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Question about gunsights


ZnarF

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Hey all!

A question that consumes me since a while... When we fly a plane since ww2 (and sometimes ww1), we use -with the gunsight or lately the hud- a system of lens projection that show us a point situated optically at the infinity: because "anything at the focus of a lens or curved mirror (such as an illuminated reticle) will appear to be sitting in front of the viewer at infinity".  But! On my car, the indication of speed projected on the windshield [situed approx. 60cm of my eyes] appears to be nor at the infinite, nor at 60cm, but at approx. 130cm... [I would have prefered >4m so no eye accomodation would be needed] What is the optical magic operating here???!!!!


Edited by ZnarF
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On 4/30/2022 at 6:35 PM, ZnarF said:

Hey all!

A question that consumes me since a while... When we fly a plane since ww2 (and sometimes ww1), we use -with the gunsight or lately the hud- a system of lens projection that show us a point situated optically at the infinity: because "anything at the focus of a lens or curved mirror (such as an illuminated reticle) will appear to be sitting in front of the viewer at infinity".  But! On my car, the indication of speed projected on the windshield [situed approx. 60cm of my eyes] appears to be nor at the infinite, nor at 60cm, but at approx. 130cm... [I would have prefered >4m so no eye accomodation would be needed] What is the optical magic operating here???!!!!

 

Why do you think it's anything other than a design decision. Keep in mind that for a gunsight you want the gunsight and the target airplane (which is essentially at optical infinity) to be in focus at the same time. Your car designer probably thinks that you will see the road ahead better if the speed indication is NOT in the same focal plane as objects on the road ahead.

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It doesn't have to be focused at infinity, with gunsights it is, because that's where the target is, optically speaking. If your local shooting range has a gun with a holosight, then you can see the effect for yourself, it's the same principle. The car's designer probably thought it'll be better to have it floating behind the windshield.

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On 5/1/2022 at 12:35 AM, ZnarF said:

Hey all!

A question that consumes me since a while... When we fly a plane since ww2 (and sometimes ww1), we use -with the gunsight or lately the hud- a system of lens projection that show us a point situated optically at the infinity: because "anything at the focus of a lens or curved mirror (such as an illuminated reticle) will appear to be sitting in front of the viewer at infinity".  But! On my car, the indication of speed projected on the windshield [situed approx. 60cm of my eyes] appears to be nor at the infinite, nor at 60cm, but at approx. 130cm... [I would have prefered >4m so no eye accomodation would be needed] What is the optical magic operating here???!!!!

 

Research,

car companies don't invest much in HUD, they just did it the cheapest way possible while military did tons and tons of research to make the HUD on a plane look great and not mess with your focus,

 

I was researching how to make a HUD on a motocycle helmet, and to my surprise I found out that if I want to make it good and functional I need to spend lots of money on research cause that tech is not for free.

 

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Thanks for you answers - @sthompsonof course, the result of having the speed indication projected approx. 80cm behind the windshield IS a wish of the car builder (but honestly, about 1m further far should have been best, requiring less accommodation / accommodation time, optimal for elderly). But the question is more: HOW to optically control the perceived distance, while the system seems to be the same as a wwII gunsight, which projects at infinite: which parameter differs?

 

(@furiz: yeah, should be nice such hud in a motorcycle helmet!!)

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WTF does your car have anything to do with DCS?

Apples & Oranges.

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Not at all. The question is about real physics of the HUD optic, which is actually the same between aircraft HUDs, car windshield displays and even reflex and holographic sights for firearms (newer HUDs are holographic, older are reflex). They all work according to the same optical principle. The difference is where exactly does the image appear.

I've never really dug into holographic optics and so on (microscopes are more my thing), so I'll leave it to someone who did to explain, but the underlying physical phenomenon is the same. 

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Hi, 

(for discussion)

in some case , in cars, it is only a matter of using mirrors (the aspheric one allow to have a larger image), whoch are obviously much much cheaper (<1USD) and easier to manufacture than (large diameter) lenses:

Head-Up Display (HUD) | Instrument Clusters | Allegro MicroSystems

while in "our" HUD, you must have a collimated (meaning as seen at an infinite distance) view of the grid/sight, and this is done with a (large and expensive ) lens 

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Edited by gabuzomeu
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Excellent - that's the key: by using different lenses, you can produce various focals: object at infinite, or at any various degree of nearness... thank you!!

image-5.jpg


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23 hours ago, rayrayblues said:

WTF does your car have anything to do with DCS?

Apples & Oranges.

They both have huds???  Calm down...

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