another way, the energy flux is the integral of the luminance over all directions.

[inownsuipsy]

Energy flux doesn't specify direction. Luminance does. To put it another way, the energy flux is the integral of the luminance over all directions.


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The other major difference is that energy flux is about energy.

Perhaps you meant luminous flux, morrie.