Uit Practical Sailor, 25 september 2020 door Drew Frye:
“As the sun goes down your eyes begin to adapt. At first your pupils dilate. Then photo-sensing chemicals needed in daylight drain away, and vision becomes over 50 times more sensitive, often without our notice until the sun is well below the horizon.
In order to preserve night vision, referred to as dark-adapted vision, safe reading lights should fall in the mescopic range (see the table with the adjacent story, “Night Vision and the Aging Eye”). This is the same range specified for military use.
Cone receptors, located primarily in the center of the retina become less efficient in this range. Because they are the cells that provide sharp focus and color, reading becomes difficult and fine details fade. Smaller type on charts becomes a blur and colors begin to flatten.
Rod receptors, more sensitive in low light but unable to distinguish color, then take over, allowing us to move about and see larger objects, like ropes, until it gets quite dark, perhaps 4,000 times darker than mid-day.
In the mescopic range, we are just entering the realm of fully dark-adapted vision that sailors need in order to keep watch across a moonless sea. The eye’s adaptation to this low level of light requires time, rigorous avoidance of bright light, and even relearning how you look at things.
Rods are far less sensitive to deep red light, so there is no question that properly dimmed red lights help preserve dark adapted vision. This is why for decades the military broadly adopted red lighting for infantry, naval, and aircraft night operations. “Rigging for red,” was the mantra.
Red light installations were also considerably more dim by design. But there are also downsides to red light, and the military standard for chart reading (MIL-STD-1472F) now allows either dim white or red light, at the option of the user.
We’ve used red headlamps and sailed boats with red cockpit lighting, and we’ve never been fully satisfied. We’ve gone so far as to temporarily cover cockpit chartplotters that could not be sufficiently dimmed. Based on our ongoing research in this area, we are convinced that most red “night vision” lights designed for use at the nav station are actually many times too bright to protect night adapted vision. As a result, we’ve modified our headlamps for reduced output (see “Make Your Own Cheap Diffuser,” above), but never made a detailed study of what is best and why.
What We Tested
We tested using Eveready Universal Plus headlamps, but everything we learned is transferable to fixed lighting. The Eveready has two colors, a white light that has an output of 100 lumens, and the red light that has an output of 30 lumens. At a normal working distance of about 12 inches, this produces about 1,000 and 300 candela of illumination per meter squared (Cd/m2) respectively.
Without a light meter, determining a light’s actual illumination (Cd/m2) based on lumens is difficult. In theory, 12.6 lumens of output from spherical light source produces 1 candela per square meter of light flux (illumination) at a distance of 1 meter. This is because the area of a 1-meter radius sphere is 12.6 square meters. However, most lights on our boat have a focused beam, so the total output is concentrated in a small area. A flashlight used at the normal chart reading distance of 12-18 inches, will have a cd/m2 rating as much as 10 times greater than its lumen rating, because the total output is focused on an area that is much less than one square meter.
If the light output is not given, white LEDs are about 75 lumens per watt and red LEDs are about 40 lumens per watt.
If your headlamp or nav station light is too bright, you don’t have to run out and buy a new one.
We found a cheap and easy solution: athletic tape. Sold in any pharmacy, or even in the supermarket, athletic tape used to tape ankles and elbows does an admirable job of diffusing the focused beams produced by common headlamps, and even by some fixed lights for navigation stations. Add more layers to get the right level.
2. Athletic tape produces a more diffuse and more useful reading light than do neutral filters.
How We Tested
We narrowed the range of tasks to just two; reading charts and untying knots. The first tests our ability to see detail, the second tests our ability to see larger object. First, we allowed our eyes to adapt in a pitch black windowless room. Using a range of light intensities, adjusted by adding either neutral filters or tape to the headlamp in both red and white modes, we would try to read a specific set of chart details. We would then locate a number of ropes scattered on the floor, and tie and untie knots.
We dimmed each light until it was sufficient to tie knots but impossible to read chart detail. After accomplishing the test tasks, we would attempt to read magazine headlines in an adjacent room that was illuminated to the equivalent of starlight. In each instance, light output (lumens/m2) was measured with a photographic meter.
Really dim lights. In daylight, a headlamp that is dim enough for dark adapted viewing is barely noticeable as on. You should be able to stare into it, open-eyed. For operating at night, the military standard (MIL-STD-1472F) calls for a range of 0.35-3.5 cd/m2. Because of its focused beam, the 30 lumen red “night” light on our test lamp was about 20-100 times too bright by this standard, depending on how far the light was from the illuminated surface.
This is why after using non-shaded red lights for our test tasks we couldn’t even read the headlines on the magazines in the simulated starlight room until several minutes had passed. Based on our observations, many of the off-the-shelf red lights are too bright for eyes in a dark-adapted state.
Color. Light wavelength impacts color perceptions. The MIL-STD-1472F wavelength requirement for red lights for dark adaptation protection is > 625 nanometers (nm). Red LEDs (635-700 nm) are a suitable. If you use a filter to convert white lights to red through the use of filters, you will likely need to reduce the bulb wattage and use darkroom filters meeting this wavelength requirement.
Red Lights. Red light considerably degraded our ability to read a chart. Magenta lettering contains enough blue to be visible under red light, but that required a brighter light, which impaired night vision. (See below, “How a Red Chart Lights Can Lead You Astray”) Red and green aids to navigation are difficult to distinguish.
In the end, we needed nearly three times the intensity of red light to read the smallest magenta print. The literature suggests vision in the range of 20/200 is typical in dim red light, and that is close to what we experienced.
White light. White light, on the other hand, can be dimmer, because the cones are more far sensitive to it, and the cones pick up detail. Because you can “read” the chart more quickly with a white light, your eyes will quickly readapt, because your eyes’ exposure to the light was short.
In our test, we could immediately read the headlines in our “starlight room” if either red or white light was dim enough. In both cases we need about 5-10 minutes to recover our full night vision. What differed was the amount of time we needed to spend looking at our chart. We could read the charts far more quickly in the dimmer white light.
Because you are reading this under typical room illumination, the accompanying photos understate the differences between the appearance of a chart read with dim red light versus one with a white light. The most notable difference is color.
As we continued dimming the lights, we concluded that having a red nav light did not interfere with moving around on the boat—a priority for a boat off soundings and away from traffic. We could see what we needed to tie a few knots in very dim light. We couldn’t see detail, but we weren’t “night blind.”
We also noticed the hallmarks of true night vision. When your eyes have fully adapted to low light, you develop a blind spot in the center of your view, the same area where you normally focus and perceive detail. That is because there are not enough rods in the fovea, or center of the retina, to be effective. Colors go away.
This is why scanning a horizon at night is better than staring at an object like a distant running light. If you stare, objects can fade from view in 2-3 seconds. Objects may also appear to move (autokinesis) if you look at them too long, because the usual visual clues about relative motion and location are missing. Instead, slowly scan your surroundings and the horizon using a regular pattern so that you don’t miss anything.
Military research is based on young service members. The average sailor, based on our reader demographic, is about 57. Myopia and wearing glasses further impairs night vision.
The military spec says 0.3-3.5 cd/m2 is all the light you need to read a chart, but as you age, the minimal amount of light required increases significantly. A person with good night vision requires no more than 15 cd/m2 to see, but as you age this number increases. An person with average to above average night vision is likely to need 15-25 cd/m2.
The theory of using dim lights to protect the eye’s ability to adapt to darkness involves keeping light levels in the mescopic range. However, the amount of light required for older eyes to read is well into the photopic range, meaning every time a light is used to read a chart, your eyes will need a much longer time to recover. Small wonder we have become more dependent on radar and plotters. We can’t see beans.
The solution? Better light discipline. Clear the decks and cabin so that you don’t need light to move about below. Lay out any gear or snacks that will be needed. You should be able to find the head or grab your rain gear in pitch black conditions. Turn off instruments you don’t need and cover instruments you aren’t actively using.
Do you really need the compass and sounder full-time during an autopilot passage? Leave the radio on, but cover the display. Ask that crew members not wear unshaded headlamps; one flash in the eyes at short range and you can’t see right for 30 minutes.
If you must use cabin lights, reduce the number and block them from view on deck. Warn people before you use a bright light or open the companionway. Avoid alcohol and even worse, tobacco. Both degrade night vision. Wear sunglasses during the day; prolonged exposure to overly bright light reduces dark adaptation for 12-24 hours.
Multi-function displays (MFDs), chart plotters, and smart phones can usually be dimmed. Smart phones can be dimmed and set to red for night use. However, if you can read them, they are almost certainly far too bright to fully protect dark adapted vision and should be covered or turned off whenever it is not actually necessary to look at them.
You can read a chart under dim light, because this is only for a few minutes and your eyes will re-adapt in 10-15 minutes. But a display is in front of you constantly, subtly blinding you, without your noticing. Note that the MIL standard for instrument reading is just as dim as for chart reading, with either red or white permitted.
Methods of dimming
We tried several methods to control brightness and were quite pleased when we found one that worked great and didn’t cost an arm and a leg.
Dimmer Switch. This is a good solution for fixed lighting. The risk, of course, is setting it too bright. Also, dimmers on LEDs can be prone to flickering unless properly mated to the lamp.
Window tint. We like tinted film for instruments that would not dim. A simple solution is to take an extra cover, cut out the viewing area and slip the window tint in. The tint is widely available in various levels of shading for about $10 for a 12 square foot roll. (Avoid the reflective film—just plain neutral density). On black and white displays red film is even better.
Tape. One of the handiest ways to dim a light, red or white, is to use two to eight layers of common athletic tape. It’s easy to tear, stays on in wet conditions, is reliably translucent (we measured 50 percent transmission), and produces a comfortably diffuse light with less glare.
The dimmest red light we could use for chart reading required three layers of tape and was 3.1 cd/m2. Seven lumens (about 70 cd/m2) was more suitable for our old eyes, or about 20 times the military standard. In our view, athletic tape produces a more diffuse, usable reading light than neutral filters, with less glare.”
bron: Practical Sailor
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