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Post by ampingu on Apr 22, 2009 7:03:24 GMT -4
Hello, Image AS11-40-5903 appears to show a 'fall-off' effect - an effect similar to that labelled in software such as 3d studio max as 'light decay' or 'drop-off'. Ive searched the interweb, such as it is, and have yet to find a simple explanation for this. Im hoping you guys can help. Ive read in a few places that the widely published version of this image is generations removed from the original and that this 'drop-off' effect is less apparent in the more pristine copies*. However, even AS11-40-5903HR appears to show 'light decay' reminiscent of a point light in relatively close proximity to the lunar surface. I understand that the sun was given to be the only source of light on the mission (other than indirect illumination, ie reflected light from surrounding objects and the surface itself), so could someone explain in simple terms how this effect occurred? Here are a few images to demonstrate: This is an example of 'drop-off' - a spotlight simulated in cg graphics software (inverse square):  A copy of AS11-40-5903HR. The effect is more obvious with extreme contrast (right). To my eye, this looks like an elliptical 'drop-off' pattern, exactly what Id expect to see in a scene lit by a spotlight:  A cg approximation:  *Ive read suggestions that copies such as AS11-40-5903HR have been retouched to lessen the effect. I dont believe this is true. AS11-40-5903HR(right) adjusted to resemble AS11-40-5903(left):  Ill be grateful for any response. Thanks. Ps: Im not an expert, nor am I a hoax advocate or conspiracy theorist. This is my first post here. Forgive me if this topic is already covered elsewhere on the site. nssdcftp.gsfc.nasa.gov/miscellaneous/planetary/apollo/as11_40_5903.jpghistory.nasa.gov/alsj/a11/AS11-40-5903HR.jpg |
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Post by Grand Lunar on Apr 22, 2009 7:37:27 GMT -4
IIRC, the appearent effect is caused by the nature of how the lunar surface reflects light.
It's highly directional, tendind to reflect most light toward the point of origin (in this case, the sun).
There is an original of the photo were the background isn't as dark as some copies of the photo show.
Like you, I'm no expert. I'm just recalling what someone else on this board had written.
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Post by Kiwi on Apr 22, 2009 8:29:28 GMT -4
To my eye, this looks like... exactly what Id expect to see in a scene lit by a spotlight. Welcome to ApolloHoax, Ampingu. I've only used the bulb-type and flash spotlights that are used in small studios, but I have to disagree with the above remark. Do you really understand the effect spotlights produce? As their name describes, they produce a fairly small spot with a fairly hard edge. Exactly what this spot looks like and how big it can get depends on the power of the light and whether it can be focussed to control the size of the spot. Many spotlights wouldn't make a spot any bigger than the small crater in which Buzz is standing, and if a spotlight was the only source of light, beyond that crater the surface would be dark even in the low-contrast version of the photo. The foreground LM footpad and the three small craters that form a rough line to the left and right of Buzz's right hand would not be lit like they are. There are plenty of other images, both still and movie, that show the visible lunar surface extends at least five if not many tens of metres beyond where Buzz is standing, so I cannot agree that it is at all lit by a spotlight. And if it was lit by multiple spotlights we would see multiple spots and possibly multiple shadows cast by the same object. If you study a few hundred or thousand lunar surface photos you will probably see similar effects -- and more -- in some of them. One example is the darkness out near the horizon -- this is quite common in cross-sun or up-sun lunar surface photos. For some reason, quite a few HBs have looked no further than AS11-40-5903. It's the favourite. If you were an experienced and observant photgrapher you would probably see similar effects in your own photos taken with a wide-angle lens in low sun on well-trampled beaches or rough grass. Many of the shadows are simply caused by the slope of the surface -- note the forward slope in the foreground. And in the brightest area, to our right of Buzz's left leg, the brightness is caused by a slope in the other direction and no visible objects casting shadows. There are bright areas in many of the footprints for the same reason -- the smoothness and angles of the surface to the light. There are long-winded technical descriptions of the reasons for the vignetting you mention (phase angle, lunar surface structure etc.) but I'm not familiar-enough with them to explain them. JayUtah illustrates some of them on this page at his web site, Clavius. Note the darkness of the grass on the right of the first photo. www.clavius.org/trrnshdow.html |
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Bob B.
Bob the Excel Guru?
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Post by Bob B. on Apr 22, 2009 8:41:53 GMT -4
There is no reason we should expect the surface to appear evenly illuminated everywhere. Surface texture and the phase angle will effect how bright the surface appears.
For instance, the foreground surface has a lot of disturbed soil caused by foot traffic. On the other hand, as I recall, the area right behind Aldrin was in the path of the LM as it landed. As such, this surface has had some of the dust blown away by the engine exhaust. These two mechanisms have created different surface characteristics resulting in different appearances.
Phase angle also changes the appearance. The angles between the line-of-sight, the light source, and the surface changes at different locations in the frame, thus we can’t expect to see a uniformly bright surface. Some angles favor light being reflected toward the camera while other angles are less favorable.
Furthermore, the area right behind Aldrin is receiving additional illumination from light reflecting off the LM.
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Post by echnaton on Apr 22, 2009 10:17:40 GMT -4
Image AS11-40-5903 appears to show a 'fall-off' effect - an effect similar to that labelled in software such as 3d studio max as 'light decay' or 'drop-off'. Welcome to the board! Is the feature you mentioned designed to mimic a naturally occurring lighting effect? |
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Post by Kiwi on Apr 22, 2009 10:48:32 GMT -4
Ampingu, I have looked at a few hundred lunar surface photos on my hard drive, and that darkening at the horizon isn't visible in many of the photos from Apollos 15 to 17 because of the mountains in the background, but it can be seen in the other missions' photos. You can quickly view thumbnails via the links in this thread. But have you simply looked at all of Roll 40's photos, including the assembled panoramas? They're all captioned in the Apollo 11 Image Library at the Apollo Lunar Surface Journal -- link at the bottom of every page here. This handy diagram www.hq.nasa.gov/office/pao/History/alsj/a11/a11photomap.gifshows the directions in which all the Apollo 11 EVA photos were taken. AS11-40-5903 was taken facing roughly northeast, 45 degrees from the sun. Others facing a similar direction are 5886, 5911, 5934 and 5954. Another group, taken at right angles to 5903, facing southeast, are 5888, 5913, 5936 and 5937. This last group are fascinating because they show the same darkening at the horizon, and the important point about that is that no spotlight is going to reach 90 degrees from NE to SE across the landscape on that sort of scale. We are seeing a natural effect, not that of a spotlight, which you say is "exactly what I'd expect." |
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Post by JayUtah on Apr 22, 2009 12:29:03 GMT -4
Im hoping you guys can help.
Probably. This is a common question, even if the answer may not be easy to find.
Ive read in a few places that the widely published version of this image is generations removed from the original and that this 'drop-off' effect is less apparent in the more pristine copies*.
Yes, but only because you get more subtle gradations in contrast in the original.
...so could someone explain in simple terms how this effect occurred?
Yes: the texture of the lunar surface. Lambertian fall-off is a true principle, but not everything that looks like it is really caused by it.
You're looking roughly up-sun -- that is, toward the sun. As such, you're seeing largely the shaded portions of texture elements. The proportion of light to dark that you see depends on the phase angle and the amount of texturing present. You can demonstrate this yourself with CG by lighting a highly textured surface from a low angle and then viewing it from different angles. Then try a surface with a varying amount of texture across some area. You'll be able to reproduce it with a purely directional light source (i.e., not a proximal point light).
Aldrin is standing on the portion of the surface that was swept by the LM descent engine. We know from other photographs that the sweeping tended to remove texture. We also know that the LM was drifting significantly to the left at touchdown. (Aldrin is at the right rear of the vehicle.)
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Post by JayUtah on Apr 22, 2009 13:28:03 GMT -4
Do you really understand the effect spotlights produce? As their name describes, they produce a fairly small spot with a fairly hard edge.
A "spot" is defined by its narrow field angle, typically 20-30 degrees. Even ellipsoidal luminaires can be defocused to produce a soft-edged beam, but typically not that soft-edged. The problem with lighting this scene using a spot luminaire to create the hot-spot is that you have to figure out where the rest of the light is coming from. If from a series of floods or scoops (such as to create a wash) then where are the multiple reflections that come from such a setup? If the hot-spot is from a wide-angle luminaire behind the astronaut (so as to create both the hot-spot at the shadow) then why aren't the texture elements behind the hot-spot showing the effects of small phase angle?
No -- the similarities between this photo and Lambertian falloff are only superficial. Many conspiracists have tried to imagine an artificial lighting design that would produce this and other Apollo photographs. To this point, none has demonstrated a lighting setup that produces all the observed results. And to this point, experienced people have been able to explain (and demonstrate) how the sun alone can create these effects.
There are long-winded technical descriptions of the reasons for the vignetting you mention...
To be scrupulous, vignetting occurs in the lens and does not vary by phase angle or subject.
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Post by ampingu on Apr 22, 2009 14:40:08 GMT -4
I've only used the bulb-type and flash spotlights that are used in small studios, but I have to disagree with the above remark... Hi. Yes. You disagree that I (personally) would expect to see this drop-off effect? No, im certainly not an expert in real life spotlights, and please dont assume I have any credentials. Im just an amateur - thats why Im here looking for answers. I think I agree with you, its not what Id expect from a spotlight, so much as its what id expect to see in simulated inverse square 'fall-off', as used in 3d modelling software to imitate real world lighting. Thats not to say Ive reached a conclusion that the source of the effect is a close proximity light source. Im simply saying that I recognise the light gradient as resembling the 'light decay' effects Im familiar with through (re)creating light in a cg 3d environment. In the cg examples I included above, I chose a simulated spotlight (with a cone) to demonstrate light 'drop-off' as opposed to other lighting effects such as vignetting*. I made use of a setting they call 'overshoot' and it may not be a good representation of any real word spotlight. I dont think the type of light used is important for the sake of demonstrating the concept. Here is the same scene having substituted the 'spotlight' for an omni-directional light in the same position with the same 'decay' setting. [never mind] *I think vignetting is something slightly different. The image I chose seems to exhibit light fall-off across the lunar surface. (I chose AS11-40-5903 because its the famous one). Is the feature you mentioned designed to mimic a naturally occurring lighting effect? Yes, I believe so. If youll forgive a link to wikipedia: en.wikipedia.org/wiki/Inverse-square_law#Light_and_other_electromagnetic_radiationIts my understanding that the mathematical models in software such as 3d studio max are designed to simulate real world effects. Thanks for the diagram, Kiwi. I guess I could use that in a mathematical analysis of the image. Id like to know if the light values conform to an inverse square drop-off around this, what appears to be, elliptical illuminated area. Im still hoping for a simple explanation that will save me the trouble**. Thanks all for looking at this. **thanks Jay, thats good enough for me. id already written the above, so i posted it anyway. Aldrin is standing on the portion of the surface that was swept by the LM descent engine. We know from other photographs that the sweeping tended to remove texture. We also know that the LM was drifting significantly to the left at touchdown. (Aldrin is at the right rear of the vehicle.) |
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Post by gwiz on Apr 23, 2009 6:48:46 GMT -4
Furthermore, the area right behind Aldrin is receiving additional illumination from light reflecting off the LM. ...and from light reflected off Aldrin's white spacesuit. As Jay says, you are looking up-sun into a textured surface, so you see the shadowed side of of the texture. Light from the LM or the suit reflected into these shadows makes a lot of difference to the overall brightness. |
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Post by ampingu on Apr 23, 2009 13:59:13 GMT -4
Phase angle also changes the appearance. The angles between the line-of-sight, the light source, and the surface changes at different locations in the frame, thus we can’t expect to see a uniformly bright surface. You're looking roughly up-sun -- that is, toward the sun. As such, you're seeing largely the shaded portions of texture elements. The proportion of light to dark that you see depends on the phase angle and the amount of texturing present. I may be misinterpreting, but I think all youre really saying is that the surface varies in colour and that the amount of light you see reflected depends on your viewing angle. This may allow for a kind of glint effect, a bit like a wet car park (as in this picture I pinched from google images):  The light/colour gradient in '5903 is amazingly similar to the kind of light 'drop-off' demonstrated in my first post - but Im happy with the explanation that the module was drifting towards the right of the image and removed surface material. Thats plenty good enough for me. I can certainly see how this effect has fuelled hoax theories. If the hot-spot is from a wide-angle luminaire behind the astronaut (so as to create both the hot-spot at the shadow) then why aren't the texture elements behind the hot-spot showing the effects of small phase angle? Could they have painted the shadows/surface of the set? Im kidding. Thanks all for the insight. As such, this surface has had some of the dust blown away by the engine exhaust. These two mechanisms have created different surface characteristics resulting in different appearances. (thanks) |
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Post by JayUtah on Apr 23, 2009 16:33:37 GMT -4
I may be misinterpreting, but I think all youre really saying is that the surface varies in colour and that the amount of light you see reflected depends on your viewing angle. Not exactly. By "texture" I don't mean variations in color such as is understood by texture-mapping in computer graphics. That's a badly named feature anyway. By texture I mean the actual variations in contour geometry in the lunar surface, more akin to what CGI people call "bump-mapping." Where such texture is present, the overall impression of brightness depends on the phase angle, which is the difference between the direction of view and the direction of illumination. Where the phase angle is small, your light comes from more or less directly behind you. The portions of the texture elements that stick up are seen fully illuminated. Their shaded sides and shadows are behind them, where they can't be seen. The overall effect is of a very bright surface. When the phase angle is large, your light comes from the side or from behind the surface. In that case you see the shaded sides of the texture elements and their shadows. The most brightly lit portions of the texture are behind the elements themselves and not as visible. The overall "macro" effect is of a dark surface. Where that texture is relatively absent, the overall apparent brightness is governed more by Lambertian illumination principles. Which is to say, the apparent brightness doesn't vary much by phase angle. Now imagine a surface in which the amount of texture varies. If seen from certain phase angles, portions of the surface will appear brighter. Figure 2 on this page www.clavius.org/bibzz2.html shows tire tracks across a textured desert floor, lit from a very large phase angle (i.e., from behind). The only light source in the image is, in fact, just out of frame where indicated. Because the tires have mashed the desert flat and removed the texture, that area appears brighter. AS11-40-5903 includes the area swept by the DPS exhaust plume. But undisturbed elements of the surface are also visible. The phase angle is very large, as you can see from the rough direction of Aldrin's shadow. Hence the surface appears brighter where it is flatter.
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Post by ampingu on Apr 24, 2009 4:04:51 GMT -4
Thanks. Thats very thorough. Im with you on the concept of the viewing angle altering the amount of light reflected back to the eye or camera. I think thats the basis for effects such as specular highlights (widely used in cg modeling). And of course the topography of the surface will affect the angle. But im not sure im entirely on the same page regarding phase angle re a relatively flat surface. Where that texture is relatively absent, the overall apparent brightness is governed more by Lambertian illumination principles. Which is to say, the apparent brightness doesn't vary much by phase angle. Now imagine a surface in which the amount of texture varies. If seen from certain phase angles, portions of the surface will appear brighter. Id suggest that its exactly because apparent brightness does vary a lot with viewing angle, even on a flat surface, that you see these highlights. ie, the surface is not a 'lambertian reflector' AS11-40-5940:  Ps: ive had to google a bit to brush up on some of this stuff. (thanks for the link. AS14-64-9057 is a good one. id not seen that before.) |
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Post by Kiwi on Apr 24, 2009 8:16:09 GMT -4
To be scrupulous, vignetting occurs in the lens and does not vary by phase angle or subject. Touche! That resulted from: 1.) When writing the post, thinking of lenses I bought in the 70s, and still have, that don't vignette at all. (Mamiya C330 6x6 TLR -- a Melico enlarging computer couldn't detect any variation in a negative of an 18% gray card from centre to corner). 2.) Thinking of more expensive lenses I once owned that vignetted terribly (Bronica ETR 6x4.5 SLR -- the drop-off was over a stop from center to edge and worse in the corners). 3.) Posting after midnight. 4.) Meaning to write "vignetting" effect (which isn't the best of terms anyway), and forgetting to do so. Thanks Jay. Im with you on the concept of the viewing angle altering the amount of light reflected back to the eye or camera. I think thats the basis for effects such as specular highlights (widely used in cg modeling). Specular highlights in photographs before CG always meant washed-out, overexposed areas produced by light sources or their barely-diminished reflections from surfaces such as glass, chrome, or water. |
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Post by gwiz on Apr 24, 2009 9:10:04 GMT -4
Id suggest that its exactly because apparent brightness does vary a lot with viewing angle, even on a flat surface, that you see these highlights. ie, the surface is not a 'lambertian reflector' Far from it, and you can get an impression by observing the moon from earth. A full moon is much more than twice as bright as a quarter moon, and fairly evenly bright across the disc. This shows that the light reflected from the moon peaks in a direction straight back to the source. |
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