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post #42 of Old 09-24-2013
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Re: Dangers of LED navigation lights

Pardon this new member chiming in (I joined because I'm researching buying a sailboat and you have to sign in to search), but I'm a pretty big astronomy buff. I don't really understand the original claim. LED lights are less like starlight than incandescent lights, not the other way around.

1. Stars aren't one singular color of white. They come in a variety of colors, ranging from blue (e.g. Deneb) to yellow (our sun) to red (e.g. Aldebaran). Some of these colors (especially Aldebaran) are very obvious to the naked eye. Incandescents bulbs tend to be stronger in the yellow/red range (why your indoor photos frequently look yellow) while white LEDs typically have a color somewhere between daylight and blue. That just means LEDs look like some stars, while incandescents look like other stars.

2. Stars are broad-spectrum, emitting light across the full visible light spectrum. This is also the case for incandescent lights. Fluorescent and LED lights are different. They inherently produce narrow-spectrum light. In order to produce "white" light, they have different phosphors (fluorescents) or doping materials (LEDs) which emit light of different colors. The combination of these colors approximate white, but the spectrum is never as even as an incandescent light. This is why incandescents are still preferred for artwork. They have what's called a high CRI (color rendering index) - their emission spectrum is flatter, like the sun's, and lacks the spikes which characterize fluorescent and LED lights.

3. The intensity (brightness) of the light drops off as distance squared. So regardless of type of light, there will always been a certain distance at which the light's brightness blends in with background stars. Having a brighter light shifts this distance further away, thus minimizing the risk of a collision. The type of light is irrelevant. All that matters here is brightness.

4. LEDs can be turned on and off instantly. They're frequently dimmed by making them flicker on and off hundreds of times a second. If the USCG really wants to make them easily distinguishable from stars, they should mandate the LEDs be strobed at around 25-50 Hz. At that frequency the light will appear constant if you look at it, but if you scan your eyes side to side, the LED will show up to your eye as a dotted line instead of a continuous streak like stars and incandescent bulbs. That would make it obvious the light is artificial.

5. Stars twinkle. They're so far away the light we see from them covers a very tiny angular width. Consequently, a tiny speck of interstellar dust or a slight disturbance in the atmosphere can momentarily cause the star to dim. Thus they twinkle. Planets and navigation lights do not twinkle.
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