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PostPosted: Sat Apr 27, 2019 8:27 am 
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I was looking at an online page about human vision and that stated something I never really though of before. If you shine a bright light in only one eye of a person in a dark room the pupil of the other eye that is shielded from the light also contracts.

That got me thinking - dark adaption involves chemical changes in the retina at the back of the eye and takes 10's of minutes to become fully effective. Following the above if you then shine a bright light in one eye does the other eye lose it's dark adaption ? I can't find an answer online.

Woud members like to experiment and let us know the results ?

One think I have noticed is that loss of dark adaption is not immediate, you can have a very short exposure to bright light and your dark adaption returns immediatly. What seems to happen is that you almost close your eyelids to avoid dazzle much as the same as you look at a very bright light during the day as this is a faster response than the pupil or loss of dark adaption.

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PostPosted: Sat Apr 27, 2019 3:13 pm 
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Wearing red-tinted goggles when we go indoors can help to preserve night vision. They used to be on sale for that purpose, but we don't seem to see them these days. Of course, there's the other option of having a dim red light on indoors

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PostPosted: Sat Apr 27, 2019 3:21 pm 
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BE FAST

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PostPosted: Sat Apr 27, 2019 5:00 pm 
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Hi John,
I don't think that the other eye loses dark adaption, except that it is likely some light will reach both eyes, unless the exposure is very short. Seconds at most.
My right eye saw stars 0.5 magnitude fainter than my left.
Green grass was also different in my two eyes.
I always saw 11 Pleiads from town. Then 13 from a town by the Baltic. It had lots of lights.
I saw 15 or 16 Pleiads on La Palma at 7,900 ft in my late 40s. Not a good night according to the locals because of Saharan dust.
I saw M33 with direct vision even though rather low. My zenithal mag limit was mag 7.2.
I forgot to look for the Geggenschein.
A fellow astronomer told me he saw 19 Pleiads, but didn't tell anybody, because of disbelief.
24 have been recorded unaided eyes.
The 12th and 13th are mag 6.2 to 6.4.
8.3 and 8.4 mag stars were seen from inside a professional high elevation observatory with a small opening.
Veronica Seider, an optical student in Stuttgart, was tested in a controlled environment when she was 21. She is supposed to have detected 10th mag stars, but I cannot get the report because of medical confidentiality.
Aoriginal Australians were tested when young and had 6/2 and 6/3 eyesight typically, with a best of 6/1.4 or 20/4.7.
They could also see very faint stars, way beyond the testers.
Almost as good results probably occurred with south American native sailors and hunter gatherers in Africa.

It is known that in healthy eyes both close down if one is exposed.

As I get older dark adaption takes longer and it not as complete.

In fact dark adaption carries on for at least a day.
Deep sky astronomers do not expose themselves to bright sunshine in the previous 24 hours.

In a case where a family lived in sewers in total darkness for a year, being fed by an old employee even when the money they gave him ran out, the effect was strange. They became sensitive to the faintest light. They occasionally had a small candle for light.
When they surfaced eventually everything looked red for quite a time.
I think they eventually recovered. From memory one family member died in the time underground.

In the book of the film Catch me if you can. Frank Abignale Jr. was imprisoned without clothes in total darkness for about six months. He was about 18. They realised he could not complete his sentence without losing his life.
He was first exposed to faint light for a day or so, and slowly the guards increased the light. They stated that he would have gone blind if suddenly exposed to full sunshine.

I visited Harold Ridley at his home, delivering the Zeiss 120cm f/7 Aero Triplet that he used for his comet photos. It took over a year before I persuaded the previous owner to let it go.
It was a 2 hour drive each way. I arrived back about 3 a.m.
He found me sitting outside his home half an hour after we parted as I just sat looking at a really dark sky.
He mentioned that exposing dark adapted eyes for a short period to indoor white lights does not affect dark adaption. I agreed. But only for a few seconds.
The curve of dark adaption slowly changes after the initial 20 or 30 minutes.

There is I think an insect which basically has image intensifier eyes.

Regards,
David


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PostPosted: Sun Apr 28, 2019 8:37 am 
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There is a page about dark adaptation of the eye here https://webvision.med.utah.edu/book/part-viii-psychophysics-of-vision/light-and-dark-adaptation/. It iassumes some medical knowledge in places however there are some interesting curves (figure 2) about adaptation particually those that show how the time taken depends on how bright the previous exposure to light was.

They use a unit new to me the Troland see https://en.wikipedia.org/wiki/Troland for more details. The Troland is deprecated see https://www.wolframalpha.com/input/?i=troland for more information.

This page also contains curves and details for light adaptation.

No answer to my question though of whether dark adaptation and the loss thereof is a single eye process or there is a connection ( by the central nervous system ?) bettween the two.

John Murrell

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PostPosted: Sun Apr 28, 2019 10:47 am 
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Hi John,
When observing with a telescope, if I needed to go indoors I shielded my observing eye and only allowed light to enter the other eye.
When returning to the telescope I did not notice a loss of sensitivity.
However, my observing sites were not truly dark.

The brain may not be able to ignore the image from the eye with less dark adaptation with binocular vision.
But just using a telescope it could be the brain just deals with the image presented, and can disregard the other eye.

One of the few really dark places I have been was Tenerife near the observatory.
It was so dark I could not detect the ground and I nearly fell into the caldera in the dark.

In Finland in a forest in a National Park there were so many stars above the trees that I could not make out the constellations. I do not know my ZLM there but probably fainter than mag 7.0.
However, the starlight itself meant the sky was not truly dark.

Then there is airglow.
Outdoors it may be impossible to find true darkness. Image intensifiers see well by starlight alone.

When testing a friend's Aurorae camera in a seemingly totally dark large hall the system clearly saw the full extent of the hall and things in it. It used a Canon 50mm f/0.95 transfer lens and 6 Konica 135mm f/2.8 imaging lenses and a British image intensifier c. 1980. It detected visible light although it may have had infra red capabilities also.

On La Palma at the zenith I saw repeated very faint meteors with short trails. I don't think that these were imagined as they all led to a central radiant. Others in the group did not detect these.

Yet with the 6 inch Newtonian some clearly saw the Pleiades nebulosity where I struggled to see it. So I saw better with unaided eyes but not with the telescope,

Thanks for the links.

Regards,
David


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