Safe viewing of the Transit
of Venus
Biman Basu
bimanbasu@gmail.com
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Global visibility of ToV 5/6.6.2012
Unlike the transit in 2004, this year’s transit will
begin long before sunrise in India. So only part of
the transit will be visible.
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Why the Sun is dangerous to
look at
• The Sun is an extremely hot and bright object.
• It emits 40% visible light; 58% infrared (heat)
radiation; and 2% ultraviolet (UV) radiation.
• The visible light that provides us with colour
vision represents just a small part of the
electromagnetic spectrum.
• The UV and infrared radiations are invisible to
the human eye, but can have dangerous effects
on the retina.
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The Sun in many colours
Sun in
ultraviolet
Sun in visible
light
Sun in infrared
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The human eye
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Structure of the retina
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The risk of directly viewing the
Sun
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Sun induced damage to the eye
• After just one direct viewing of the Sun, a
person can attain a retinal ‘burn’ creating a
blind spot known as ‘solar retinopathy‘, which
often results in impairment or loss of vision.
• Solar retinopathy is damage to the eye's
retina, particularly the macula, which is the
region of keenest vision, from prolonged
exposure to solar radiation.
• It usually occurs due to staring at the Sun, or
watching a solar eclipse or a planetary transit
without eye protection.
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Solar retinopathy
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How the Sun damages the
retina
• Exposure of the retina to direct sunlight 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.
• 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.
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Retinal damage is painless
• Depending on the severity of the damage, an affected
observer may experience either a temporary or
permanent loss of vision.
• The important thing to remember is that both
photochemical and thermal retinal injuries occur
without the victim's knowledge, because 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.
• So it is always safer to take protection to avoid
damage to the retina.
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Damage to the cornea
• Staring directly at the Sun can also damages the
cornea, leading to a condition known as ‘photokeratitis’,
which is a painful inflammation of the cornea.
• Typically, the lens of the adult human eye absorbs
some UV rays to protect the eye.
• But, prolonged exposure to the Sun overwhelms these
natural defences, often leading to photokeratitis.
• This condition usually heals on its own in about a week.
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Photokeratitis
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Safe viewing of the transit of
Venus
• A safe solar filter should transmit 0% of the harmful
UV radiation (315 and 380 nm), less than 0.003% of
visible light (380 to 780 nm), and no more than 0.5% of
the near-infrared radiation (780 to 1400 nm).
• Filters made from certified aluminised Mylar sheets,
black polymer sheets, or dark welder’s glass (no. 14)
are safe for viewing the progress of the transit.
• Use of pinhole camera or small mirror, although good
for viewing solar eclipses, may produce fuzzy image of
the Sun in which Venus may not be clearly visible.
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Unsafe solar filters
• Unsafe solar filters include all colour films,
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 used
in photography, and polarising filters.
• Most of these transmit high levels of
invisible infrared radiation which can cause
a thermal retinal burn
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Spectral response of solar filters
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Safe solar filters
CAUTION! Never put on the
eclipse goggles and then look
through binoculars or a telescope!
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Watching through a telescope
When watching the transit through a telescope,
the Mylar filter must be fixed in front of the
objective. Never use "eyepiece" solar filters!
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The projection method
The best way for amateur astronomers to observe the
position of Venus during transit is to project the image
of the Sun on a white card using a small telescope.
CAUTION! NEVER LOOK AT THE SUN DIRECTLY!
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DON’Ts while observing transit
of Venus
• Don’t attempt to observe the Sun with naked
eyes.
• Never look at the Sun through a telescope or
binocular without a proper filter, placed in
front of the objective; not behind the eyepiece.
• Don’t use smoked glass, colour film, sunglasses,
exposed black-and-white film, or polarising
filters, as they are not safe.
• Don’t look at a reflection of the Sun from
water, even coloured water.
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Watch the transit safely!
It’s a once-in-a-lifetime
opportunity to see one of
Nature’s rarest spectacles.
HAPPY VIEWING!
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Thank You!
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