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Q: Properites of Light from Outer Space ( No Answer,   4 Comments )
Question  
Subject: Properites of Light from Outer Space
Category: Science > Physics
Asked by: gadgetmad69-ga
List Price: $50.00
Posted: 02 Nov 2004 09:09 PST
Expires: 03 Nov 2004 17:18 PST
Question ID: 423467
Hello.

I'm interested in learning more about the properties of light.

In particular, I would like to know the Lux measurement of our sun /
surrounding stars from outside the Earths atmosphere.  For
information, it is widely reported that a Summers day on the equator
would be 100,000 Lux so I guess that the figure from outer space would
be much greater.

Once this has been ascertained, I would like at least 500 words about
the properties of light from outer space along with links to
appropriate web pages.  This may include current factual or
theoretical viewpoints.  E.g does the Lux increase the nearer one is
to a star and what effect does this have on humans ability to process
visual information at extreme light levels? (a comparison between low
light levels and high light levels might be appropriate here)

The answer to this question will be posted on the web and credit given
so thanks in advance for your help and good luck!
Answer  
There is no answer at this time.

Comments  
Subject: Re: Properites of Light from Outer Space
From: racecar-ga on 02 Nov 2004 10:52 PST
 
The atmosphere does not absorb much in the visible range, so if you
get 100,000 lux at noon on the equator, that's very close to what you
get just outside the atmosphere.

Yes, the illuminance is a function of distance from the source.  It
varies as one over the distance squared.

Humans have two types of light sensing cells.  Only the rods (which
cannot see color, as the cones can) are sensitive enough to pick up
very weak signals, but there are few or no rods in the center of the
retina.  So if a star is very faint, you may not be able to see it
when looking straight at it, even though you can see it if you look in
a slightly different direction.  For the same reason, it may be
impossible to tell the color of a very weak light source, even though
you can see it.  Also, rods are insensitive to red light, so at very
low light levels we can see blue or green light much better than red.
Subject: Re: Properites of Light from Outer Space
From: gadgetmad69-ga on 02 Nov 2004 11:08 PST
 
Racecar-ga

Thank you for these comments, expecially "Yes, the illuminance is a
function of distance from the source.  It varies as one over the
distance squared."
Subject: Re: Properites of Light from Outer Space
From: hfshaw-ga on 02 Nov 2004 14:06 PST
 
Measurements of light intensity are reported in a somewhat bewildering
array of units, which makes things tough to sort out at first.  The
first thing to know is that when dealing with measurements of light
intensity, there are two types of units:  photometric units, which are
measures of *visible* light intensity, and radiometric units, which
are measures of the total electromagnetic energy (including those
wavelenghts that the human eye is not sensitive to).  "Lux" is a
photometric unit that takes into account the sensitivity of the human
eye to different colors (wavelengths) of light.  See
http://www.physlink.com/Education/AskExperts/ae409.cfm.

The total energy emitted by a light source per unit time is measured
in units of power and is called either the "radiant flux "when all
wavelengths are included, or "luminous flux" (sometimes "photometric
flux") if only the human response to the light is considered.  Radiant
flux is expressed in units of Watts, while luminous flux is expressed
in units of "lumens".  See
http://www.intl-light.com/handbook/flux.html.  When talking about the
sun or stars, scientists use the term "luminosity" to refer to the
total radiometric power emitted by the the object.  The sun's
luminosity is about 3.9 * 10^26 Watts.  Empirically, the masss and
luminosities of stars are correlated, and the luminosity for "typical"
stars (what are called "main sequence" stars) varies as the the mass
raised to a power somewhere between 3 and 4 (usually taken as
luminosity ~ M^3.5).  See
http://csep10.phys.utk.edu/astr162/lect/binaries/masslum.html.

Another way to describe the amount of light given off by a source is
in terms of "radiometric intensity" or "luminous (or photometric)
intensity", which is defined as the flux (see above) per unit solid
angle.  Intensity, therefore, has units of power per unit solid angle;
 Watts/steradian for radiant intensity, or lumens per steradian for
luminous intensity.  One lumen per steradian is defined as one
candela.  See http://www.intl-light.com/handbook/intens.html.  There
are 4*pi steradians in a sphere.  See
http://whatis.techtarget.com/definition/0,,sid9_gci528813,00.html for
a description of solid angles a the definition of a steradian.

Typically, a light source (like the sun or a star) emits a constant
amount of energy per unit time (more or less).  Now, think of a hollow
sphere with radius R surrounding the light, and centered on the light.
 All the energy emitted by the light must pass through the surface of
that sphere.  As the sphere gets larger, the surface area of the
sphere also gets larger.  In fact, the surface area grows as R^2
(formula for the surface area of a sphere is A = 4*pi*R^2).  The
amount of energy per unit area at a given distance, R,  from a light
source is then the total power of the source (in Watts or lumens)
divided by the surface area of the sphere with radius R.  Obviously,
as one gets further and further from the light source, the energy
emitted by the light must be distributed over larger and larger
surface areas, and the "flux density" decreases as 1/R^2.  The
radiometric flux per unit area is called the "irradiance" and is
expressed in units of Watts/meter^2, while the photometric flux per
unit area is called the "illuminance" and is expressed in units of
lumens/meter^2.  One lumen per square meter is defined as one "lux". 
See http://www.intl-light.com/handbook/irrad.html

Finally, one can report the intensity of a light source in terms of
the flux density per unit solid angle.  Radiometric measurements are
given in units of Watts/Meter^2/steradian and this measure is called
the "radiance".  Photometric measurements are given in units of
lumens/meter^2/steradian, and this measure is called the "luminance". 
Unlike flux density measurements given in terms of irradiance or
illumance, the luminance and radiance of a source are independent of
the distance from the source.  See
http://www.intl-light.com/handbook/rad.html.
Subject: Re: Properites of Light from Outer Space
From: gadgetmad69-ga on 03 Nov 2004 17:15 PST
 
Hfshaw-ga

This is fantastic, I have even more questions now but I think the
links that you have provided will answer them.  The way you wrote the
penultimate paragraph allowed me to visualise the topic extremely
well.

Thank you very much for your time and help.

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