[OT] Looking at the sun - Safe distance?

Henry Spencer wrote:

In article ,
Bruce Palmer wrote:

On some afternoons, depending on clouds and atmospheric dust, the sun
looks like a giant orange ball that you can actually look at without eye
damage as it sets...


A note of caution: safely low visible-light intensity doesn't necessarily
equate to safely low UV and IR intensity (especially the latter, which has
a tendency to penetrate clouds etc. better than visible light). Your eye
is not a reliable guide to what's eye-safe.

(I'll admit to having yielded to the temptation to look in such situations,
especially when there were naked-eye sunspot groups visible... but only in
brief glances, never looking steadily.)

Interesting. I hadn't thought of that. Luckily I can't ever
remembering actually staring at it that way for more than a short while.

Nevertheless, as you move away from the sun, beyond 1 AU, there must be
a point at which the intensity of harmful radiation falls below the
level that will damage your eyes. I'm going to try to find an
ophthamologist (sp?) who might be able to tell me the level of radiation
(at whatever wavelength) considered "safe". From there it should be
simple to derive a distance from the sun. I realize that the "safe"
distance in an ocular sense might not be "safe" in terms of other
energized particles that could cause radiation damage of other sorts.

I was watching a hockey game the other night and one of the opposing
team members' last name was Van Allen. At one point I yelled at the TV
"How's your radiation belt?" and got the most peculiar glance from my
wife. She sort of laughed. I thought it was funny. Guess you had to
be there.

--
bp
Proud Member of the Human O-Ring Society Since 2003

In article ,
Bruce Palmer wrote:
Nevertheless, as you move away from the sun, beyond 1 AU, there must be
a point at which the intensity of harmful radiation falls below the
level that will damage your eyes.

It might be a long way out. The apparent brightness, in photons per
square degree, doesn't change with distance -- the amount of light
received by a given collector (e.g. your pupil) drops off according to the
inverse-square law, but so does the apparent area of the Sun. So the spot
of concentrated light on your retina gets smaller, but the light intensity
within it doesn't change. Eventually, second-order effects like optical
imperfections in the eye will start to blur it, and conduction cooling
will get more effective as the heated area gets smaller, but you might
be well out of the solar system before the combined effects make the Sun
eye-safe.

Indeed, the danger to your eyes may be greater in the outer solar system,
because the focused spot will still be damaging, but the total brightness
won't be high enough to trigger the argh-that's-too-damned-bright reflex
that prevents you from staring at the Sun without deliberate effort here.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

(Henry Spencer) writes:

In article ,
Bruce Palmer wrote:
Nevertheless, as you move away from the sun, beyond 1 AU, there must be
a point at which the intensity of harmful radiation falls below the
level that will damage your eyes.

It might be a long way out. The apparent brightness, in photons per
square degree, doesn't change with distance [...] you might
be well out of the solar system before the combined effects make the Sun
eye-safe.

This seems unlikely. Laser beams can be focused to points even smaller
than the sun, and 1 mw lasers are considered almost completely safe.
Normal eye reflexes are enough to protect you, though the Class 2A laser
definition states that they can still cause damage if you overcome your
reflexes and stare into the beam for more than 1000 seconds. This is
the safest laser class, used for bar code readers and other completely
unprotected applications. No safety measures of any kind are
required - not even a warning sticker.

I don't know the maximum pupil area, but it should be about 1/2 cm^2 at
most, so 20 w/m^2 should be safe. Given that the sun's output at 1AU
is about 1400 w/m^2, you need about a factor of 70 reduction, so at 8.5 AU
and beyond should be safe enough for all practical purposes.

There must be some further distance where even deliberate staring of *any*
duration will not cause problems. I suspect (though the laser specs
don't state such a limit) that it's not a lot further out, since the
damage mechanism is thermal and you must be pretty close to the final
equilibrium temperature after 1000 seconds.

Lou Scheffer

In article ,
Louis Scheffer wrote:
It might be a long way out. The apparent brightness, in photons per
square degree, doesn't change with distance [...] you might
be well out of the solar system before the combined effects make the Sun
eye-safe.

This seems unlikely. Laser beams can be focused to points even smaller
than the sun, and 1 mw lasers are considered almost completely safe...
I don't know the maximum pupil area, but it should be about 1/2 cm^2 at
most, so 20 w/m^2 should be safe...

Depends somewhat on what duration you assume. Winburn's "Practical Laser
Safety" says that for visible light, continuous exposure for 1s, the
threshold of retinal damage seems to be about 10mW/cm^2 = 100W/m^2. But
you want to crank a healthy safety factor into that, not least because
the threshold is rather lower for short-wavelength IR and sunlight has a
fair bit of that.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Louis Scheffer wrote in message ...
(Henry Spencer) writes:

In article ,
Bruce Palmer wrote:
Nevertheless, as you move away from the sun, beyond 1 AU, there must be
a point at which the intensity of harmful radiation falls below the
level that will damage your eyes.

It might be a long way out. The apparent brightness, in photons per
square degree, doesn't change with distance [...] you might be well out
of the solar system before the combined effects make the Sun eye-safe.

This seems unlikely. Laser beams can be focused to points even smaller
than the sun, and 1 mw lasers are considered almost completely safe.

This leads to a curious numerical coincidence. If you are X AU from
the sun, then you can safely view the sun through optics of size X mm
in diameter. (At 1 AU, 1 mw comes through a 1 mm circular aperature).
So if you are the distance of TAU, the mission to one thousand AU,
then you can safely view the sun through anything up to and including
a 1-meter telescope. So at that distance the sun could surely be
considered eye-safe. (And at 1 mw/m^2 irradiation, that mission will
not be using conventional solar panels. Although you could imagine a
large solar sail that doubles as a concentrator...)

Lou Scheffer

In message , Henry Spencer
writes
In article ,
Bruce Palmer wrote:
Nevertheless, as you move away from the sun, beyond 1 AU, there must be
a point at which the intensity of harmful radiation falls below the
level that will damage your eyes.

It might be a long way out. The apparent brightness, in photons per
square degree, doesn't change with distance -- the amount of light
received by a given collector (e.g. your pupil) drops off according to the
inverse-square law, but so does the apparent area of the Sun.

Drat. You beat me to it. But continuing to the logical conclusion, the
Sun will continue to be a hazard until it's too small to resolve (1
minute of arc ??)
--
Rabbit arithmetic - 1 plus 1 equals 10
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