APOD: Solar Flare in the Gamma ray Sky (2012 Mar 15)

[Chris Peterson]

You're thinking about particle accelerators; I'm talking about light.

Actually, I'm talking about natural sources of photons, be they visible light or gamma rays. I'm specifically not talking about particle accelerators, since I have no idea what sort of casual language particle physicists use around them. I'm just talking about the lingo of astronomers.

[alter-ego]

I disagree. In practice, "brightness" is usually casually synonymous with "intensity", are refers to the particle (typically photon) flux.

Yes, brightness and intensity are synonymous. They both refer to intrinsic source emission characteristics that are independent of distance, but in the photonics related sciences, I believe the units are more commonly expressed using power or energy. Of course, the brightness electromagnetic radiation can be expressed in terms of photon flux, but correlation to SI units, publications, and device senstities, as examples, require conversion to physically meaningful parameters which ultimately reduce to power and energy. For example, in astronomy, cosmic background source brightnesses (radio through gamma ray) are mostly referenced in units of Watts or energy (Joules, eV)/second. Only in special conditions have I seen photon flux describe these brightness constants. It's easier and more practical to utilize these numbers directly than have to make additional conversion calculations to get values that are most wanted most of the time. True, I've seen, and used, photon flux (photons/second) used for threshold irradiance (photons/[cm² sec]) dependencies, but generally speaking, I can't remember when I last saw brightness constants described by photon flux.

My experience suggests a contrary view to yours, but I admit I'm not in astronomy professionally, and college is the last time I knew any astronomers. Astronomy has been a passion of mine for most of my life while my profession has been in laser R&D and more fundamental research using lasers to explore plasma characteristics. I've mostly seen formal brightness characterization in terms of power or energy, not as a particle flux.

[neufer]

In practice, "brightness" is usually casually synonymous with "intensity", are refers to the particle (typically photon) flux.

I've seen, and used, photon flux (photons/second) used for threshold irradiance (photons/[cm² sec]) dependencies, but generally speaking, I can't remember when I last saw brightness constants described by photon flux.

Chris is probably thinking more along the lines of source brightness (photons/[steradian sec]) than of than of illumination intensity (photons/[cm² sec]) since he often has to remind us that while telescopes can collect photons over an extended apertures and also make extended sources seem closer (by expanding the steradians) they cannot increase the apparent source brightness (#photons per steradian) of those extended sources (e.g., planets, nebulas, galaxies, etc.).
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Note, however, that Webster's Revised Unabridged Dictionary (1828) says absolutely nothing about photons :
............................................................
BRIGHTNESS, n. briteness. Splendor; luster; glitter.

• 1. Acuteness, applied to the faculties; sharpness of wit; as the brightness of a man's parts. (Sorry ladies.)

............................................................
BRIGHT, a. brite. [Heb. to shine.]

• 1. Shining; lucid; luminous; splendid; as a bright sun or star; a bright metal.
  
  2. Clear; transparent; as liquors.
  
  3. Evident; clear; manifest to the mind, as light is to the eyes.
  
  4. Resplendent with charms; as a bright beauty; the brightest fair.
  
  5. Illuminated with science; sparkling with wit; as the brightest of men.
  
  6. Illustrious; glorious; as the brightest period of a kingdom.
  
  7. In popular language, ingenious; possessing an active mind.
  
  8. Promising good or success; as bright prospects.
  
  9. Sparkling; animated; as bright eyes.
  
  • [list]The Men Who Stare at Goats (2009)
  Lyn Cassady (George Clooney): Like all Shaman before him, he had traversed the wilderness. Now he was returning to his people, a changed man. He brought with him his confidential report, which he called: "The New Earth Army manual." The New Earth Army is a banner under which the forces of good can gather. The courage and nobility of the Warrior, blended with the spirituality of the Monk. The Jedi Warrior will follow in the footsteps of the great imagineers of the past: Jesus Christ, Lao Tse Tung, Walt Disney. The role of The New Earth Army is to resolve conflict world-wide. Jedis will parachute into war zones, utilizing sparkly eyes technique, carrying symbolic flowers and animals, playing indigenous music and words of peace...
  
  Bob Wilton (ex-Jedi Warrior Ewan McGregor): What's... What's the sparkly eyes technique?

[/list]

[Chris Peterson]

Yes, brightness and intensity are synonymous. They both refer to intrinsic source emission characteristics that are independent of distance...

That is definitely NOT the case. As a photometricist, I am concerned with both physical and instrumental intensities. More often than not, intensity as I work with it is independent of the actual source intensity, and concerns only the intensity I record. That intensity does, of course, vary with distance to the source.

, but in the photonics related sciences, I believe the units are more commonly expressed using power or energy.

In most areas of astronomy (and certainly in photometry), intensity is a simple photon count. In spectrometry, intensity is also just a photon count... given a pair of peaks, one of long and the other short wavelength, the intensity stated for each would not typically be adjusted for the different intrinsic energies of the photons recorded in each.

The usual simple metrics are intensity, the photon count; wavelength, the photon energy; integrated energy, the product of the photon count and photon energy over some particular wavelength range.

Please keep in mind here that my earlier response was a simple answer to a simple question. In a highly technical discussion, everybody would agree on terms, definitions, and units from the outset. Doing that was not necessary to my previous answer, however.

[neufer]

This 1932 promotional photo of Joan [b][color=#FF0000]Blondell[/color][/b] was later banned under the now unenforceable Motion Picture Production Code.

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, but in the photonics related sciences, I believe the units are more commonly expressed using power or energy.

In most areas of astronomy (and certainly in photometry), intensity is a simple photon count. In spectrometry, intensity is also just a photon count... given a pair of peaks, one of long and the other short wavelength, the intensity stated for each would not typically be adjusted for the different intrinsic energies of the photons recorded in each.

The usual simple metrics are intensity, the photon count; wavelength, the photon energy; integrated energy, the product of the photon count and photon energy over some particular wavelength range.

O.K., I give up:

Radio astronomers define the brightness (a.k.a., luminance, flux density) of extended radio sources in terms of jansky per steradian (1 Jy/ [4π steradian]) ~ 1,200,000 photons per second/[m² steradian]).

Optical astronomers define the brightness (a.k.a., luminance, flux density) of extended visible light sources in terms of brils ~ 130,000,000 photons per second/[m² steradian]). The sun at noon has luminance of about 5 trillion brils (= 500,000 blondels = 50 lamberts).

[Beyond]

Hey Art, does the 500,000 blondels take into consideration the shiny gold shoes Nice chair btw, although a bit dated.

[ta152h0]

The klingons are here, to serve mankind !!

[neufer]

Hey Art, does the 500,000 blondels take into consideration the shiny gold shoes

I hadn't noticed the shoes.