Ask HN: How is the Eiffel tower blue light beam implemented?

Probably a collimating parabolic reflector with a high-luminance light source (like Xenon arc lamps).

Edit: This article [0] says it's 6000W Xe lamps.

[0] https://www.connexionfrance.com/French-news/Eiffel-Tower-s-l...

Edit 2:

I think the 'lighthouse' beam installation is made up of 4 collimated searchlights [1] custom-made by Francis [2]. They've got close-up photos of the Eiffel tower searchlights in their promotional material for their architectural lighting.

[1] https://www.wonders-of-the-world.net/Eiffel-Tower/Lighting-o... [2] https://francis.co.uk/en_US/about-us/

It's just a normal spotlight, it looks like a beam because the top of the tower touches the cloud cover.

The Luxor light in Las Vegas is similar, here's a walk-through!

https://www.youtube.com/watch?v=lS2wFpHFoXA