Fleet Submarine Periscopes

[LukeFF]

I want to know if I am understanding something here correctly about the periscopes used in fleet submarines:

The diameter of the exit pupil for both the attack and observation periscopes is listed as being 4mm. Now, if I'm reading the description of exit pupil diameter correctly, (such as from this link), the brightness of an image viewed through a lens will increase as the exit pupil diameter increases.

Now, with that said, given that both fleet sub periscopes had the same exit pupil, does that mean the brightness of the image was the same in both periscopes?

[JMV]

A tad too technical for me, but see following link if that can be usefull in any way :

http://www.maritime.org/fleetsub/pscope/index.htm

[Jan Kyster]

"Brightness" is a matter of how much light your system gathers = first lense. The Mt. Palomar telescope probably have the same pupil exit

[Rockin Robbins]

Actually, Jan Kyster is incorrect. The brightness of the image is only partially determined by the diameter of the objective lens. It is possible to put enough magnification on a larger objective lens to make the image dimmer than a lower power view through a smaller objective lens, so objective diameter alone is not responsible for image brightness. The other factors are light transmission efficiency through the system (clarity of glass, number of optical elements, dirt and foreign matter in the system) and the magnification.

The exit pupil is the diameter of the total field of view that your eye is looking at though the eyepiece. The image will appear brighter until it reaches the diameter of a fully dark-adapted eye pupil, about 7mm for a young adult and decreasing to about 5mm for an older adult. Any image of over 7mm exit pupil will not look any brighter to your eye than a 7mm or whatever is the diameter of your pupil at the time.

The way you calculate the exit pupil is to take the diameter of the objective lens (the first lens in the system at the top of the periscope) and divide that by the magnification. For instance, have you ever wondered why 50x7 binoculars are such a popular size? 50mm/7x=just a hair over a 7mm exit pupil. Personally I own a 50x10 pair of binoculars because for me a 5mm exit pupil is just as bright and I can use the extra magnification.

So exit pupil equals diameter of objective lens/magnification. Brightness at the same exit pupil will be greater for a larger diameter objective lens because there is more light condensed into that little diameter.

[Fish40]

Very interesting topic. I have a question concerning scope useage. In reality, was the Obs. scope ever used, or could it even be used to conduct an attack? I know the attack scope had a higher magnification and smaller profile for the obvious reasons. What about the range markings? Did the Obs. scope have similar markings as the attack? I think you know where I'm headed (at least I hope you do). In case you don't, It's my feeling in the interest in realism, that if the Obs. scope was and could only be used as an observation medium, it should be portrayed as such by removing (if possible) the fire controls and associated TDC inputs from the screen. I'm thinking along the lines of what OLC did for his GUI Mod in SH3.

[Munchausen]

Originally Posted by Fish40

In reality, was the Obs. scope ever used, or could it even be used to conduct an attack?

See Calvert's "Running Silent," page 79 of the hardbound copy, for a discussion on how (and why) he used the observation scope to conduct a submerged attack.

[Hitman]

If I understood correctly the concepts -forgive me here but I might be saying something stupid- the idea is that the periscope is a tube with ain input (Lense) and an output (exit pupile) through which the light/image travels.

The larger the input (Light admission), the more light you are able to go through the tube BUT the exit will again limit the total amount.

Responding to your question, unfortunately this brings us to a problem with an unknown variable. Let's put it like this:

Supppose light admission is 10 (Arbitrary vakue picked solely for this example) in observation periscope and 6 in Attack scope.

*If* exit pupil of 5 mm is fully used with 6, then YES you would not notice any difference at all with an observation scope when looking through both.

*If* exit pupil of 5 mm is NOT fully used with 6, then NO, you would not see the same with both, because the larger observation scope captures a larger amount of light (10) and outputs more through the exit pupil.

Unfortunately we do not have a reply for that question: Wether the 5mm exit pupil was fully used or not.

BUT

the logic says that it wasn't because otherwise it would be stupid to mount a second periscope with different characteristics (You could benefit from a second periscope as back-up, but what would be point in making it with different specs then?).

So my bet would be that there was a noticeable difference when you looked through both. In several accounts from WW2 fleet subs commanders I have readed that looking through a periscope was like doing it with sunglasses, and that they were totally useless at dark nights. If that can serve as guide.

In the intelligence reports on the captured U-570 you can see that both british and american examiners were favourably impressed by the german zeiss optics, but the british much less so when comparing with their Barr&Strout items. The americans in turn liked a lot the periscopes in U-570, probably because their Kollmorgan ones were not really very good.

Would like to know what happened after the war when they got german technology, and wether Kollmorgen did improvements for the post war fleet boats that were improved with the experience gained (Guppy conversions).

[LukeFF]

Originally Posted by Fish40

Very interesting topic. I have a question concerning scope useage. In reality, was the Obs. scope ever used, or could it even be used to conduct an attack? I know the attack scope had a higher magnification and smaller profile for the obvious reasons. What about the range markings? Did the Obs. scope have similar markings as the attack? I think you know where I'm headed (at least I hope you do). In case you don't, It's my feeling in the interest in realism, that if the Obs. scope was and could only be used as an observation medium, it should be portrayed as such by removing (if possible) the fire controls and associated TDC inputs from the screen. I'm thinking along the lines of what OLC did for his GUI Mod in SH3.

In short: yes, the observation scope had the same magnification and markings as the attack one. One big difference was that it did not tilt as high as the attack scope.

[LukeFF]

Originally Posted by Hitman

Unfortunately we do not have a reply for that question: Wether the 5mm exit pupil was fully used or not.

Yes, about the closest I can find to how much light came through both scopes is Norman Friedman's discussion about the subject in his book U.S. Submarines Through 1945. It seems that light transmission at its best was about 30%.

Say, here's maybe another guess as to how efficient each scope was at transmitting light: the Fleet Submarine Manual states the observation scope had a "Minimum outer diameter of reduced section" of 1.99 inches, while the attack scope had a diameter of the same section of 1.414 inches. Does that play any role in light transmission?

[Hitman]

Yes, about the closest I can find to how much light came through both scopes is Norman Friedman's discussion about the subject in his book U.S. Submarines Through 1945. It seems that light transmission at its best was about 30%.

That would be correctly translated to the game by giving the Alpha Channel in the mod a 70% darkness. Which is easy enough to do This would ensure that you can only see in the scope screen 30% of light intensity as compared to being in the open bridge.

But would that be attack or observation scope?

here's maybe another guess as to how efficient each scope was at transmitting light: the Fleet Submarine Manual states the observation scope had a "Minimum outer diameter of reduced section" of 1.99 inches, while the attack scope had a diameter of the same section of 1.414 inches. Does that play any role in light transmission?

It should, but it's almost impossible to quantify. Taking those numbers alone, this just tells us that the attack periscope tube had a diameter of 71% what the observation scope had. If the lenses are reduced accordingly (Which might, or might not be correct) this would mean that we should give the attack scope a 29% less light transmission of what the observation one had. It's probably not an exact number, but it is sure much closer to the real thing than any wild-ass guess

In any case, if we pick 30% light transmission for observation scope and scale that back to a 71% in the attack one, that would give us a mere 21,3% of light transmission in this last one. That's REALLY dark

I'll try to make some examples later with screenshots for you. Stay tuned.

[Hitman]

OK, here we go.

For this samples I have used RFB2, setting up a mission in the situation where you would have MOST possible light in the game: In the equator, zero fog, zero clouds, zero wind, midday (12AM) June the 24th (Although this is not much relevant at zero latitude). I have minimized the task bar in the screenshots, so that the black surround is complete and doesn't make contrast. Gamma settings is at 50% (Neutral)

In the first image you see the FULL light transmission, as you would see it with naked eye from the bridge.

In the second, 30% light transmission (Observation scope)

In the third, 21% light transmission (Attack scope)

FULL LIGHT:

30% (OBS SCOPE):

21% (ATT SCOPE):


Honestly, I think this is too exagerated. With the attack scope you would only barely see the silhouette at daytime, and it certainly looks VERY different to the numerous pictures taken through periscopes that are around the web (Not just classic but also modern). But I think, the best way of solving doubts would be to ask Gino or any other curator of USS Cod to take a look at them and tell us if they feel correct

[Nisgeis]

Obs periscope was also used by the OOD. Later it housed the ST radar and was used during submerged attacks to get radar range (which could also calibrate the vertical stadimeter). Also, the observation scope had a horizontal stadimeter, which could measure angular length, for AoB, but I don't think that was ever used in that way.

[Hitman]

BTW Luke,

if you can tell me wether the markings were fluorescent at night (Dim green), then I think that I can make them work like that in the game thanks to a trick discovered by Karamazovnew.

[LukeFF]

Originally Posted by Hitman

BTW Luke,

if you can tell me wether the markings were fluorescent at night (Dim green), then I think that I can make them work like that in the game thanks to a trick discovered by Karamazovnew.

I'll try to see if I can find that out.

Meanwhile, I actually do like that effect above. I did some more searching in the Submarine Periscope Manual and found this information:

(Referring to the observation scope):

3. Image brightness. Omit Section 4U4 and substitute the following details. Since there are five fewer lenses in the Type III, we may expect this periscope to transmit more light than the Type II. Less light is lost by absorption and reflection.

...

By multiplying the transmission (= 100 percent minus the percent of loss) values together, we find that the overall theoretical transmission of the periscope is 19.2 percent in low power and 24.4 percent in high power. These values may also be called the transmission efficiencies, since the incident light was taken as 100 percent.

So, we can conclude there that (1) the observation scope let in more light than the attack scope and (2) the max light transmission was 24.4 percent. Data was taken from page 351, BTW.

As for the attack scope:

Light transmission was 11.1% in low power and 14.2% in high power (page 210).

Thus, we see that the transmission efficiency of the Type II is only about 11 percent, about 89 percent of the incident light is lost when the optical elements have not been coated.

Makes one wonder, though, if the optics ever were coated. Light transmission with coated optics at low/high power was 33.9% and 43.9%, respectively.

Would you mind modifying the scopes in RFB and seeing what kind of effects we can come up with? If so, might be good to take this to PMs.

[Harmsway!]

Be careful what you do with these figures. Transmission is only about the efficiency of light making its way through the glass. It is not the end all in brightness. I have spent lots of hours looking through scopes and I can tell you the higher the magnification the dimmer the image.

The brightness intensity seen at the eyepiece is inversely proportional to the square of the magnification.

When you double the magnification, you get one quarter the brightness. ie, 1/2 squared.

I find the scope images above at 30 and 21% are not at all representative. It is not as simple as photo-shopping the brightness numbers into an image. The truth is our minds eye plays a major roll in the apparent brightness. Those of you who have taken photographs with a fast lens know that film or CCD will capture light different then the natural eye sees.