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prop-head
01-12-2005, 05:20 PM
I'm prompted to raise this topic because of the different ways gunsight displays are rendered in different sims. Especially after seeing what happens in CFS3 when one moves one's virtual viewpoint a significant distance in the L/R and up/down directions (via either the mapped keys or TrakIR's 6DOF mode).

In this area Oleg has it right -- well, not quite entirely, but I'll get to that later.

So, how does a gunsight work? In your basic WWII-era gunsight there are three main components:

1) Illuminated reticle - an opaque material having the reticle pattern etched or cut out so that light will pass through. Behind, or underneath, the reticle lies a lamp for a light source.

2) Collimating lens - placed at a distance from the reticle equal to the lens's focal length, so as to collimate, or make parallel, the light emerging through it. This makes the reticle pattern appear to be at infinity. In other words, if you aimed a camera into the collimating lens so as to photograph the reticle within the assembly, you'd have to set your camera's lens to the infinity focus position.

3) Reflector plate - simply a piece of plate glass, positioned immediately above the collimating lens, and tilted at about a 45 degree angle so as to reflect the image of the reticle toward the pilot's eye (I say "eye", and not "eyes", because most gunsights are not large enough to look through with both peepers simultaneously.)

In any sim the reflector plate is obvious, of course. And most times the collimator lens is represented as a rounded dome of glass immediately under the reflector. As it should be.

Now, many folks have the mistaken notion that the reticle pattern is projected ONTO the reflector plate, as though the plate were a screen. Not so. The reflector simply transfers the image so that it's conveiently seen against your intended victim. You're really seeing a virtual image of the reticle which lies far beyond the reflector, suspended in space, as it were.

Let's say you move your head back and forth in the L/R direction (as though you were trying to touch your temples against the sides of the canopy). As long as the reticle remains visible, it will remain motionless WITH RESPECT TO THE WORLD OUTSIDE -- it is not tied to the reflector in any way! Beyond a certain point, when your eye has moved too far from the ideal viewing position, the reticle will begin to disappear as it gets clipped because of the limits imposed by the physical size of the reflector and/or collimating lens.

Another thing... Irrespective of the distance of the pilot's eye from the reflector, the angular size of the reticle pattern DOES NOT CHANGE. You can see this when you toggle the "Gunsight mode", wherein your virtual pilot alternates between resting back in his seat and hunching forward and lining up more properly with the gunsight. In either position, even if the pilot has moved fore/aft a fair bit (as in some planes) the diameter of the reticle pattern remains the same, while the gunsight assembly does appear to change in size (along with the instruments, etc.) somewhat due to the change in distance.

Now, let's look at the bad. Take, for example, the CFS series of flight sims. They represent the gunsight very incorrectly, as though the reticle is "painted" on the reflector plate! http://forums.ubi.com/images/smilies/34.gif So no matter where in the cockpit the virtual pilot's head might be (in CFS3, that is), the reticle always remains fixed in place on the reflector, and hence can sometimes be very mis-aligned with respect to the direction the guns are pointing and shooting! http://forums.ubi.com/images/smilies/53.gif

IL2 et al do a very much better job, in that the reticle is correctly modeled as a virtual image lying at infinity. And the use of alpha blending makes it properly transparent, too! http://forums.ubi.com/images/smilies/11.gif

But there is room for a further, small improvement. Look at the various gunsights in a variety of cockpits. In many cases the reticle pattern is larger than the collimating lens diameter, sometimes very much so. This is impossible. The diameter of the collimating lens is the window through which the reticle is seen, so the reticle can not be larger than this.

The bottom line: no matter how large the reflector may be, the reticle's area of visibility on the reflector is restricted to a circle of diameter equal to that of the collimating lens. (To be most accurate, this circle is actually a little smaller because of the difference in distance from the pilot's eye direct to the collimator versus the larger distance eye-reflector-collimator. But we needn't nit-pick, eh?) http://forums.ubi.com/groupee_common/emoticons/icon_rolleyes.gif

It's probable that in some cases the collimating lens is represented as being too small, for to limit the reticle to such diameter would be unrealistically restrictive.

By way of bona fides, I'll mention that I design telescopes and large astronomical binoculars, and fabricate optics for laser machine vision systems.

No601_prangster
01-12-2005, 05:25 PM
Great info prop-head. Thanks! http://forums.ubi.com/images/smilies/11.gif

WTE_Dukayn
01-12-2005, 05:34 PM
indeed, quite an interesting read.

Chuck_Older
01-12-2005, 05:52 PM
I thought you were asking how they worked

whew! you know more about it than me, easily

VF-3Thunderboy
01-12-2005, 05:53 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR> Now, many folks have the mistaken notion that the reticle pattern is projected ONTO the reflector plate, as though the plate were a screen. Not so. The reflector simply transfers the image so that it's conveiently seen against your intended victim. You're really seeing a virtual image of the reticle which lies far beyond the reflector, suspended in space, as it were. <HR></BLOCKQUOTE>


This is incorrect Prophead, the reflection is seen on the reflector GLASS, which has a super thin coating of metal on it to increase the reflection. Id show you mine, but they dont photograph well with a digital. Im having a heckof a time getting some coated glass for a Mk 8mod 6 conversion to a mk8 mod 8.(Navy standard issue, PTO )

The reflection goes onto the glass... No doubt there m8! http://forums.ubi.com/images/smilies/cry.gif

ALSO, the size of the actual gunsight Image is about the size of a quarter or less, the lense (or colminator as you say) MAGNIFIES this image ( a yellowish glass with silkscreened gunsight image on it.) and in turn it REFLECTS off the reflector glass. Any glass will actually work, but the metal coating really helps, and it helps with glare a bit too.


http://forums.ubi.com/images/smilies/16x16_smiley-tongue.gif

StG77_Stuka
01-12-2005, 06:00 PM
Great info! http://forums.ubi.com/images/smilies/25.gif

Athosd
01-12-2005, 06:03 PM
Most interesting - thanks for that Prop-head.

Cheers

Athos

prop-head
01-12-2005, 07:10 PM
Thunderboy,
Cool... you're actually playing around with these things!

What I'm saying is this: the reticle is not projected ONTO the glass plate, but is reflected OFF the plate. Very different processes -- but I'm certain we're differing only on semantics, wot? http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif But just to clarify further for all...

If the reticle was projected ONTO the plate, you'd be able to see it from any position, just like Uncle Schmoe's vacation slides projected onto a screen (the glass plate would have to be roughened, or ground, for this to happen, though). The plate simply redirects the path of the image of the reticle being collimated by the lens.

And yes indeed, the collimator does magnify the actual reticle pattern, as must happen when a positive focal length lens is employed. But in no way can the image of the reticle as seen by the pilot appear larger than the collimator lens itself. Verify this yourself with a simple mock-up of a gunsight. Materials required:
- magnifying lens.
- mirror at least a bit bigger than the magnifyer, or glass from a picture frame if no suitable mirror is available.
- a reticle-like pattern drawn on paper.

Draw a bullseye pattern with crossline, about one-quarter the diameter of the magnifying lens.

Measure the (approximate) focal length of the lens by projecting the (upside-down) image of a distant scene or object on your wall. The focal length is the distance between lens and projected image.

lay your "reticle" on a table.

Rig some kind of stand to hold the lens above the paper at a distance equal to the focal length.

Support or simply hold the mirror a short distance above the lens and at a 45 degree angle. (A gunsight's reflector plate is nothing more than a low-reflectance mirror, n'est pas?)

Look into the miror and find the position which centers the "reticle". Now move your head about, including toward/away from the mirror. Observe the results.

Replace the "reticle" with a small light bulb, and the mirror with a clear glass plate. Now you're closer to making a real gunsight.

It should become apparent that your view of the reticle is ultimately limited by the physical size of the collimator lens, no matter how much it might magnify. Indeed, simply view any object through a magnifier -- you cannot see the magnified image beyond the egde of the lens.

I didn't mention glass coatings because I only wanted to convey the more important principles. I wouldn't be surprised if some early models of gunsight had no partially-reflecting coatings on the reflector plate. In any case, a pretty bright lamp had to be used so that the reticle could be seen against a daylit scene, and the necessarily high electrical current often resulted in short lamp life.

Of course you're aware that in the sim a few Axis planes are modeled with a so-called reticle dimmers. This device is mis-named. It should be called a sky dimmer. It's a dark-ish filter placed in front of the reflector so as to dim a bright sky background while not affecting the brightness of the reticle. In the game, flick it into position when your target flies into the sun and voila! You can now see the reticle more easily (otherwise you couldn't see the reticle at all in the glare).

ZG77_Lignite
01-12-2005, 07:34 PM
Extremely interesting information, in a much mis-understood area of WWII, thanks Prophead. Very few people realize how these sights work (myself included) and don't understand how they are better than a 'ring-and-bead' type sight, no doubt due to the long mis-use the 'reflector' sights have recieved in sims. I expect to see a 'new standard' be given with the advent of Oleg's BoB, due to the 6DoF support. Thanks again very much.

If your up to it, please explain to us how the thick armoured 'windscreen' glass affects vision out of the cockpit, for example, say a FW190. Oh My God did I just say that out loud? (it would pay big dividends to start a whole new thread on that subject, so as not to ruin your excellent work so far)

prop-head
01-12-2005, 08:05 PM
Lignite,
On the effect of thick glass... In the case of flat, plane parallel panels which are not perpendicular to the line of sight, the view of the world beyond is somewhat displaced, by an amount that scales with glass thickness and angle of tilt. Thicker glass and greater tilt means more displacement. In the case of your FW example (and other similarly-equipped a/c) the thick front screen displaces the view downward because the glass is tilted back toward the pilot.

You can verify this yourself. Sight a horizontal edge or line in the distance while looking through a pane of glass tilted back in the manner of a fighter's windscreen. Observe what happens at the edge of the glass -- the horizontal line will appear to be "split", the segment seen through the glass a little lower than that seen directly.

In a real FW, etc., with the significant thickness of the armoured panel, there would be a discernable discontinuity between it and the perspex sidelight adjacent to it (taking into account the framing separating the two, of course). But modeling this in a sim would most likely add unnecessary computational overhead! http://forums.ubi.com/images/smilies/16x16_smiley-sad.gif

VF-3Thunderboy
01-12-2005, 08:38 PM
Bloodey Prophead, yea, semantics. I wish you Brits would learn to speak prop-er Englith..

But the light (bulb) reflects off the GLASS...Not projected into the air. Your jsut seeing the light bulb/ image in the reflector glass, which is why they call in reflector glass.It looks like a mirror from certain angles, and you cant see through it. You need to be right in line.

BUT, there is a "light dimmer" on these things, and the light bulbs were about 25 watt. Between the dimmer and the dark reflector glass you could adjust the thing.

I keep track of all the WW2 Gunsights on E-bay.

A Japanese Zero gunsight in mint condition sold for a bit over US $10,000.

German Revi sights bids go up to around $1000.00, but most never sell, as the owners want more.There is one on there now for 1800.00

Britt early BOB gunsight, possibley Hurricane, (as seen in CFS3): I think it went for $700.00, not sure.

Mk 8 Mod 8 (US navy /corsair etc..)250.00 +

Mk (6-23A,B,C ?-I forgot) (B-24, bombers, P-40's) regularly on for as little as $15.00

Mk 12 "Mailbox" gunsights (Torpedo bombers?)
from $5.00 up, got mine for $18.00 Not sure how they aimed with this little thing, but its cool..Has the tinted glass window that comes up...

Ive got a bomber gunsight too, its has a little "iron " crosshairs thing on the side if the bulb goes out. Very cool, has the glass, but no gunsight image. http://forums.ubi.com/images/smilies/16x16_smiley-sad.gif

Bloodey, look at this German Revi :

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4078&item=6503807751&rd=1

See how the lens itself just reflects off the glass? Thats the bloody mirrored glass.
$3000?? Yea, check is in the mail...

Czunik
01-13-2005, 01:17 AM
I add few things.

The lens does not have to be focused to infinity - it can be focused to any distance - the virtual reticle will then seem to be in that distance. For aircraft infinity is pretty much what you need.

Similar sight system is used these days on hand-guns. It looks like small window - almost vertical glass of size about one inch. It has some metal framing. You can see it sometimes in action movies.

Shooter can see virtual red reticle (usually simple dot) thru the window at the target distance (which is adjustable sometimes). Advantages against laser pointing devices are that only the shooter sees the dot, that the dot has same size and brightness regardless of the target distance and that it is simple to change shapes of the reticle (sometimes shape of the target is used on shooting competitions). It also takes less power from the batteries.

In Half life 2 there is submachine gun with this sight. And it is of course modeled wrong. Reticle is visible in the window even if the gun is sideway and the reticle does not aim to the target. Since there is no 'thru the sight' view in HL2, the reticle should never be visible. This could of course look booring.

I also saw this sight in anime movie Appleseed 2004. There is nice sequences where they had perfect chance to show it right (but they didn't). You can see bad guy aiming with it on the heroine (female hero) and camera slowly changes the view behind the guy and then thru his reticle. The sight is empty in this sequence. Little later the reticle is showed from the opposite direction (from the 'victim' side of the gun) - and you can see nice red dot in the center of the sight regardless the view angle again ! Very funny !

Freycinet
01-13-2005, 07:26 AM
I didn't fly CFS3 long enough to look at the gunsights, but if they simply "painted" the gunsight image onto the reflector plate then that is just ridiculous...

Maddox did a great job. As for the collimator glass sometimes being smaller than the reticle image, well, that's just a small aesthetic issue, which doesn't influence gameplay. But good observation.

Great initial posting, more like a simHQ-posting than what is usually to be read here in the ubizoo. Thanks!

plumps_
01-13-2005, 10:02 AM
Here you can download a little movie that shows a Revi "in action". (http://home.arcor.de/frischluftduft/Revi.zip)

---

This is a diagram I made to show a big advantage of the reflective sight: You can move your eye position a little bit, and you will still see the reticle point at the target. Which is important in a shaking aircraft:

http://home.arcor.de/rayluck/sturmovik/revi-scheme.gif

In my understanding the collimating lens ideally doesn't make the reticle pattern appear at infinity but at the target distance. <span class="ev_code_BLACK">That's why we see a setting ring at the Hurricane's sight (http://www.partizanska-eskadrila.com/reference/hurry.htm) where the pilot can adjust the range. Only when the correct distance is selected the reticle will point at the target correctly even when the eye moves away from the target line (I guess).</span>
But my guessing was not very good and the last paragraph needs to be corrected, see next page.

Jungmann
01-13-2005, 10:22 AM
I don't have the optical know-how of prop-head but I do have a pretty good collection of WWII gunsights, including most of those mentioned by Thunderboy (minus the Japanese sight--they cost a ****ing fortune).

And observing how they work, lighting them up--sometimes with DC power, sometimes simply removing the bulb and shining a flashlight up through the collimating lens, I think prophead has it right when he says the image is reflected off the 45 degree glass.

In the 20 or so sights I have, propboy, only one has anything but plain old tempered glass--a late war Mark 14 (like the one on the P-51D in the game) that has a slight rosy tint to the reflector. Maybe it's a metal coating, but it seems to me it's just for glare control, like a light pair of sunglasses, not to increase reflection.

My sense of the connection between the glass and the projected image--well, imagine you're sitting in a movie theater about half way back and a movie is being shown. You take a long stick (I know, this is a little Mr. Wizard--bear with me) with a small pane of glass attatched to the end and you raise it into the beam of light travelling from the projector behind you to the screen in front. You would see, on that glass, a piece of the projected image, the movie--the glass would, in essense, intercept the light being transmitted. But the projector beam would be focussed not on your little square of glass, but the picture screen there in front of you.

This is my sense of how reflector sights work. The generated image is focussed at infinity, out there, towards your target. The 45 degree glass intercepts it, so the pilot can see it. Because the image is focussed at infinity, as the pilot tracking a target moves his head around (from G forces, etc.) the generated image, the reticle, may move around on the glass but the reticle pipper stays in the same place on the target. The glass is simply intercepting the reticle image--it's not stopping it, it's not the movie screen that the image is projected onto.

The advantage of reflectors over the old ring and post sight was this specific ability--for the pilot to be able to keep a bead on his target even as his head moved around in the cockpit, instead of having to keep his eyes firmly in alignment with a ring and a post, no matter how violent his maneuvering, like somebody firing a rifle with iron sights.

Very interesting thread. Cheers to all.

VF-3Thunderboy
01-13-2005, 10:36 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>That's why we see a setting ring at the Hurricane's sight where the pilot can adjust the range. <HR></BLOCKQUOTE>
The ring probably adjusts the size of the circle. It seems this is being way over thought. Its a lightbulb that reflects off the glass. In my Mk 8Mod6, it actually reflects off the windscreen (of a Hellcat etc). They took the glass out. If you have a ZIP disk case (clear plastic) or even a CD case, you can see how this works. Shine a flashlight strait up into it, and angle it back at you, you can see the reflection, but still see through it. The colminator lens is just a magnifing lens that magnifies the origional gunsight image, which is black and clear where the gusnight image is, so what you are really seeing is the light bult shine through where there is no black ink on the image.

If you shine a flashlight onto the CD case the light reflection you will see would be the gunsight image. If you move your head, you will be out of line. You need to keep you head strait.

There is nothing in these things that has anything to do with "distance". The distance is between the lightbulb, and the Reflector glass. It is butt-ugly simple. As I said, you can see for this yourself, flashlight, and the CD case from PF and your in business! http://forums.ubi.com/images/smilies/16x16_smiley-surprised.gif

Jungmann
01-13-2005, 10:40 AM
Plumps__--a bit of info. The Mark II reflector, standard on RAF fighters from '39 to '44 or so, has two adjustment rings on it, but neither affects the size of the reticle.

The rings affect two horizontal lines of light that intersect the arc of the reticle at 3 and 9 o'clock. One adjustment ring is labelled "base feet"; that's for the wingspan of the target--it runs for 35 to 100 feet. Say your Jerry fighter target has a wingspan of 35 feet--turning the ring pulls the ends of those two horizontal lines closer together. The other ring is "range"--it runs from 150 to 600 yards. Turning it also makes those same two lines move closer or further apart.

Toghether, they turn the sight into a range finder. Say the convergance on your a/cs guns is 300 yards. By setting that on the range ring and 35 feet on the base ring, it means when the wingtips of your target touch the ends of those two horizontal lines on the reticle, the gunsight's telling you you're 300 yards behind it. Time to fire.

VF-3Thunderboy
01-13-2005, 10:47 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>The generated image is focussed at infinity, out there, towards your target. The 45 degree glass intercepts it, so the pilot can see it. <HR></BLOCKQUOTE>

This is also incorrect!!

The "Generated image" is "focused" strait UP. The reflector glass reflects it back to you!

Butt ugly simple....Flashlight, CD case, everyone can have a reflector gunsight. Look outside with it.


[QUOTE](minus the Japanese sight--they cost a ****ing fortune)./QUOTE]


They are practicly non-existant, so you cant get one anyway..

German Revi sights are very plentiful, and still in the hands of most of the would be sellers! $950.00 tops. http://forums.ubi.com/images/smilies/16x16_smiley-tongue.gif

VF-3Thunderboy
01-13-2005, 10:53 AM
Technicaly speaking, you can build one for like 5-10 bucks or just stuff lying around the house.

Get the gunsight recticle image (s) black and white only, copy them to clear plastic, tape over a flashlight, get a magnifing glass, and a piece of glass or CD case, BOOM youve got the poor mans reflector gunsight..

http://forums.ubi.com/images/smilies/16x16_smiley-tongue.gif

ronison
01-13-2005, 10:59 AM
Quote:
------------------------------------------------------------------- What I'm saying is this: the reticle is not projected ONTO the glass plate, but is reflected OFF the plate. Very different processes -- but I'm certain we're differing only on semantics, wot? Wink2 But just to clarify further for all...
-------------------------------------------------------------------

Funny any image projected is reflected back. That is how optics work. If a projected image were not reflected then no one would see it. And if the "Projector screen" ie... glass plate was not there then the image would not be reflected back. So in reality you are both right but the image is projected and reflected.

plumps_
01-13-2005, 11:08 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Jungmann:
My sense of the connection between the glass and the projected image--well, imagine you're sitting in a movie theater about half way back and a movie is being shown. You take a long stick (I know, this is a little Mr. Wizard--bear with me) with a small pane of glass attatched to the end and you raise it into the beam of light travelling from the projector behind you to the screen in front. You would see, on that glass, a piece of the projected image, the movie--the glass would, in essense, intercept the light being transmitted. But the projector beam would be focussed not on your little square of glass, but the picture screen there in front of you. <HR></BLOCKQUOTE>
Actually the piece of the projected image is there, but we're not interested in it. I think what we're looking for in your example is the very small reflection of the projector's bright lens somewhere in the background of the movie theatre.

plumps_
01-13-2005, 11:24 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by ronison:
Quote:
------------------------------------------------------------------- What I'm saying is this: the reticle is not projected ONTO the glass plate, but is reflected OFF the plate. Very different processes -- but I'm certain we're differing only on semantics, wot? Wink2 But just to clarify further for all...
-------------------------------------------------------------------

Funny any image projected is reflected back. That is how optics work. If a projected image were not reflected then no one would see it. And if the "Projector screen" ie... glass plate was not there then the image would not be reflected back. So in reality you are both right but the image is projected and reflected. <HR></BLOCKQUOTE>
There are two different images:

1. A projection -- what you see is the projection screen 'modified' by the light coming from the projector. Of course the light is reflected, but what you see is the screen. The way it looks depends on the light that's shining onto it, and you wouldn't see it at all if there was no light shining onto it.

2. A reflection -- what you see is the projector itself, not the screen.

plumps_
01-13-2005, 12:24 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR> There is nothing in these things that has anything to do with "distance". The distance is between the lightbulb, and the Reflector glass. It is butt-ugly simple. As I said, you can see for this yourself, flashlight, and the CD case from PF and your in business! <HR></BLOCKQUOTE>

Here's an experimental set-up with a CD case that will show you that distance does matter. Hold the case vertically and open it so that the angle is 45?. The centre of the CD is your target:

http://home.arcor.de/rayluck/sturmovik/revi-cdcase.gif

So, in my understanding, what the collimating lens basically does is keep the distances a and b equal -- in an optical sense.

plumps_
01-13-2005, 12:55 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Jungmann:
Plumps__--a bit of info. The Mark II reflector, standard on RAF fighters from '39 to '44 or so, has two adjustment rings on it, but neither affects the size of the reticle.

The rings affect two horizontal lines of light that intersect the arc of the reticle at 3 and 9 o'clock. One adjustment ring is labelled "base feet"; that's for the wingspan of the target--it runs for 35 to 100 feet. Say your Jerry fighter target has a wingspan of 35 feet--turning the ring pulls the ends of those two horizontal lines closer together. The other ring is "range"--it runs from 150 to 600 yards. Turning it also makes those same two lines move closer or further apart.

Toghether, they turn the sight into a range finder. Say the convergance on your a/cs guns is 300 yards. By setting that on the range ring and 35 feet on the base ring, it means when the wingtips of your target touch the ends of those two horizontal lines on the reticle, the gunsight's telling you you're 300 yards behind it. Time to fire. <HR></BLOCKQUOTE>

OK, then I'm probably wrong when I say that the optical range needs to be adjusted. But still the distance does matter. So I guess that at such large distances the difference is marginal, and a lense focused to infinity will just be allright to hit at 100, 200 or 300 m as well, even if your eye moves slightly out of the target line.

plumps_
01-13-2005, 01:13 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by VF-3Thunderboy:
The colminator lens is just a magnifing lens that magnifies the origional gunsight image, which is black and clear where the gusnight image is, so what you are really seeing is the light bult shine through where there is no black ink on the image. <HR></BLOCKQUOTE>
Why would you need a magnifying lens when the reticle is right below the reflective plate and you're so close to it?

UKPsycho
01-13-2005, 01:25 PM
Great thread! Very interesting reading. Please keep it coming! http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif http://forums.ubi.com/images/smilies/11.gif

VF-3Thunderboy
01-13-2005, 04:37 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>So, in my understanding, what the collimating lens basically does is keep the distances a and b equal -- in an optical sense. <HR></BLOCKQUOTE>

The Colminator lens is a fancy magnifying glass.
Its probably optically designed to put out perfect right angles, but its still a fancy magnifying glass.


I see what they are getting at, I think. Im not sure what A is though. Im not sure if this works or not. You could try it out simple enough if you measured the CD case, and flashlight top. But if you move your head, I would think the reflection will also 'move'.

Actually all it says is that the image will be in the center of the case, nothing to do with where your head moves. There is NO TRACKING going on here. Its a light bulb and a reflection.Period...!Get a flashlight and a CD case.. Really try it...

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Why would you need a magnifying lens when the reticle is right below the reflective plate and you're so close to it?
<HR></BLOCKQUOTE>

The reticle is about the size of a quarter or less, probably to do with the lightbulb SIZE.

A larger image would be less illuminating I guess. They use 25 watt light bulbs.


That is a good demonstration, if you look at the REVI sight, you can see the lightbulb, the reticle sort of, and the magnifing lense.


And the big one:

if you move your head, you will be out of the line of sight...!! http://forums.ubi.com/images/smilies/blink.gif

Sulky
01-14-2005, 01:13 AM
Thunderboy,
Roger that. Its a simple bit of kit indeed.
The art is in its use. With a known wingspan it gives you range. With a known speed it gives you lead.
The Mk8 and Mk9 were fundamental to the success of US combat tactics. We read endless threads on BnZ and high speed slashing attacks but nothing on how the Allied flyers hit their targets, whilst screaming bye.
After studying the Mk8 usage manual and applying that knowledge, I now find deflection shooting extremely accurate. Great in sim, but a much greater insight into the world of the WW2 US Aviator.
Sulk'

plumps_
01-14-2005, 01:56 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by VF-3Thunderboy:
I see what they are getting at, I think. Im not sure what A is though. Im not sure if this works or not. You could try it out simple enough if you measured the CD case, and flashlight top. But if you move your head, I would think the reflection will also 'move'.

Actually all it says is that the image will be in the center of the case, nothing to do with where your head moves. There is NO TRACKING going on here. Its a light bulb and a reflection.Period...!Get a flashlight and a CD case.. Really try it... <HR></BLOCKQUOTE>
I did not only try it, I also drew that diagram. Now stop thinking and just try it yourself. There's not much to measure, simply hold the flashlight directly below the CD case for the first try and two feet below the case for the second try, and then watch the difference. This is no rocket science, it's simple geometry: When the distance between "target" (CD) and reflective plate equals the distance between light bulb and reflective plate, and the reflective plate is inclined by 45?, the reflection of the light bulb will not move in relation to the target when the observer moves.


<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR> Why would you need a magnifying lens when the reticle is right below the reflective plate and you're so close to it?
<HR></BLOCKQUOTE>
The Colminator lens is a fancy magnifying glass.
Its probably optically designed to put out perfect right angles, but its still a fancy magnifying glass.
...

The reticle is about the size of a quarter or less, probably to do with the lightbulb SIZE.

A larger image would be less illuminating I guess. They use 25 watt light bulbs. <HR></BLOCKQUOTE>
If all they needed was a bigger reticle they would simply have used a bigger light bulb.

Remember what prop-head wrote about the collimating lens:

2) Collimating lens - placed at a distance from the reticle equal to the lens's focal length, so as to collimate, or make parallel, the light emerging through it. This makes the reticle pattern appear to be at infinity. In other words, if you aimed a camera into the collimating lens so as to photograph the reticle within the assembly, you'd have to set your camera's lens to the infinity focus position.

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>And the big one:

_if you move your head, you will be out of the line of sight...!!_ http://forums.ubi.com/images/smilies/blink.gif <HR></BLOCKQUOTE>
No. As long as you still see the reticle on the reflective plate you can still aim. Otherwise you wouldn't need a reflective sight at all.

Remember what prop-head wrote, he's correct:

Let's say you move your head back and forth in the L/R direction (as though you were trying to touch your temples against the sides of the canopy). As long as the reticle remains visible, it will remain motionless WITH RESPECT TO THE WORLD OUTSIDE -- it is not tied to the reflector in any way! Beyond a certain point, when your eye has moved too far from the ideal viewing position, the reticle will begin to disappear as it gets clipped because of the limits imposed by the physical size of the reflector and/or collimating lens.

gmot_ka
01-14-2005, 03:08 AM
Great info!

Especially plums firs driagram helped me a lot! http://forums.ubi.com/images/smilies/11.gif

So, the collimator lens produces a 'virtual' image of the cross (piper) behind the pilot. This image can be seen throu the reflector plate at the right position in front. Hopefully at your target!

Maybe you can make an other diagram for clearification. How a slide projector works, with a 'not virtual' image of the cross in front, at the reflector plate.

So you can see the differenc: in one case the piper is moving when you move your haed, in the other case it is not.

One more remark to your first drawing: the 'virtual' image behind you is the image of the piper (reticle) not the image of the plane. Anyway, it help me to unterstand how it works.

plumps_
01-14-2005, 04:55 AM
OK, I made a diagram that illustrates the WRONG idea some people have of how it should work. This is what would happen if the reticle was simply projected to a fixed position of the reflective plate. You'd have to align directly behind the pipper to hit the target. And you wouldn't even know if your head is in the correct position because there's not a combination of notch and bead, but only a notch. So this can't work:

http://home.arcor.de/rayluck/sturmovik/revi-scheme-wrong.gif

And once more the correct diagram:

http://home.arcor.de/rayluck/sturmovik/revi-scheme.gif

Sulky
01-14-2005, 07:09 AM
In the ring and bead sight the pilot must align the bead of the fore sight in the inner ring of the back sight with his eye, for every type of shot, otherwise his "line of sight" will be incorrect. For instance if the eye is 1/8th inche out of line it will give an error of 20feet at 400 yards. Again this is not so with the reflector sight which, so long as its image is visable on the windshield, it again automaticly compensates for the position of the pilot's head.
The "again"s in that quote refer to the precedeing para which covers fixed (ring n bead) as against floating (reflector sight) eye position in the fore and aft direction.
And yes the reflector sight does have an infinate focus apparent to the pilot, otherwise the comparison of aircraft features to gun sight would involve a refocussing and be extremely difficult. Its an aid not a handicap.
Sulk'

VF-3Thunderboy
01-14-2005, 10:30 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>This is no rocket science, it's simple geometry: When the distance between "target" (CD) and reflective plate equals the distance between light bulb and reflective plate, and the reflective plate is inclined by 45?, the reflection of the light bulb will not move in relation to the target when the observer moves.
<HR></BLOCKQUOTE>

This is not true, it will not move in relation to the center of the glass, NOT the target.

You are implying that the gunsight magicly tracks the target no matter where your head is. This is totally incorrect. It is a light bulb and a reflection, butt - ugly simple. it is not a lazer beam, you need to be directly behind it to have the line of sight to hit the target.

This is why the Navy taught deflection shooting, as it was not automatic tracking. In a high G situation, you need to pull ahead of the target. US Navy gunsights were designed for this task. US Air force gunsights were not (a simple dot in a circle.)


If you move your head, you will be out of the line of sight with your guns,and your gunsight no ifs about it, unless you can prove some optical trick here, like some actual background data, or description by a military pilot who used em...There is no way that the reflection tracks the target, and remember it projscts UP, not OUT...then back to you , not the target

http://forums.ubi.com/images/smilies/16x16_smiley-surprised.gif http://forums.ubi.com/images/smilies/16x16_smiley-tongue.gif

plumps_
01-14-2005, 10:53 AM
VF-3Thunderboy, you obviously haven't understood the principle...

What else can I do if you don't understand what I say?

I do have a CD case and a flashlight in front of me and I see with my own eyes that the reflection moves in relation to the glass and not in relation to the target.

If you don't believe me then maybe at least you believe your own eyes? Watch the movie I had linked to on page 1: http://home.arcor.de/frischluftduft/Revi.zip

There you can see how the reflection moves in relation to the glass.

Once more, it's not a matter of magically tracking the target, it's a simple case of "angle of incidence is equal to angle of reflection".

plumps_
01-14-2005, 11:25 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by VF-3Thunderboy:
You are implying that the gunsight magicly tracks the target no matter where your head is. This is totally incorrect. It is a light bulb and a reflection, butt - ugly simple. it is not a lazer beam, you need to be directly behind it to have the line of sight to hit the target.

This is why the Navy taught deflection shooting, as it was not automatic tracking. In a high G situation, you need to pull ahead of the target. US Navy gunsights were designed for this task. US Air force gunsights were not (a simple dot in a circle.)


If you move your head, you will be out of the line of sight with your guns,and your gunsight no ifs about it, unless you can prove some optical trick here, like some actual background data, or description by a military pilot who used em...There is no way that the reflection tracks the target, _and remember it projscts UP, not OUT...then back to you , not the target_

http://forums.ubi.com/images/smilies/16x16_smiley-surprised.gif http://forums.ubi.com/images/smilies/16x16_smiley-tongue.gif <HR></BLOCKQUOTE>
You're over-interpreting it.

To be more precise: Of course the sight doesn't track a moving target the way a lead computing sight does. What it does is track the aiming point the sight and guns are adjusted to.

VF-3Thunderboy
01-14-2005, 11:36 AM
Ok I did watch the clip, it obvoulsy moves when you move your head, so if your not lined up, youll be off target. You can plainly see that, so basicly you need to center the image on the glass, fairly simple.

That being said, this is my MK 23 gunsight which is very similar to the revi. Same thing.
http://img83.exs.cx/img83/1562/gunsightdemo1bx.jpg


If your head MOVES, you will still be out of line of sight with your GUNS, and your sight image, but its EASY ENOUGH to center the light ring on the glass...

To reiterate, its a reflection of the light bulb, the "optical lens" is so that you get a perfect image of the very small origional image. That is why you can see to infinity with the lense, so there is no distortion, IE: a "Fancy" magnifing glass..

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR> The BASIC CONCEPT of a reflection gunsight is so you can see through the light reflection with nothing metal BLOCKING YOUR LINE OF SIGHT. <HR></BLOCKQUOTE>


That is what makes WW2 flight sims so fun is the SKILL in deflection shooting...


Blooody@ I think Im gonna mount that thing in the car for the heck of it now!! http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

plumps_
01-14-2005, 12:02 PM
So you own a reflective gunsight but you don't understand it. I may own a bible but that doesn't make me a priest.

Your pictures are worthless as long as the bulb isn't switched on.

Maybe someone else is ready to teach you the basics. It's becoming too time-consuming to say all this in a language that isn't my own.

Fortunately there seem to be a few persons who have understood the principle.

VF-3Thunderboy
01-14-2005, 12:34 PM
Ok- I put a light in it, yes with a light in it it actually does stay on the target with a slight head movement,within reason (6 inches side movement it looks like). It is not perfect at all, but the "concept" is sound!

Very cool!

The optical lens seems to be crucial. So the diagrams were correct! My bad


http://forums.ubi.com/images/smilies/blink.gif http://forums.ubi.com/images/smilies/35.gif

plumps_
01-14-2005, 12:47 PM
For those who don't have a real Revi at home I've got one more idea to demonstrate the principle. Sometimes a simulator can be quite useful. http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

Start Forgotten Battles. Go to the QMB, select a BF-109 E or F type. Switch to the view where the sight is not centred (Shift + F1). Fire the guns and observe where the bullets go to. Then hit Shift + F1 again and repeat the firing. What do you see? The bullets will go to the centre of the reticle, no matter if the view is centred or not.

S!

prop-head
01-17-2005, 05:52 PM
Whew! I didn't know I'd stir up such a hornet's nest!

Now that Thunderboy has "gotten it" http://forums.ubi.com/images/smilies/35.gif, I'll add a wee bit more... In the following I'll use as an example the simple reticle pattern comprising a central dot and outer ring, for simplicity's sake.

As has been mentioned already, the advantage of the optical gunsight -- indeed, the reason it was invented -- is that it eliminates the necessity of trying to stay precisely aligned with a ring and bead sight in a bouncing cockpit. Alignment on the target is now possible in spite of head shake -- within limits.

Here's the crux. The diameter of the collimating lens limits the amount of displacement off-axis of the pilot's eye before the reticle pattern center disappears. Irrespective of the eye's distance from the reflector, a 3-inch diameter collimator allows the reticle's CENTER to remain visible as long as the eye remains within 1.5 inches of the optimal axial position, for a total allowable lateral (i.e., L/R, up/down, etc., NOT fore/aft) displacement of 3 inches, or the collimator's diameter. A 5-inch diameter collimator would allow the eye to wander laterally a total of 5 inches.

Now, you might ask, why aren't gunsight collimators made way bigger then? Well, for two reasons mainly. Most important, the whole gunsight assembly would have to be bigger too, and could end up obscuring more of the panel's gauges. And making a large diameter lens with an ideally short focal length (so as to keep the lamp/reticle housing as short as possible) would necessitate a more difficult-to-figure aspheric curve on one or both of the lens surfaces -- and the lens would be quite thick, too. Even then there could remain undesirable distortion in the projected reticle pattern.

I emphasized the CENTER of the reticle in the preceding discussion for this reason. Depending on the eye's distance from the reflector, the ring portion of the reticle pattern will disappear from view more or less quickly as the eye wanders off-axis. By way of a somewhat extreme example, suppose a dive bomber pilot moved out of the way so that his rear gunner could align his eye properly with the forward gunsight, putting the dot dead center in the reflector. In all likelyhood he wouldn't be able to see the outer ring at all. But, like the pilot, he could still move his eye laterally a total distance equal to the collimator's diameter and still keep the reticle's dot in view. And probably PORTIONS of the outer ring would come into view as the dot moves toward one or another edge of the reflector. Those of you with working gunsights can easily verify all this.

By the way, the movie link given earlier in this thread which illustrates the Revi gunsight in action is excellent. But it would have been even more potent if, instead of a low-contrast, nearly featureless background, a clearly identifiable distant target were positioned behind the reticle. Then it would be obvious that lateral displacement of the camera (or eye) indeed does not cause the projected reticle to drift off-target.

In a nutshell, the optical gunsight negates the problem of parallax in aiming. With the old fashioned ring and bead sight, a very small lateral displacement throws off the alignment of eye-bead-ring, and moreover the nearness of the aiming device to the eye causes it to appear to move with respect to a distant target. But the PARALLEL light transmitted by the collimator means that as long as the reticle can be seen, the eye can wander about while the direction in which it is looking toward the image of the reticle remains unchanged (read: PARALLEL), hence fixed with respect to a distant target.

The question of target distance has come up, but in the final analysis is irrelevant. Even when lining up on your victim from 50 meters/yards distance (which is rather close if he blows up!), the parallax across across a 3-inch diameter collimator is 1/10 degree, or one part in 600 -- quite negligible!

I have built a form of gunsight as an aiming device for a large astronomical binocular. Its reticle was simply a small hole in sheet aluminum, behind which was placed a red LED. The collimator was about 30mm in diameter. Placed near the collimator, and tilted at a 45 degree angle, was a small mirror adjustable in tilt for final alignment. The mirror directed the collimated image to the usual clear glass reflector plate, also titled at 45 degrees and parallel to the mirror (think of a tank periscope-like arrangement). Looking through the reflector was a small wide-field aiming telescope of aperture 30mm and magnification 5X. Once this device was mounted on the binocular, a star was centerd in the binocular and the "gunsight" mirror was adjusted until the red dot was placed against the star as seen in the small finder.

But for years now amateur astronomers could buy an inexpensive, commercially made finder essentially identical to the WWII-era gunsight. It's called the Telrad. My own homemade unit is functionally similar, differing only in the addition of a small finder scope, enabling stars to be seen which are too faint for the unaided eye.

VF-3Thunderboy
01-17-2005, 06:30 PM
Thanks for bringing this up Prophead, http://forums.ubi.com/images/smilies/mockface.gif I had no idea the whole reflector sight concept was so complex. http://forums.ubi.com/images/smilies/351.gif

You are right on the money with the REVI movie. It confirmed MY (old http://forums.ubi.com/images/smilies/cry.gif) perspective perfectly the way it was displayed...
Myabe I can make a proper movie when I make a recticle for my Mk23..

US gunsights on average have lenses about twice as large as the revi,about 4.5 inches in diameter, theyre very big in comparison...

Also, my relfector glass gives a mirror like reflection at about 15 degrees angle...Its not just "glass" its "reflector glass".

The Mk 23 i have was probably used in bombers(turret?) So if they have door positiions in sims, this and the Mk6a unit should be incorporated in the sim for mid-late war aircraft...
I should post pics of that Zero Gunsight. They are almost non-existant... http://forums.ubi.com/images/smilies/16x16_smiley-tongue.gif

Jungmann
01-17-2005, 07:40 PM
One other advantage of the reflector over the old ring and bead: you don't have to constantly shift your focus between the ring (maybe a foot and a half in front of your eyes) and the target (maybe 300 yards). The fact that the lens is focussed at infinity gives the appearance of the reticle being out there, at infinity, on top of the target, at the same range as the target. The pilot can keep his eyes focussed at the target out there at infinity and still have an aiming reticle superimposed optically on top of if.

The two-D sim doesn't reproduce this.

Cherz,

prop-head
01-17-2005, 08:11 PM
Jungmann, you obviously have a good grasp on this! On the idea of differential focus between near and far, no sim models this, to be sure. In reality once one has focused on the target/reticle, the gunsight assembly (and anything else in the cockpit) would be slightly out of focus. This would be more apparent in low light conditions when the eye's pupils would be dilated, hence offering a smaller depth of field (just as for a camera lens). I wouldn't be surprised if some future sim models this different focus effect, either automatically or by user input, by focussing/defocussing depending on whether you're looking down in the 'pit or looking out at the wider world. And imagine this in 3-D via hi-res HMD (Head Mounted Display).... http://forums.ubi.com/images/smilies/icon_twisted.gif