Comment by bleepblorp | Hacker News Reader

Comment by bleepblorp | Hacker News Reader

Parent Full thread

LED car headlights do use this principle; it does have some applications.

However, the diameter of each LED optic would need to at least equal the diameter of the single optic used for a point source light. The result would be an enormous searchlight with a very broad beam.

I can't find any solid information on the quantitative light output of the AN/TVS-3, but xenon arc lamps typically produce 30 lumens per watt. Therefore, the 20KW AN/TVS-3 should produce around 600,000 lumens.

Readily available high-power LEDs are capable of 1700 lumens, so equaling the AN/TVS-3 would take--as a very rough ballpark that does not take into account the different light distribution between LEDs and discharge lamps--350 LEDs and optics. The AN/TVS-3 uses 30" optics, so those optics would also need to be at least 30" in diameter.

350 30" optics is a lot more like a wall of light than a searchlight. Aiming that many separate optics would also be a nightmare.

> I can't find any solid information on the quantitative light output of the AN/TVS-3, but xenon arc lamps typically produce 30 lumens per watt

https://en.wikipedia.org/wiki/Luminous_efficacy

What I'm not sure of is if the lumins/watt rating is given for the light output or the power input.

The input power rating is 450 Amps * 200 Volts * 1.732 = 155880 electrical watts. That's nearly 7.75 times as much electrical power as light output. That makes sense since the peak efficiency is about 7.3 for a xenon arc lamp.

Why does the optic need the same diameter? For gathering all of the light, with a mirror at least, I think you could place a smaller optic closer to the light and still capture the same fraction of the light?

I probably am missing something about optics though.

The ability of an optical system (a lens, mirror, or combination) to collimate the output of a light source into a narrowly diverging beam is directly proportional to the diameter of the optical system's final element and inversely proportional to the size of the light source.

Bigger optics = less beam divergence = more useful range.

This is a function of the diffraction limit[0] and is a property of physics.

If you want to match the beam divergence of a 30" optical system, you have to use 30" optics; there are no shortcuts.

[0] https://www.edmundoptics.com/knowledge-center/application-no...

I know that we can create very large virtual apertures for receiving signals by using arrays of many receivers. The basic diffraction physics presume a single source, so it isn't totally obvious that they generalize to a multi-source system.

I can, however, imagine that a multi source system would require in-phase light sources to kind of mimic a phased-array receiver on the send side.

Reply on news.ycombinator.com