Can the use of a Sextant cause eye damage?

[noelex 77]

Dennis if you search the forum you will find this topic has been done to death. However, I will try once more because I feel the the notion of the red light myth is very bad advice for cruising sailors standing night watch.

So if you believe:

Originally Posted by denverd0n

The color has nothing to do with it.

You need to explain data such as this:

[Adelie]

Originally Posted by noelex 77

I think we would be better to agree to disagree on that point Dennis .

I’m not sure that arguing with an eye doc about their area of expertise is a winning move.

Originally Posted by noelex 77

Dennis if you search the forum you will find this topic has been done to death. However, I will try once more because I feel the the notion of the red light myth is very bad advice for cruising sailors standing night watch.

So if you believe:

You need to explain data such as this:

Night vision is a very complex subject and a single graph is not going to be sufficient basis to support any side in an argument. Especially since such a graph means you are arguing from first principles rather than empiracly.

Talk to A64pilot about this. He had a long discourse in some thread in the last year. If I recall correctly the USArmy went to white lights for instruments in helicopters after doing a lot of research.

I’ve got a paper on my laptop about this I’ll post later after work.

[noelex 77]

Originally Posted by Adelie

Especially since such a graph means you are arguing from first principles rather than empiracly.

The graph is produced from experiments on real subjects. Thus it is empirical data rarher than theoretical predictions based on first principles.

[hellosailor]

Plain glass, such as the multiple optical glass filters and optical glass lenses in sextant telescopes, will block an extremely high amount of UV light. IR tends to pass through glass more, but it is focused differently (as are all wavelengths) and if the image of the sun is being focused on your retina, the "IR image" of it will not be in focus, not be as sharp, regardless of the extant that it gets through. If anyone is really concerned there are IR blocking glass types (HAG, thin heating absorbing glass, is sold for optical and theatrical use) and window films like 3M Crystalline, which block about 99% of the IR and UV without blocking more than a few percent of visible light, clear as glass.

The traditional most effective means of blocking IR in glass is to layer it lightly with a gold film (gold tinted glacier glasses being the example) but that's still expensive.

On the myth of red light and night vision...the point has been made that this is well past any news. Since LEDs were invented and pure colors of light are now so cheaply available, many issues from expensive days of colored glass are moot. The USN conducted extensive tests on the use of colored lights for night vision and as a result, I am told that the last couple of generations of ships use BLUE lighting for night vision. Among the reasons for this, is that if someone catches a glimpse of your "interior" night lighting leaking out, they are much less likely to see the blue light, than to notice a RED light somewhere out there. Aircraft and automakers also differ in results and reasons, but the hard science comes down to the mystical attributions of red, and the way it really doesn't need to be adjusted much from one person to the next. A DIM white light is just as good as red, but there's higher variation from one person to the next in what is "dim enough".
And then there are magenta lines on navigational charts. Anyone ever see what happens to them in different colors of light? Ahuh, not a military concern.

[thinwater]

Originally Posted by hellosailor

....A DIM white light is just as good as red, but there's higher variation from one person to the next in what is "dim enough".
And then there are magenta lines on navigational charts. Anyone ever see what happens to them in different colors of light? Ahuh, not a military concern.

This is the key point to me. For my personal eyes, I need a LOT more red light to function than I need white light. But it is easier to regulate the amount of light by going to a designated colored light.


Change in subject.


My last chart plotter had the brightness control on one of the menus. If you turned it down at night to a suitable level, there was no possible way you would be able to reset it in daylight, because you couldn't read the menus! I ended up placing three beach towels over the plotter and me to get it dark enough. After that, I learned to turn it up when I was finished.

[Adelie]

Originally Posted by noelex 77

The graph is produced from experiments on real subjects. Thus it is empirical data rarher than theoretical predictions based on first principles.

The graph shows real measured data and in that sense is empirical, but it does not measure light exposure doing a shipboard task then seeing how long before night vision recovers enough to perform another shipboard task requiring night vision.

The graph is measuring how long it takes for the blank spot at the center portion of one's night vision to shrink to 1* after red or white light exposure. So you are arguing from the 1st principal that the faster recovery rate of the center portion of the eye after exposure to red light to the conclusion that red light allows faster night vision recovery sufficient for shipboard tasks. That's why I say that you are arguing from 1st principles. If you wanted to empirically measure night vision recovery on a vessel, you would do different tests.

Also, the graph seems to have caveats that were not conveyed in the text.
Not really sure, the description is hard to read and the citations seem to be behind paywalls. Science publishing needs to get it's act together, but that's another issue altogether.
I did find a discussion of this topic including the graph above. The discussion for one of the related graphs indicates:
"3) At low brightness (the bottom graph), white and red are very close.
[sidenote: One should note that white exposure is always lower than red. That's been done to take into account that people need a little bit more red brightness to see than with white]" It's not clear if exposure times were the different for white and red or what exactly was done for one graph or for all of them.

The prescription to use red light is a very simple one and I believe that in reality the problem is very complex and has multiple answers.

On the one side you have the question of what shipboard task do you need light for? Plotting your position on a chart? Changing a sail? Finding a small lost object on the sole? Do you need to discern red coloring during any of these activities?
For charting you may want to use white light so you can see magenta lines. Even if you don't need to see VTS lanes compass roses in the US are printed in magenta. Yeah, you can use protractors but I really don't like messing with variation so I always use the inside rose that shows magnetic north. If I just want to plot a GPS position then a dimmed red light might be better, depends on how much detail I need to plot the position. In any case I'm going to need brighter red light to do the same plotting. How much difference in recovery time does that make?

On the other side what are you doing that needs night vision? Looking out for the navigation lights of other vessels? Trimming a sail? Looking for unlit lobster pots so you don't hit them?
Looking for other vessels' navigation light seems like the most important. So how long after light exposure before my eyes are recovered enough to see those lights. How long the blank area at the center of my vision persists doesn't tell me how soon I can discern navigation lights with my peripheral vision.

My experience with red lights is fixtures with 3 position switches where the options are White-Off-Red and there is no option for dimming anything. Even the above graph makes it clear that if the red light is bright enough you can recover from a dimmed white light faster.

Middle link below has recommendations for what light to use and when:
-No matter what your color choice it must be fully adjustable for intensity.
If you need the fastest dark adaptation recovery and can adjust to the limitations, or everyone in your group is using night vision equipment then blue-green.
-If you must see detail (reading a star chart, or instrument settings) and can lose peripheral vision (see note 1), then a very long wavelength red at a very low level. Red really only has an advantage at very low levels (were the night blind spot is very obvious).
-A general walking around light so that you don't trip over the tripod, knock over equipment or bump into people, then blue-green with enough red added to get rid of the night blind spot, or maybe just use white. Blue-green at higher brightness also works very well and at a lower intensity than white.
-If you need to see color and detail then likely the best choice is the dimmest white light for the shortest amount of time.

https://biology.stackexchange.com/qu...r-night-vision
Night Vision - The Red Myth
Is using red light for "preserving night vision" simply a myth?

[Adelie]

Originally Posted by Adelie

...
Talk to A64pilot about this. He had a long discourse in some thread in the last year. If I recall correctly the USArmy went to white lights for instruments in helicopters after doing a lot of research.

I’ve got a paper on my laptop about this I’ll post later after work.

Found it: http://www.cruisersforum.com/forums/...ml#post2440072

And in another thred it turns out he cited the same paper I have: http://www.cruisersforum.com/forums/...ml#post2294828

[seandepagnier]

so... If I use welding filters I can burn my eyes from IR?

I read that any single color, red, green or blue is better because it only disrupts some cones. I don't really make sense of this or know if it's true.

Night vision is most sensitive in a circle around the center, which I notice often I can detect faint lights by not looking directly at them.

I'm pretty sure younger eyes have the biggest advantage compared to special colors if that even helps.

[noelex 77]

Originally Posted by Adelie

On the other side what are you doing that needs night vision?

This is a good point. Red light is the best colour for preserving night vision, but this does not automatically make it always the best night time choice for everything. When using red light all colour information is lost and complex visual tasks will be slower, more difficult and more error prone.

So for example when planning a passage, calculating bearings, distances, tides, currents etc would generally be better done under white light. The greater reduction in dark adaptation would usually be justified.

On the other hand, a quick look at the chart to verify a depth for example is practical to do with dull red illumination. If dark adaptation was at a premium, for example when coming into an anchorage at night and trying to spot unlit boats, crab pots etc then using red light for a brief look at the chart would be sensible.

There are also many uncritical visual tasks on boat. Making coffee getting dressed etc. When doing these tasks it is silly not to preserve as much night vision as possible and red light is ideal.

So don’t think of red as a panacea, its practical use is for non complex visual tasks, especially when the deeper levels of dark adaptation are needed.

A red fitting with adjustable levels is better.

Sorry for the thread drift, but it an important subject that many cruising sailors don’t understand.

[noelex 77]

Originally Posted by boat_alexandra

so... If I use welding filters I can burn my eyes from IR?

I dont think the risk is high, but welding filters have been made to block harmful radiation from welding not from the different wavelengths produced by the sun.

In recent years welding fiters have become more sophisticated and tailored more specifically. So the risk is greater.

The difficulty is most of the damage from the sun is produced from wavelengths that cannot be seen. Therefore when looking through the filter there is no way to tell if the filter is doing an adequate job. Comfort, or darkness in the visable spectrum is not a good indication of suitability. Using a filter that has been designed for solar viewing is better than trying to use something that has been disigned for a different purpose.

[Snowpetrel]

Good technique is important when using a sextant. Occasionally you can get a flash in your eyes even with the shades correctly set as it can sometimes shine outside the shades and still get into the telescope somehow. As you get better at shooting the sun this tends to be much less common.

Or if the horizon shade is set too low you can get a bright burning flash of sun through it when . With practice if you set the horizon shade up correctly for the horizon, then add the full shades to shoot the sight. When you get the sun near the horizon you need to flick the extra shades off so you can see the horizon clearly.

Other issues can be in partly cloudy conditions when the sun comes out from behind a cloud. Then you need to be able to quickly add a few shades. Sometimes to both horizon and index mirrors. Ive often wonder if an adjustable polarising lens could be used for quickly fine tuning the shade level. Unfortunately sextant development stopped in the 80's.

But you will find that your first few sights will be tricky, and you might get the odd slight flash. With a bit of practise manipulating and selecting shades you should find it's not a big issue.

[hodgmo]

“Ive often wonder if an adjustable polarising lens could be used for quickly fine tuning the shade level.”

The US Navy tried this, and (at least) some David White Mark II sextants used adjustable polarizers rather than colored glass for the index and horizon filters. Here’s a picture of one: https://photos.app.goo.gl/UwBGMEygVHchMb3A9

A US Navy comparative test of sextants (1958-1961) resulted in a report (“PRECISION CELESTIAL NAVIGATION EXPERIMENTS”) that includes some discussion of the use of polarizers for daylight star observations: http://www.dtic.mil/dtic/tr/fulltext/u2/259559.pdf

Regarding the OP question regarding eye safety: The above Navy report, and other authoritative documents I am aware of, eg, Bowditch’s “American Practical Navigator,” include little or no discussion of eye damage being a problem. In my experience, as I find the sun for a sight, there’s plenty of warning that more filter is required if I am using insufficient shades.

[gjordan]

The OP was asking about a Davis sextant. Davis is in Calif and sells replacement parts. They may have better quality filters available or just add one filter to the set. Google them and ask. I found that the Davis lifeboat sextant would not gather enough light to do star or planet sights but the slightly more expensive Ebco with its 4 power scope worked fine for evening shots. The 25 should be fine and has more (better selection) of filters than the early lifeboat sextant. Just my thoughts. Grant.

[seandepagnier]

what about using a few sunglasses shades?

What else can work? I am skeptical of welder shade now. How to test for IR to know if it's safe? Use a magnifying glass and see if it can still burn something or get very hot?

Maybe a digital camera can work (I know it's electronic) but no risk of eye damage.. and it can potentially take much more accurate sights.

[Cpt Pat]

Originally Posted by noelex 77

Eye damage is almost always caused by the non visible portions of the spectrum.

With any filter the concern is that the visible portion of the spectrum can be attenuated more than the UV and IR portions. In this case looking at the sun can be comfortable, but the unseen radiation can cause significant damage.

This is easily prevented with well designed filters, but it is impossible to tell by looking at, or through the filters if they provide adequate protection.

I think cases of damage while using a sextant are rare, but you are dependent on the manufacturer producing a suitable filter. When purchasing a sextant second hand there is the concern that the filters have been replaced perhaps with a DIY alternative.

I concur. I worked for years with IR lasers and was taught that one cannot depend on eye pain to detect damaging levels of light. The pain is caused by the muscles straining to contract the pupils. Non-visible wavelengths of light (IR and UV) don't cause the pupils to contract, and the retina has no pain nerves.

If pain were a reliable deterrent to eye damage people wouldn't burn their retinas from staring at a solar eclipse.

If it doesn't hurt, that doesn't mean it isn't hurting your eyes. Having said that, I have the same Davis sextant and have spent many cumulative hours staring at the sun through it with my right eye. The vision in my right eye is still the same as my left eye. As the other posters here suggest: start with all the filters and then subtract them only as needed. Never sight the sun with all the filters removed, even when sighting through clouds.