[OT] Looking at the sun - Safe distance?

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. |

On Mon, 5 Jan 2004 14:21:10 -0600, Bill Higgins
wrote:

Next time, try saltines as well.

....****, when I did this in High School, everyone thought I was nuts.
Good to see someone else thought of that one as well :-)

OM

--

"No ******* ever won a war by dying for | http://www.io.com/~o_m
his country. He won it by making the other | Sergeant-At-Arms
poor dumb ******* die for his country." | Human O-Ring Society

- General George S. Patton, Jr

On Mon, 05 Jan 2004 09:37:54 GMT, John Beaderstadt
wrote:

I was getting sunburned over only the left side of my face.

....Then quit following those UFOs with the window down :-P\

OM

--

"No ******* ever won a war by dying for | http://www.io.com/~o_m
his country. He won it by making the other | Sergeant-At-Arms
poor dumb ******* die for his country." | Human O-Ring Society

- General George S. Patton, Jr

Henry Spencer wrote:


If memory serves, total eclipses are safe for naked-eye viewing *during*
*totality*. But caution has to be exercised during the partial phases
before and after totality.

I'm pretty sure the corona's UV emissions can damage the eye even at
totality; I know my eyes hurt after I looked at a total eclipse for a
minute or two (and anyone who doesn't think that they are also going to
look at it during totality is fooling themselves- if there was a cage
with a tarpaulin over it that said "Warning! Medusa having sex with a
Basilisk! DO NOT LOOK!" around 95% of people would promptly take a peek
under that tarpaulin inside of ten seconds.)

Pat

"OM" om@our_blessed_lady_mary_of_the_holy_NASA_researc h_facility.org wrote
in message ...
On Mon, 05 Jan 2004 09:37:54 GMT, John Beaderstadt
wrote:

I was getting sunburned over only the left side of my face.

...Then quit following those UFOs with the window down :-P\

Funny you'd mention that. I just got that on DVD and watched it like a week
ago.



OM

--

"No ******* ever won a war by dying for | http://www.io.com/~o_m
his country. He won it by making the other | Sergeant-At-Arms
poor dumb ******* die for his country." | Human O-Ring Society

- General George S. Patton, Jr

(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 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
Remove spam and invalid from address to reply.

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