Chris Peterson wrote:K1NS wrote:
[If] the sun emitted a billion times more energy in gamma rays than in visible light-
-it seems to me that the planet would be fried to a cinder!
Yes, it would! The total energy of the gamma rays was only a fraction of the total energy output of the Sun.
"Brightness" is determined by the photon flux, not the photon energy, and the Sun didn't emit very many gamma ray photons compared with the much less energetic ones is emits all the time.
Brightness = lumens per steradian = energy flux/steradian as observed visually.
The
most common use of brightness involving "
invisible light" radiation
refers to
energy flux/steradian and
NOT to photon flux/steradian.
http://en.wikipedia.org/wiki/Candela wrote:
<<Prior to 1948, various standards for luminous intensity were in use in a number of countries. These were typically based on the brightness of the flame from a "standard candle" of defined composition, or the brightness of an incandescent filament of specific design. The term candlepower was originally defined in England by the Metropolitan Gas Act 1860 as the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. Spermaceti is found in the head of sperm whales, and once was used to make high quality candles. Germany, Austria and Scandinavia used the
Hefnerkerze, a unit based on the output of a
Hefner lamp.
The candela (symbol: cd) is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function (a standardized model of the sensitivity of the human eye to different wavelengths, also known as the luminous efficiency function). A common candle emits light with a luminous intensity of roughly one candela:
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10
12 hertz and that has
a radiant intensity in that direction of 1⁄683 watt per steradian.
A 25 W compact fluorescent light bulb puts out around 1700 lumens; if that light is radiated equally in all directions, it will have an intensity of around 135 cd. The luminous intensity of light-emitting diodes is measured in millicandela (mcd), or thousandths of a candela. "Ultra-bright" LEDs can reach 15 000 mcd, or higher.>>
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1 KING HENRY VI Act 1, Scene 1
Messenger: A Talbot! a Talbot! cried out amain
And rush'd into the bowels of the battle.
--------------------------------------------------------
1 lumen = 1 talbot per second = 1⁄683 watt
1 candela = 1 lumen per steradian
1 nit = 1 candela per square meter
Code: Select all
Quantity Symbol SI unit Symbol Dimension Notes
-------------------------------------------------------------------------------------------------------
Luminous energy Qv lumen second lm⋅s T⋅J [nb 3] units are sometimes called talbots
Luminous flux Φv lumen (= cd⋅sr) lm J also called luminous power
Luminous intensity Iv candela (= lm/sr) cd J an SI base unit, luminous flux per unit solid angle
Luminance Lv candela per m^2 cd/m^2 J/L^2 units are sometimes called nits
Illuminance Ev lux (= lm/m^2) lx J/L^2 used for light incident on a surface
Luminous emittance Mv lux (= lm/m^2) lx J/L^2 used for light emitted from a surface
Luminous exposure Hv lux second lx⋅s T⋅J/L^2
Luminous energy density ωv lumen second per metre^3 lm⋅s/m^3 : T⋅J/L^3
[quote="Chris Peterson"][quote="K1NS"]
[If] the sun emitted a billion times more energy in gamma rays than in visible light-
-it seems to me that the planet would be fried to a cinder![/quote]
Yes, it would! The total energy of the gamma rays was only a fraction of the total energy output of the Sun.
[b][color=#FF0000]"Brightness" is determined by the photon flux, not the photon energy[/color][/b], and the Sun didn't emit very many gamma ray photons compared with the much less energetic ones is emits all the time.[/quote]
[b][color=#0000FF]Brightness = lumens per steradian = [u]energy flux[/u]/steradian as [u]observed visually[/u].[/color][/b]
The [b][u]most common[/u][/b] use of brightness involving "[u]invisible[/u] light" radiation
refers to [b][color=#0000FF]energy flux/steradian[/color][/b] and [b][color=#FF0000]NOT to photon flux/steradian[/color][/b].
[quote=" http://en.wikipedia.org/wiki/Candela"]
<<Prior to 1948, various standards for luminous intensity were in use in a number of countries. These were typically based on the brightness of the flame from a "standard candle" of defined composition, or the brightness of an incandescent filament of specific design. The term candlepower was originally defined in England by the Metropolitan Gas Act 1860 as the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. Spermaceti is found in the head of sperm whales, and once was used to make high quality candles. Germany, Austria and Scandinavia used the [url=http://en.wikipedia.org/wiki/Hefner,_Hugh_M.]Hefner[/url]kerze, a unit based on the output of a [url=http://en.wikipedia.org/wiki/Hefner,_Hugh_M.]Hefner[/url] lamp.
The candela (symbol: cd) is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function (a standardized model of the sensitivity of the human eye to different wavelengths, also known as the luminous efficiency function). A common candle emits light with a luminous intensity of roughly one candela:
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10[sup]12[/sup] hertz and that has [b][color=#0000FF]a radiant intensity in that direction of 1⁄683 watt per steradian[/color][/b].
A 25 W compact fluorescent light bulb puts out around 1700 lumens; if that light is radiated equally in all directions, it will have an intensity of around 135 cd. The luminous intensity of light-emitting diodes is measured in millicandela (mcd), or thousandths of a candela. "Ultra-bright" LEDs can reach 15 000 mcd, or higher.>>
[c][size=150]--------------------------------------------------------
1 KING HENRY VI Act 1, Scene 1
Messenger: [i][b][color=#0000FF]A Talbot! a Talbot![/color][/b] cried out amain
And rush'd into the bowels of the battle.[/i]
--------------------------------------------------------[/size]
[size=200][i][color=#0000FF]1 lumen = 1 talbot per second = 1⁄683 watt
1 candela = 1 lumen per steradian
1 nit = 1 candela per square meter[/color][/i][/size][/c]
[code]Quantity Symbol SI unit Symbol Dimension Notes
-------------------------------------------------------------------------------------------------------
Luminous energy Qv lumen second lm⋅s T⋅J [nb 3] units are sometimes called talbots
Luminous flux Φv lumen (= cd⋅sr) lm J also called luminous power
Luminous intensity Iv candela (= lm/sr) cd J an SI base unit, luminous flux per unit solid angle
Luminance Lv candela per m^2 cd/m^2 J/L^2 units are sometimes called nits
Illuminance Ev lux (= lm/m^2) lx J/L^2 used for light incident on a surface
Luminous emittance Mv lux (= lm/m^2) lx J/L^2 used for light emitted from a surface
Luminous exposure Hv lux second lx⋅s T⋅J/L^2
Luminous energy density ωv lumen second per metre^3 lm⋅s/m^3 : T⋅J/L^3[/code][/quote]