[ASC-list] solar safety

Ian ianw2 at exemail.com.au
Sat Jun 2 05:16:54 UTC 2012

Last week's "Catalyst" on ABC TV said that it was safe to take photos of the
Transit of Venus if you put a "solar density 5" filter on the front of your

If you are not familiar with solar photography, this (above) might be

a "solar 5 filter" means that the light is reduced by 10 to the power of
minus 5;

i.e. 1/100,000.

Solar filters can be made with a metal coating on glass or on plastic, or
from a black polymer;  and should be certified to AS1338.2 and 1338.3, or
the equivalent ISO, UK, or USA Standards.  They also block infra-red and
ultra-violet wavelengths.  These filters are usually supplied by
astronomical/telescope shops for observing sunspots, solar eclipses etc..

However, if you go to a camera shop and ask for a dark "number 5 filter",
you will probably be offered a "neutral density 5" (ND5) filter.  This
filter reduces the light equivalent to 5 aperture stops:

i.e. 1/2 x 1/2 x 1/2 x 1/2 x 1/2 = 1/32

Notice the difference !!!!!!!

* There are black polymer solar filters, mounted in rings designed to be
screwed onto camera lenses ...

* There are black glass neutral density filters, mounted in rings designed
to be screwed onto camera lenses ...

ooops, to an inexperienced person they may look to be the same thing.

I tried to get a thread-on solar camera filter recently ... they were sold
out, because of the recent annular solar eclipse and the approaching Transit
of Venus.  However, ND camera filters are fairly common.  

A neutral density filter is usually used when you want to use a slow shutter
speed, but the lens aperture cannot close far enough.  (Its called "neutral"
as it darkens all colours evenly;  but it may have no effect on IR or UV).

If an ND filter is used for solar photography it will probably get very hot,
expand, and break.  If the camera has plastic parts inside they will
probably soften and distort.  If the user has a display screen for pointing
and focusing the camera then they might only burn their fingers on a hot
camera body;  but if they look through an optical viewfinder, (e.g. on an
SLR/DSLR camera), they risk significant eye damage.

N.B. the retina has no pain receptors, so you can only recognise eye injury
after it has happened.

"Catalyst" also suggested using a number 12 welder's filter.  A number 14 is
usually recommended, if it is made of green glass.  However, most are now
made of plastic, and these would only be suitable if they had a sputtered
gold coating.

So, if you have no certification etc., how can you tell what's what?

If you have an antique tungsten filament light bulb, turn it on, and look at
it through the filter.  Through my solar filters I can see the red hot
filament as a mid, even dull, orange/red;  everything else is TOTALLY black.
Through my best solar filters I can see the individual coils in the filament
... wow.

If you want further solar safety information try

www.transitofvenus.org  and click on the [Eye Safety] link.  Ralph Chou's
video lecture is good.

Any astronomical observatories/societies that regularly observe the Sun (for
sun spots, flares, eclipses etc.) should also know how to do it safely.

;-)  Ian

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