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Seeing at night

Most of us don’t do a whole bunch of night sailing, except for the occasional distance race or when trying to make good distance on a cruise. Sailing at night opens up a whole bunch of new issues, and since we don’t get much practice doing, the solutions aren’t always obvious.

One tremendous issue is the visibility (of lack thereof). People rarely realize how dependent they are on sight as the primary sense until it is taken away by darkness. We key so much of our world to sight, and being deprived of that sense interferes with our ability to operate normally. It’s at best annoying, and at worst, terrifying.

Being able to see your instruments helps alleviate some of that terror. You can at least be reassured by the same numbers that you see while sailing during the day, and compare your situation against that. Any aircraft pilot will tell you that no matter what you think you feel while flying, the instruments rarely lie. Your body might tell you that you’re in a slow ascent, but the instruments will tell you for certain if that’s true. Basing your decision off a gut feeling while flying can easily get you killed.

For instruments to be visible in the dark, some sort of lighting is required. On old analog type displays, there was often a dimable incandescent bulb inside the unit that provided a nice dull glow. Some analog instruments also had luminescent paint to augment the visibility. Most digital instruments (ours included) use LEDs for back lighting.

The problem with instrument back lighting is that your eyes adapt to the dark, but the lighting level typically doesn’t change unless you control it. Human eyes reach one plateau of dark adaptation around 9 minutes, and another hours later. Instrument lighting levels at one point in the cycle may be completely wrong for a later point. Fortunately, it is possible to control the system lighting level (see “Set Light Level”) and the lighting level for individual disaplays (see “Remote displays commands: @Jn” section) on the Ockam system. Note that the system light level will control the maximum brightness of all the displays, while the individual display control can dim individual displays down from the system maximum.

Another problem is wavelength perception. The human vision system perceives some wavelengths of light as brighter than others. Two light sources of identical physical luminosity but different wavelengths are perceived as having different brightnesses. Pure red light appears to be dimmer than other colors, but that’s not the reason that most night lighting is red. Most night lighting is red because that’s what was used in photography dark rooms (remember those?). Red light was used in dark rooms because black and white film had poor response in the red part of the spectrum, so being exposed to red light would not ruin developing photographs like other colors of light. Red lighting for night use came into common use during World War 2, but the reasoning behind it was not well understood – they just did it that way because it worked in the dark room. It was serendipity that the human eye didn’t change dark adaptation as quickly when exposed to red light.

All displays on the Ockam system now use red back lighting. Some of the older Matryx displays have a green backlight. The Matryx displays with green backlights can be converted to red back lighting if desired however. The green back light shouldn’t interfere too badly with night vision, as the Matryx displays can be individually dimmed lower than the system lighting level.

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