National Aeronautics andSpace Administration
Goddard Space Flight CenterGreenbelt, Maryland 20771

NASA / TP-2008-214169

Total Solar Eclipse Bulletin- excerpt -OBSERVING THE ECLIPSE

the meter range. The tissues in the eye transmit a
substantial part of the radiation between 380 –
400nm to the light-sensitive retina at the back ofthe eye. While environmental exposure to UV
radiation is known to contribute to the accelerated
aging of the outer layers of the eye and thedevelopment of cataracts, the primary concern
over improper viewing of the Sun during aneclipse is for the development of "eclipse
blindness" or retinal burns.
Exposure of the retina to intense visible lightcauses 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 aloss of visual function, which may be either
temporary or permanent depending on theseverity of the damage. When a person looks
repeatedly, or for a long time, at the Sun without
proper eye protection, this photochemical retinaldamage may be accompanied by a thermal injury– the high level of visible and near-infrared
radiation causes heating that literally cooks theexposed tissue. This thermal injury or
photocoagulation destroys the rods and cones,
creating a small blind area. The danger to vision issignificant because photic retinal injuries occur
without any feeling of pain (the retina has no pain
receptors), and the visual effects do not become
apparent for at least several hours after thedamage is done (Pitts 1993). Viewing the Sun
through binoculars, a telescope, or other opticaldevices without proper protective filters can resultin thermal retinal injury because of the highirradiance level in the magnified image.
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 theSun. It is never safe to look at a partial orannular eclipse, or the partial phases of a total
solar eclipse, without the proper equipment

Eye Safety and Solar Eclipses by Dr.Ralph Chou
A total solar eclipse is probably the mostspectacular astronomical event that most people
will experience in their lives. There is a great dealof interest in watching eclipses, and thousands ofastronomers (both, amateur and Professional)
and other eclipse enthusiasts travel around theworld to observe and photograph them.
A solar eclipse offers students a unique
opportunity to see a natural phenomenon that
illustrates the basic principles of mathematics andscience that are taught through elementary andsecondary school. Indeed, many scientists(including astronomers!) have been inspired tostudy science as a result of seeing a total solareclipse. Teachers can use eclipses to show how
the laws of motion and the mathematics of orbits
can predict the occurrence of eclipses. The use of
pinhole cameras and telescopes or binoculars toobserve an eclipse leads to an understanding of
the optics of these devices. The rise and fall ofenvironmental light levels during an eclipse
illustrate the principles of radiometry andphotometry, while biology classes can observe theassociated behavior of plants and animals. It isalso an opportunity for children of school age tocontribute actively to scientific researchobservations of contact timings at different
locations along the eclipse path are useful inrefining our knowledge of the orbital motions of
the Moon and Earth, and sketches andphotographs of the solar corona can be used tobuild a three-dimensional picture of the Sun'sextended atmosphere during the eclipse.
Observing the Sun, however, can be
dangerous if the proper precautions are not taken.
The solar radiation that reaches the surface of theEarth ranges from ultraviolet (UV) radiation atwavelengths longer than 290nm, to radio waves in1

and techniques. Even when 99% of the Sun'ssurface (the photosphere) is obscured during thepartial phases of a solar eclipse, the remaining
crescent Sun is still intense enough to cause aretinal burn, even though illumination levels arecomparable to twilight (Chou 1981 and 1996, andMarsh 1982). Failure to use proper observing
methods may result in permanent eye damage
and severe visual loss. This can have important
adverse effects on career choices and earningpotential, because it has been shown that mostindividuals who sustain eclipse-related eye
injuries are children and young adults (Pennerand McNair 1966, Chou and Krailo 1981, andMichaelides et al. 2001).
The same techniques for observing the Sun
outside of eclipses are used to view andphotograph annular solar eclipses and the partly
eclipsed Sun (Sherrod 1981, Pasachoff 2000,
Pasachoff and Covington 1993, and Reynolds andSweetsir 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 ascreen placed about a meter behind the opening.
Multiple openings in perfboard, a loosely woven
straw hat, or even between interlaced fingers canbe used to cast a pattern of solar images on ascreen. A similar effect is seen on the groundbelow a broad-leafed tree: the many "pinholes"
formed by overlapping leaves creates hundreds ofcrescent-shaped images. Binoculars or a small
telescope mounted on a tripod can also be used
to project a magnified image of the Sun onto awhite card. All of these methods can be used toprovide a safe view of the partial phases of aneclipse to a group of observers, but care must be
taken to ensure that no one looks through thedevice. The main advantage of the projection
methods is that nobody is looking directly at theSun. The disadvantage of the pinhole method isthat the screen must be placed at least a meter
behind the opening to get a solar image that islarge enough to see easily.
The Sun can only be viewed directly whenfilters specially designed to protect the eyes areused. Most of these filters have a thin layer ofchromium alloy or aluminum deposited on theirsurfaces that attenuates both visible and nearinfrared radiation. A safe solar filter should
transmit less than 0.003% (density ∼ 4.5) of visible
light and no more than 0.5% (density ∼ 2.3) of thenear-infrared radiation from 780 – 1400nm. (Inaddition to the term transmittance [in percent], theenergy transmission of a filter can also bedescribed by the term density [unitless] wheredensity, d, is the common logarithm of thereciprocal of transmittance, t, or d=log10[1/t]. A
density of ‘0’ corresponds to a transmittance of100%; a density of ‘1’ corresponds to atransmittance of 10%; a density of ‘2’ corresponds

to a transmittance of 1%, etc.). Figure 23 showstransmittance curves for a selection of safe solarfilters.
One of the most widely available filters for safe
solar viewing is shade number 14 welder's glass,which can be obtained from welding supplyoutlets. A popular inexpensive alternative isaluminized polyester that has been madespecially for solar observation. (Note that thismaterial is commonly known as "mylar," although
the registered trademark "Mylar®" belongs toDupont who does not manufacture this material
for use as a solar filter. Note that "Space blankets"and aluminized polyester film used in gardening
are NOT suitable for this purpose!) Unlike thewelding glass, aluminized polyester can be cut tofit any viewing device, and does not break whendropped. It has recently been pointed out thatsome aluminized polyester filters may have large(up to approximately 1mm in size) defects in their
aluminum coatings that may be hazardous. A
microscopic analysis of examples of such defects
shows that despite their appearance, the defects
arise from a hole in one of the two aluminizedpolyester films used in the filter. There is no large
opening completely devoid of the protective
aluminum coating. While this is a quality control
problem, the presence of a defect in the aluminum
coating does not necessarily imply that the filter ishazardous. When in doubt, an aluminizedpolyester solar filter that has coating defects
larger than 0.2mm in size, or more than a single
defect in any 5mm circular zone of the filter,
should not be used.
An alternative to aluminized polyester that has
become quite popular is "black polymer" in whichcarbon particles are suspended in a resin matrix.
This material is somewhat stiffer than polyester
film and requires a special holding cell if it is to be
used at the front of binoculars, telephoto lenses,
or telescopes. Intended mainly as a visual filter,
the polymer gives a yellow image of the Sun(aluminized polyester produces a blue-whiteimage). This type of filter may show significant
variations in density of the tint across its extent;some areas may appear much lighter than others.
Lighter areas of the filter transmit more infrared
radiation than may be desirable. The advent ofhigh resolution digital imaging in astronomy, especially for photographing the Sun, has increased
the demand for solar filters of higher opticalquality. Baader AstroSolar Safety Film, a metalcoated resin, can be used for both, visual andphotographic solar observations. A much thinnermaterial, it has excellent optical quality and muchless scattered light than polyester filters. Filters
using optically flat glass substrates are available
from several manufacturers, but are quite
expensive in large sizes.

23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; however, any
black-and-white negative with images in it is notsuitable for this purpose. More recently, solar
observers have used floppy disks and compact
disks (CDs and CD-ROMs) as protective filters bycovering the central openings and looking throughthe disk media. However, the optical quality of thesolar image formed by a floppy disk or CD isrelatively poor compared to aluminized polyester
or welder's glass. Some CDs are made with verythin aluminum coatings which are not safe – if theCD can be see through in normal room lighting, itshould not be used! 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.
Some sources of solar filters are listed below.
Unsafe filters include color film, black-andwhite film that contains no silver (i.e., chromogenic film), film negatives with images on them,smoked glass, sunglasses (single or multiplepairs), photographic neutral density filters andpolarizing filters. Most of these transmit highlevels of invisible infrared radiation, which cancause a thermal retinal burn (see Figure page 3).
The fact that the Sun appears dim, or that nodiscomfort is felt when looking at the Sun throughthe filter, is no guarantee that the eyes are safe.
Solar filters designed to thread into eyepieces
that are often provided with inexpensive
telescopes are also unsafe. These glass filtersoften crack unexpectedly from overheating when
the telescope is pointed at the Sun, and retinal
damage can occur faster than the observer canmove the eye from the eyepiece. Avoid
unnecessary risks. Local planetariums, science
centers, or amateur astronomy clubs can provide
additional information on how to observe theeclipse safely.
There are some concerns that UVA radiation(wavelengths from 315 – 380nm) in sunlight may
also adversely affect the retina (Del Priore 1999).
While there is some experimental evidence forthis, it only applies to the special case of aphakia,
where the natural lens of the eye has been
removed because of cataract or injury, and noUV-blocking spectacle, contact or intraocular lenshas been fitted. In an intact normal human eye,
UVA radiation does not reach the retina because
it is absorbed by the crystalline lens. In aphakia,
normal environmental exposure to solar UV
radiation may indeed cause chronic retinal
damage. The solar filter materials discussed inthis article, however, attenuate solar UV radiation
to a level well below the minimum permissible
occupational exposure for UVA (ACGIH 2004), soan aphakic observer is at no additional risk ofretinal damage when looking at the Sun through aproper solar filter.
In the days and weeks before a solar eclipse
occurs, there are often news stories andannouncements in the media, warning about thedangers of looking at the eclipse. Unfortunately,
despite the good intentions behind thesemessages, they frequently contain misinformation,and may be designed to scare people fromviewing the eclipse at all. This tactic may backfire,
however, particularly when the messages areintended for students. A student who heeds
warnings from teachers and other authorities not
to view the eclipse because of the danger tovision, and later learns that other students did see
it safely, may feel cheated out of the experience.
Having now learned that the authority figure waswrong on one occasion, how is this student goingto react when other health-related advice about
drugs, AIDS, or smoking is given (Pasachoff2001)? Misinformation may be just as bad, if notworse, than no information.
Remember that the total phase of an eclipse
can, and should, be seen without any filters, andcertainly never by projection! It is completely safe
to do so. Even after observing 14 solar eclipses,
the author finds the naked-eye view of the totally
eclipsed Sun awe-inspiring. The experienceshould be enjoyed by all.
Contributed by:
B. Ralph Chou, MSc, OD
Associate Professor, School of Optometry
University of Waterloo
Waterloo, Ontario, Canada N2L 3G1

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