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Douglas Denny pointed us toward a review paper on irradiation; by Haines and
Allen, in "Navigation", 1968-9..

I will make a few observations about that paper, without yet arriving at a
conclusion. There are several aspects that readers should be aware of.

It relates entirely to naked-eye observations. The intention seemms to have
been to estimate the usefulness of sextant angle observations from a space
vehicle, or perhaps from the Moon. In neither case would there be the sort
of buffeting that occurs at sea, so in practice I would expect a rather
high-magnification scope to be appropriate. Because the irradiation
phenomenon occurs in the eye, when comparing the reported observations in
this paper with our own experience at sea, we have to reduce the given
values for irradiation by the magnification of whatever scope is being used.
Presumably, the irradiation value of 0.6' + 0.6', recommended at one time to
be applied to upper-limb Sun observations taken at sea, presumed some
telescope magnification. Anyone know what that was?

Case 1 relates to work on resolution between two point sources, with
different levels of light background. Although that text surrounds fig 5,
the diagram of the testing facility, case 1 used an entirely different
setup, not described well in any detail. Presumably, to discover more we
would need to read ref 23, by Ogle (an apt name, for a student of vision).
Ogle's results, as shown in fig 4, look strange, and counter-intuitive, to
me (which doesn't, in itself, mean that they are wrong). Particularly 4a and
4b, which shows resolution getting better as the background illumination is
increased. To me, that is a surprise!

Some other things look odd. Fig 3a looks quite crazy, showing the pattern of
distribution of light on the retina becoming wider as the intensity
increases. Why should that be so? About fig 3b, the text refers to "the
overlap of the diffraction patterns on the retina..." If we take a pupil
size of 6mm, I make it that the first minimum of the Airy disc occurs at
just 20 arc-sec from the centre, far smaller than any of the effects that
are discussed. The conclusion from that is that the eye is not
diffraction-limited, or not in wide-pupil conditions anyway. Instead, other
optical imperfections predominate.

When we get to case 2, that work was done at the "Ames High Luminance Vision
Laboratory". That name, on its own, should raise some suspicions, that
perhaps what's of interest in this research may not be relevant to us at
sea. Those suspicions increase when we see the light-source that they use, a
carbon-arc lamp running at 10 kilowatts! And a condenser-lens made of
quartz, presumably so that it doesn't melt. What appears to be the aim of
the research is to see how much angular observations are upset by the
presence of a blinding light shining on a white target.

An enormous range of luminance has been investigated, but to be frank, I
don't know how to relate those luminance values to the familiar images of
the objects I'm accustomed to seeing in the sky. All that I can say about
such observations, is that if sextant images get uncomfortably bright, the
first thing a real navigator would do is to pull in a shade, to bring the
images back into his comfort-zone. There seems to be no mention of the use
of shades, anywhere in the paper.

In this note, I'm not attempting to assess the value of the Haines and Allen
work in itself; just to point out some factors that a reader should take
into account when weighing-up whether it has any relevance to the
observations he takes at sea.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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