Collimated light from red dot scopes?

[Shadow89]

I am trying to piece together how the parabolic mirror manages to reflect the "red dot" from the focal point to the eye without distortion.

I compare this with a conventional car headlight, which operates almost exactly the same way, except it has a non-transparent backing. Why does the ret dot scope manage to produce a clear image of the "dot" (focal point), whereas the car headlight apparently distributes the light evenly throughout the surface?

In other words; they have the same design, but when you look at the car headlight (perhaps with sunglasses on), you will not see the light focused in a point. It will appear as if the whole paraboloid surface is shining (no "dot").

Does anyone here care to shine some light on this? :)
(I hope you appreciate the my engineering humor).

 

[Charles Link]

In this case, just as in the case of the headlight, when you view the reflector, you will see the entire reflector illuminated, if you are a sufficient distance away, so that you are looking at all points of the reflector almost straight on. It helps, as in the case of the car headlight, if the source at the focal point is somewhat finite in size. That way, the reflector can appear to be completely illuminated when viewed even at somewhat closer range. $\\$ When viewing parallel rays, (basically from a very small source at the focal point of the reflector), and you focus at $+\infty$, yes, the lens of your eye will bring the light to focus to a small point on your retina. The point source at the focal point of a reflector will appear as a point source at infinity. $\\$ For the case of the reflector, to see whether a portion is illuminated, you draw a ray from your eye to (a portion of) the reflector, and upon reflection,(with angle of incidence=angle of reflection), the question is, does that reflected line pass through the source? If it does, that portion of the reflector will appear illuminated. For a very small source at the focal point, this only occurs at a very small portion on the reflector that is straight in front of you.

 

[Shadow89]

In this case, just as in the case of the headlight, when you view the reflector, you will see the entire reflector illuminated, if you are a sufficient distance away, so that you are looking at all points of the reflector almost straight on.

So for example, if I walk one kilometer away, and observe the RDS through a telesope, the entire RDS reflector will appear bright red?

It helps, as in the case of the car headlight, if the source at the focal point is somewhat finite in size. That way, the reflector can appear to be completely illuminated when viewed even at closer range.

I really appreciate your feedback, but I don't think I fully understand. The RDS light source is very small (finite), but it doesn't produce a totally illuminated reflector screen. Thats the point, right?
Did you mean thet the headlight bulb should be as big as possible?

$\\$ When viewing parallel rays, (basically from a very small source at the focal point), and you focus at $+\infty$, yes, the lens of your eye will bring the light to focus to a small point on your retina. $\\$ For the case of the reflector, to see whether a potion is illuminated, you draw a ray from your eye to (a portion of) the reflector, and upon reflection, the question is, does that reflected line pass through the source? If it does, that portion of the reflector will appear illuminated.

This makes sense, but whsat do you mean with focus at infinity? The focus of the eye, or the focus of the parabola? I apologize for my incompetence.

 

[Charles Link]

When your eye is focused at infinity, the retina=which contains the sensors of your eye, is in the focal plane of the lens of your eye. Parallel rays that your eye is looking at will be brought to a point in the focal plane of the lens of your eye=basically focused to a single point on your retina. What you then see is a single bright point in the far field.

 

[Charles Link]

I really appreciate your feedback, but I don't think I fully understand. The RDS light source is very small (finite), but it doesn't produce a totally illuminated reflector screen. Thats the point, right?
Did you mean thet the headlight bulb should be as big as possible?

The filament or LED source of the headlight is much larger than the red dot of this scope that you are using. The result is that the headlight will appear to be nearly completely illuminated at a much closer range.

 

[Charles Link]

So for example, if I walk one kilometer away, and observe the RDS through a telesope, the entire RDS reflector will appear bright red?

Yes, that likely would be the case. (And I should have answered all 3 questions in the same post, but I need to learn how to pick out multiple quotes, etc.) $\\$ Basically the calculation for computing the distance $s$ where this occurs is the distance $s$ needs to be such that $\frac{d}{f} >\frac{D}{s}$, where $d$ is the diameter of the source, and $D$ is the diameter of the reflector. For small $d$, the distance $s$ can be quite large.

 

[Shadow89]

Thank you very much for helping me understand these things! I really appreciate it!

Yes, that likely would be the case. (And I should have answered all 3 questions in the same post, but I need to learn how to pick out multiple quotes, etc.)

I just regular quote, then copy/paste the "/QUOTE" tags

 

[sophiecentaur]

Why does the ret dot scope manage to produce a clear image of the "dot" (focal point), whereas the car headlight apparently distributes the light evenly throughout the surface?

Firstly, have you ever heard of Pepper's Ghost? It works on the same principle as a red dot sight and the observer sees light from two different sources, superimposed. In the theatre illusion, both the objects are placed the same distances behind a semi silvered mirror. To fit the red dot illusion in the scope, the paraboloid is used to place the dot image at 'infinity' so that's where it appears in the red dot sight. A laser sight is a different thing as it actually projects a dot on the target where the red dot in a red dot sight is just 'virtual'.

 

[Shadow89]

Firstly, have you ever heard of Pepper's Ghost? It works on the same principle as a red dot sight and the observer sees light from two different sources, superimposed. In the theatre illusion, both the objects are placed the same distances behind a semi silvered mirror. To fit the red dot illusion in the scope, the paraboloid is used to place the dot image at 'infinity' so that's where it appears in the red dot sight. A laser sight is a different thing as it actually projects a dot on the target where the red dot in a red dot sight is just 'virtual'.

This makes a lot of sense and I greatly appreciate the feedback.
But with this logic, shouldn't the LED in the RDS be a laser pointer?

Lets say I had a super-powerfull LED, and a very steep paraboloid mirror, could I make a laser beam with the same diameter as the exit diameter on the mirror?

 

[sophiecentaur]

Lets say I had a super-powerfull LED, and a very steep paraboloid mirror, could I make a laser beam with the same diameter as the exit diameter on the mirror?

You would blast your eye if the optics were the ones in your (Wiki) diagram in the top post.
A laser sight has totally different optics to protect your eye and a laser doesn't need a paraboloid as it's a narrow beam anyway. Look up laser sight.

 

[Shadow89]

You would blast your eye if the optics were the ones in your (Wiki) diagram in the top post.
A laser sight has totally different optics to protect your eye and a laser doesn't need a paraboloid as it's a narrow beam anyway. Look up laser sight.

No my friend, I know how a laser sight works. Its just a laser pointer (basically). Unless you are referring to holographic sights such as the eotech, which uses laser diodes (and other optical devices) to produce a hologram at infinity.

My question about the theoretical parabolloid laser device was purely theoretical, and not intended as a sighting device. Just for understanding of the optics previously discussed.

 

[sophiecentaur]

No my friend, I know how a laser sight works. Its just a laser pointer (basically). Unless you are referring to holographic sights such as the eotech, which uses laser diodes (and other optical devices) to produce a hologram at infinity.

My question about the theoretical parabolloid laser device was purely theoretical, and not intended as a sighting device. Just for understanding of the optics previously discussed.

Oh right! The whole of the (off axis) paraboloid will appear to be red if the LED is at the focal point of the reflector. The optics are specifically made to produce an image of the dot at infinity. It is a Virtual Image (as in a mirror) because light just appears to be coming from it. You could get a virtual image that's much closer than infinity by moving the LED closer to the mirror and that could be seen from a distance with a telescope. Move the LED further away and you can project the LED (real ) image on a screen.
If you have a shaving mirror (concave) or even a spoon you can see the sort of thing I mean. Close up you see a magnified image of yourself (right way up). Further away the image explodes and you see nothing. Further away still, you get a real image that's in front of the mirror.
Google Concave Mirror images.
A car headlamp is built for a different purpose and the bulb is on axis but I seem to remember that (lamp off!) you can see an image of bits of the bulb if you go very close up. (That's the sort of thing a 10 year old nutter will do!)
Edit: I just remembered that I have a Telrad Finder on my astro telescope. That works just like a rifle sight but it projects a reticule in your view of the sky and you see concentric red rings over a small viewing angle so you can point the scope at what you want. Same arrangement of a mirror but there's no lens as you want a wider angle of vision.

 

[davenn]

Yes, that likely would be the case. (And I should have answered all 3 questions in the same post, but I need to learn how to pick out multiple quotes, etc.)

tis easy ... hilite a piece of text in a post and a tab will show

Click on the Reply ( right side of that grey button) and it will appear in your post

Yes, that likely would be the case.

add your answer

do a couple of line spaces

go back to the next bit of text you want to quote, hilite and select it ...
Click on the Reply ( right side of that grey button) and it will appear in your post

(And I should have answered all 3 questions in the same post, but I need to learn how to pick out multiple quotes, etc.)

and again, now add your response

you can do that multiple times from some one's single post or even from multiple posts from multiple people

cheers
Dave

 

[Shadow89]

Edit: I just remembered that I have a Telrad Finder on my astro telescope. That works just like a rifle sight but it projects a reticule in your view of the sky and you see concentric red rings over a small viewing angle so you can point the scope at what you want. Same arrangement of a mirror but there's no lens as you want a wider angle of vision.

For posterity, and other people reseaching this, here is a comparison of Reflector Sights (fig. 1) and Red Dot Sights(fig. 2 and 3).

I assume the aforementioned telrad sight to fall under the reflector sight category.

The apparent difference between the two, is that reflector sights have one or more lenses (spherical surfaces), whereas RDS uses a parabolic mirror (non-spherical) to achieve the same task (create collimated light from diverging light).

 

[sophiecentaur]

Good pictures. I guess my telrad is easier to manufacture and it doesn't need to be so compact. To say they are quite pricey, it is rather toy-like and not a bit rugged. But I got mine for free and it's actually pretty easy to use. and relatively easy on the neck compared with an ordinary finder scope.
I have a (Hawke?) telescopic sight on my air rifle and that has a set of illuminated dots on the reticle. I assume that they are illuminated from round the edge. They are certainly bright enough. How much better is a "Red Dot" sight, using that clever optics?

 

[Shadow89]

I have a (Hawke?) telescopic sight on my air rifle and that has a set of illuminated dots on the reticle. I assume that they are illuminated from round the edge. They are certainly bright enough. How much better is a "Red Dot" sight, using that clever optics?

I also have a Hawke 4x on my air rifle. It has a wire reticle which I find to be annoyingly thick. Since yours has an illuminated reticle I assume it has an etched reticle. As you suggested, these are illuminated radially, or possibly slightly in front. (Do you have photos of the sight picture?).

The benefit of the RDS is simply that it is parralax free when aiming at objects far away. Also you can use it with both eyes open. The reflex will give the same qualities but are often bulkier.

Both reflex and RDS can be coupled with magnifiers, but at the expense of parrallax.

 

[sophiecentaur]

I have no pictures but the sight is on quite a nice Weihrauch .22 rifle so it's probably not bottom of the range. I will have a look for Parallax next time I am using the rifle. It actually does me fine for rats and mice which deserve to DIE. when they run around in my roof. Once every few months is the only use it gets but I am always impressed by its consistency. (No special practice). I wonder if it could be to do with the Beta Blockers I take these days? I hardly shake at all - even with excitement. lol.
The eye relief is amazing and it surprises me that I rest my cheek on the stock and everything's in focus, way back from the eyepiece.