|
Many people suffer serious,
incurable eye damage when
viewing solar eclipses. To prevent
eye damage, you must be
informed of the dangers, the many misunderstood
and dangerous viewing methods, and
the few (but cheap and easy) safe
viewing methods.
The Dangers
Looking directly at the sun for more
than a second at any time (eclipse
or no eclipse) can damage your eyes.
Viewing a partial (annular) eclipse
and/or
the partial
phases
of a total
solar eclipse with unprotected eyes
is worse because you are concentrating
on looking at the sun, and you look
at it longer. It will
cause serious injury to your eyes that
will affect your vision for the rest
of your
life. Parts
of your retina may literally be cooked!
You
will feel nothing (there
are no pain receptors in your retinas.)
The sunlight may not even be very
bright. You will notice no difference
for several
hours.
But then you will notice a difference,
and your eyes will be damaged for
as long as you live.
Don't take a chance!
Safe Viewing Methods
There are only three safe viewing methods:
1. Viewing through a special
aluminized mylar film designed
specifically for eclipse viewing (examples).
(The mylar may be made into very inexpensive
paper-framed viewers or glasses and
sold cheaply, or distributed free.)
2. Viewing through very
dark welder's glass (No. 14)
3. Viewing by projection: you
use a card with a small hole in it
to project
an image of the eclipse on a screen,
and you watch the screen, not the sun.
You can use a piece of paper, or even
a straw hat with a little hole in it.
Just put something with a little hole
in it over a flat surface and you'll
see the sun's movement.
Unsafe Viewing Methods
These viewing methods, some hallowed
by folk tradition or flawed logic,
are unsafe, and you will damage your
eyes by using them:
— Sunglasses, single or multiple
layers
— Smoked glass
— All color film, black-and-white
film that contains no silver, photographic
negatives
with images on them (x-rays and snapshots)
— Polarizing
filters
— Photographic neutral density
filters
— Mylar film not of optical
grade (such as product packaging)
The Good News
If you are in the path of totality,
and during the totality of the eclipse,
it is safe (and recommended!) to view
the sun with the naked eye. But before
and after totality, or if you are not
within the path of totality, you should
use the safe viewing methods mentioned
above.
Click here for
info on photographing an
eclipse.
Here is more information, adapted
from an article entitled "Eye
Safety During Solar Eclipses" by
B. Ralph Chou, MSc, OD, Associate Professor,
School of Optometry, University of
Waterloo, Adapted from NASA RP 1383
Total Solar Eclipse of 1999 August
11, April
1997,
p. 19.
Observing the
Sun can be dangerous if you do not take the proper
precautions. The solar radiation
that reaches the surface of Earth ranges from ultraviolet
(UV) radiation at wavelengths longer than 290 nm
to radio waves in the meter range.
The tissues in the
eye transmit a substantial part of the radiation
between 380 and 1400 nm to the light-sensitive
retina at the
back of the eye. While environmental exposure to
UV radiation is known to contribute
to the accelerated
aging of the outer layers of the eye and the development
of cataracts, the concern over improper viewing of
the Sun during an eclipse is for the development
of "eclipse
blindness" or retinal burns.
Exposure of the
retina to intense visible light causes damage to
its light-sensitive rod and cone cells.
The light triggers a series of complex chemical
reactions within the cells which damages
their ability to respond
to a visual stimulus, and in extreme cases, can
destroy them. The result is a loss
of visual function which
may be either temporary or permanent, depending
on the severity of the damage. When
a person looks repeatedly
or for a long time at the Sun without proper protection
for the eyes, this photochemical retinal damage
may be accompanied by a thermal injury
- the high level
of visible and near-infrared radiation causes heating
that literally cooks the exposed tissue. This thermal
injury or photocoagulation destroys the rods and
cones,
creating a small blind area. The danger to vision
is significant because photic retinal injuries
occur without
any feeling of pain (there are no pain receptors
in the retina), and the visual effects do not occur
for
at least several hours after the damage is done
[Pitts, 1993].
The only time that the
Sun can be viewed safely with the naked
eye is during a total eclipse, when
the
Moon completely covers the disk of the Sun. It
is never
safe to look at a partial or annular eclipse, or
the partial phases of a total solar eclipse, without
the
proper equipment and techniques. Even when 99%
of the Sun's surface (the photosphere) is obscured
during
the partial phases of a solar eclipse, the remaining
crescent Sun is still intense enough to cause a
retinal burn, even though illumination levels are
comparable
to twilight [Chou, 1981, 1996; Marsh, 1982]. Failure
to use proper observing methods may result in permanent
eye damage or severe visual loss. This can have
important adverse effects on career choices and
earning potential,
since it has been shown that most individuals who
sustain
eclipse-related eye injuries are children and young
adults [Penner and McNair, 1966; Chou and Krailo,
1981].
The same techniques for observing
the Sun outside of eclipses are used
to view
and photograph annular
solar
eclipses and the partly eclipsed Sun [Sherrod,
1981; Pasachoff & Menzel 1992; Pasachoff & Covington,
1993; Reynolds & Sweetsir, 1995]. The
safest and most inexpensive method is by projection. A
pinhole
or small opening is used to form an image of the
Sun on a screen placed about a meter behind the
opening.
Multiple openings in perfboard, in a loosely woven
straw hat, or even between interlaced fingers can
be used to cast a pattern of solar images on a
screen.
A similar effect is seen on the ground below a
broad-leafed tree: the many "pinholes" formed
by overlapping leaves creates hundreds of crescent-shaped
images.
Binoculars or a small telescope mounted
on a tripod can also be used to project a magnified
image of
the Sun onto a white card. All of these methods
can be
used to provide a safe view of the partial phases
of an eclipse to a group of observers, but care
must be
taken to ensure that no one looks through the
device. The main advantage
of the projection methods is
that nobody is looking directly at the Sun. The
disadvantage
of the pinhole method is that the screen must
be placed at least a meter behind the
opening to get
a solar
image that is large enough to see easily.
The Sun
can only be viewed directly when filters specially
designed to protect the eyes are used. Most
such filters have a thin layer of chromium alloy
or aluminum
deposited on their surfaces that attenuates both
visible and
near-infrared radiation. A safe solar filter
should transmit less than 0.003% (density~4.5)[1]
of visible
light (380 to 780 nm) and no more than 0.5% (density~2.3)
of the near-infrared radiation (780 to 1400 nm).
One of the most widely available
filters for safe solar viewing is shade
number 14 welder's
glass, which can
be obtained from welding supply outlets. A
popular inexpensive alternative is aluminized
mylar manufactured
specifically for solar observation. ("Space
blankets" and
aluminized mylar used in gardening are not suitable
for this purpose!) Unlike the welding glass,
mylar can be cut to fit any viewing device, and
doesn't break
when dropped. Many experienced solar observers
use one or two layers of black-and-white film
that has
been fully exposed to light and developed to
maximum density. The metallic silver contained
in the film
emulsion is the protective filter. Some of the
newer black and white films use dyes instead
of silver and
these are unsafe. Black-and-white negatives with
images on it (e.g., medical x-rays) are also
not suitable.
More recently, solar observers have used floppy
disks and compact disks (both CDs and CD-ROMs)
as protective
filters by covering the central openings and
looking through the disk media. However, the
optical quality
of the solar image formed by a floppy disk or
CD is relatively poor compared to mylar or welder's
glass.
Some CDs are made with very thin aluminum coatings
which are not safe - if you can see through the
CD in normal room lighting, don't use it!! No
filter should
be used with an optical device (e.g. binoculars,
telescope, camera) unless it has been specifically
designed for
that purpose and is mounted at the front end
(i.e., end towards the Sun). Some sources of
solar filters
are listed in the following section.
Unsafe filters
include all color film, black-and-white film
that contains no silver, photographic negatives
with images on them (x-rays and snapshots),
smoked glass, sunglasses (single or
multiple pairs),
photographic neutral density filters and polarizing
filters. Most of these transmit high levels of invisible
infrared
radiation which can cause a thermal retinal
burn (see Figure 24). The fact that
the Sun appears
dim, or that
you feel no discomfort when looking at the
Sun through the filter, is no guarantee
that your
eyes are safe.
Solar filters designed to thread into eyepieces
that are often provided with inexpensive telescopes
are
also unsafe. These glass filters can crack
unexpectedly from overheating when
the telescope is pointed
at the Sun, and retinal damage can occur faster
than
the observer
can move the eye from the eyepiece.
In spite of these precautions,
the total phase of an eclipse can
and should be viewed
without
any filters
whatsoever. The naked eye view of totality
is not only
completely safe, it is truly and overwhelmingly
awe-inspiring!
Total Solar Eclipse in Turkey
Best Cities for Viewing
Photographing an Eclipse
Travel Details
Recommended
Itineraries
Transport in Turkey
|
