Wikipedia:Reference desk/Archives/Science/2007 October 28

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October 28

Red Onion Skinn cells

In Biology class, we observed a piece of red onion skin under a 40X objective lens and probably a 10X eye piece. A drop of salt water was added to the skin under the microscope, the red part of the cell shrunk. Is the red part the vacuole or cytoplasm of the cell? Acceptable 01:46, 28 October 2007 (UTC)

Usually, plant dyes (and other secondary metabolites and end products) are stuffed into vacuoles by the plant cell. They would wreak havoc with usual plant metabolism taking place in the cytoplasm. --85.179.20.169 08:36, 28 October 2007 (UTC)

Fall question

I got to wondering why tree leaves aren't all the same shape. You would think there'd be an ideal leaf shape that all trees would have settled down to, but the variety is practically infinite. Are there evolutionary pressures on tree leaf shape by species, or is it accidental? In other words, are certain characteristics of an organism of no importance for suvival, and do these characteristics thus more or less reflect some underlying symmetry or other order that is merely a manifestation of their peculiar origins or mechanisms? I hope that made sense. --Milkbreath 02:23, 28 October 2007 (UTC)

Few (if any) things are accidental when it comes to evolution, although there is always some 'noise', to allow a species to find a new optimum when the environment changes. On a large scale you can look at the difference between pine needles and leaves. Pine needles allow trees to survive very cold climates, whereas regular leaves allow trees to get the maximum amount of photosynthesis in more temperate climates. I expect it works the same way for differences between leaves. The shape of the leaf is the ideal solution for the area where the tree lives in terms of getting the most sunlight, surviving the cold, the heat, and so on. If two trees live in the exact same area, they may have different survival strategies. One tree may get the optimal amount of nutrition from the location, whereas the other may reproduce faster, expand it's habitat faster, or be better at surviving changes in the environment. I'm not an evolutionary biologist, but I figure that's how it works. And there is always the possibility that everything else being equal, there are several optimal solutions, and two trees arrive at different ones. risk 03:41, 28 October 2007 (UTC)

As Risk says, there's lots of different things that a tree needs to deal with: snow, wind, sun, heat, cold, rain, storms. Some need leaves that shed snow, others need leaves that aren't destroyed by cyclones, others need leaves that won't loose too much water in dry weather. Lots of variables means lots of different leaf shapes make lots of different "ideals". What about two trees in the same forest? Well they may also grow in disparate areas, and they may survive or thrive in different situations differently. (oh, and it's spring here - all our imported trees are starting to sprout their myriad leaves in myriad shapes. All our local trees kept their leaves over winter, so we here think it's funny to call that season "fall") --Psud 09:07, 28 October 2007 (UTC)

Most of the factors which affect leaf growth have been mentioned, but let me add disease, insects, and animals which eat or damage leaves. Unless the animal provides some benefit to the plant, like spreading seeds for it, the plant will want to minimize leaf damage. Having many small leaves may help here, as large animals won't bother with small leaves, and the leaves can be easily dropped if they become diseased. Small leaves would also fare better in strong winds, but wouldn't do as much photosynthesis, due to all the gaps between the leaves for sunlight to pass through. StuRat 18:26, 28 October 2007 (UTC)

Leaves are a critical weapon the the war between the trees and the grasses. Grasses and trees try to poison each other, and deciduous trees try to kill grass by annually blanketing the grass with leaves. The leaf shape and size is of critical importance in the success of this strategy. it is deeply involved with the activities of herbivores, so the evolutionary response times will ensure a large range of leaf shapes and sizes. The suburban ecosystem with Maple trees andblue grass is profoundly unnatural-Arch dude 03:06, 29 October 2007 (UTC)

An additional thought is that it is a mistake to assume that everything about a plant (or other organism) has been determined by evolution. Natural selection will only affect those traits that change the survival rate of the species. Sometimes that differential survival critically hinges on leaf shape (needles in evergreen trees with longer winters, waxy, pointy-tipped leaves in plants with areas of high rainfall, etc), but sometimes it does not. For example, a maple leaf and an oak leaf are quite distinct to our eyes, but are probably not all that different in terms of practical survival rates for the individual plants and their offspring. Matt Deres 16:29, 29 October 2007 (UTC)

Butterflies and the fate of the universe

A flap of the wings of a butterfly in India will, given enough time, alter the course of a tornado in the US. Not to put too fine a point on it, but it will inevitably change everything about the world. Will it do the same for the universe? Sappysap 03:54, 28 October 2007 (UTC)

• The way I understand it, that's not actually true. Rather than being taken literally, I think that statement is supposed to be a refernce to the fact that the causes of something like a tornado are so complex that minute events (eg the butterfly) that might not otherwise be associated with the outcome are related. 68.18.209.108 04:42, 28 October 2007 (UTC)

The butterfly effect is a vey specific illustration of the fact that the Earth's atmosphere is a complex and interconnected system in which small changes are quickly magnified. So attempts at predicting weather patterns more than a few days ahead are doomed to failure because we cannot practically collect enough starting data with enough precision to produce accurate estimates. By extension, it has become a metaphor for any natural system that exhibits sensitive dependence on initial conditions. It does not mean that the flap of the butterfly's wings will change everything about the world. Although the butterfly's wings could (in theory) change the course of a torndao, we know of no physical mechanism by which they could (even in theory) cause an earthquake, significantly affect the path of an asteroid, or change the temperature of the Sun. A belief in mysterious and undiscovered mechanisms which intimately link every feature of the universe is more akin to astrology than to science. Gandalf61 07:28, 28 October 2007 (UTC)

Let's put it in perspective. Here's the closest thing to affecting the Universe. Our Indian butterfly flaps it's wings, that causes a tiny (but important in our chaotic weather system) change in the course of a storm in Sydney. A lightning strike happens in one place rather than another and sends a radio pulse out into space. That pulse of radio probably doesn't go very far into the universe, and on the few occasions where it does encounter something, it has practically no effect. Then you consider that the lightning probably would have happened anyway, just somewhere a little way away from where it did happen, and that lightning flash would have been effectively identical from even a fraction of a lightyear away (if it was even detectable at that distance) --Psud 08:58, 28 October 2007 (UTC)

Alternatively, that new lightning strike hits the person who would later have discovered a viable way to extract energy from fusion on a small scale, which would lead on to globe-spanning revolutions in virtually every field of engineering. The technology would've allowed the human race to construct interplanetary and eventually interstellar transportation, and ultimately lead to the colonisation of the galaxy. Which would have an observable effect from that distance. "For want of a nail" and all that. GeeJo ^(t)⁄_(c) • 11:02, 28 October 2007 (UTC)

I feel I should point out that one needs a very strange view of the world for this question to even make sense. In order to pin the tornado on the butterfly you have to suppose that the butterfly's action could have been otherwise, but that everything after that just snowballs according to deterministic (if chaotic) laws. Future choices by other butterflies would presumably dilute the long-term meteorological influence of this one butterfly to the point where it didn't matter whether it flapped its wings or not. The analogy does make mathematical sense as a statement about initial conditions of chaotic systems, but it doesn't make physical sense unless you subscribe to some form of last Thursdayism. -- BenRG 14:39, 28 October 2007 (UTC)

That's a switch—pinning something on a butterfly. --Milkbreath 15:27, 28 October 2007 (UTC)

The problem here lies in one misstated word in the question: A flap of the wings of a butterfly in India will, given enough time, alter the course of a tornado in the US. - not will but could (with a very low probability). There are a bazillion butterflies and a bazillion wing beats - and many, many other sources of atmospheric turbulance - any and all of which are having some effect. It is the net effect of all of them that causes unpredictability. So had that butterfly not flapped at that exact moment, then perhaps the tornado halfway around the world and a decade off in time might not have happened. It's definitely possible - in a chaotic system such as our atmosphere, there is infinite sensitivity to these kinds of events. But for any specific wing beat of any specific butterfly, the odds are truly astronomically small. As to whether the event could cause universe-wide changes - yes, sure. I like GeeJo explanation as to how this could come about. It's definitely possible for a butterfly wing flap to deeply affect something over the other side of the universe a very long time into the future. Again - it's very unlikely - but for sure it can happen. SteveBaker 16:34, 28 October 2007 (UTC)

There are limits to the extent of the effect of a butterfly’s actions. Some parts of the universe can’t possibly ever be affected by a flap of a butterfly’s wings. Due to the inflation of the universe, most of the universe is beyond what can ever be reached by a photon leaving our galaxy now. MrRedact 19:09, 28 October 2007 (UTC)

And, depending on how you look at all this, you can even say "very likely" instead of "very unlikely". In a chaotic system, anything can affect anything; everything affects everything. So the likelihood is very high that "event X" (whatever it is) is caused (in part) by some ridiculously distant cause that you can't even imagine. What's very unlikely, of course, is that event X was caused by a particular ridiculously distant alleged cause Y. —Steve Summit (talk) 16:51, 28 October 2007 (UTC)

The answer is YES. Here is a fun homework problem from graduate statistical mechanics. Imagine that you knew the initial position and velocity of every particle in the universe, except that you misplaced one electron 4 light years away by 1 cm. How long does it take before that simple error, by virtue of the electron's gravity, gives rise to sufficient chaos that you can no longer predict the position of gas particles in front of you? It's an extremely small error, but it is magnified by each of umpteen interactions between the gazillion particles in any given air parcel so that appreciable chaos sets in after only a few minutes (if my memory serves). So yes, that damned butterfly will introduce chaotic effects in sensitive systems throughout the rest of the observable universe. Dragons flight 09:43, 29 October 2007 (UTC)

How does that chaos set in before the electron's misplacement can propagate (at the speed of light) to the origin? You might mean that the area around the electron becomes chaotic, but then what's the "4 light years away" about? Or perhaps you mean that the electron's entire electromagnetic field is appropriate to its actual position (as opposed to it, at $t=0$, suddenly being magically shifted by that centimeter), but I have a hard time seeing how you could have that field interact with everything else and yet have the wrong idea about its source's location. It's an interesting question — with what shape and speed does the "wave" of chaos from a disturbance propagate? — but I'm not sure how to get a quantitative answer from what you've said. --Tardis 16:08, 29 October 2007 (UTC)

Technically, it's an error in the local, t=0 gravitational field corresponding to a 1 cm error in what the position of an electron at 4 light year's distance would have been at time t=-4 years. For simplicity, I wasn't describing the time delay. Dragons flight 18:58, 29 October 2007 (UTC)

Anyway, how come it's always tornadoes? I want a butterfly to flap its wings in India, and a beautiful long-legged nymphomaniac to make a wrong turn and end up in my living room. —Steve Summit (talk) 16:54, 28 October 2007 (UTC)

Well, the actions of a butterfly could cause a long-legged nymph to turn up, but that's another story... Laïka 20:24, 28 October 2007 (UTC)

If you find it comforting that the likelyhood of either nymph or nymphomaniac (that's someone who is really enthusiastic about immature insects - right?) turning up in your living room is influenced by such things - then I strongly recommend butterfly collecting. SteveBaker 22:00, 28 October 2007 (UTC)

I think astronomical systems are still chaotic, but not nearly as much as terrestrial ones. For example, if two meteors pass close by each other, a tiny difference in there original position will make a much larger difference in there final path. Unlike terrestrial systems, however, these are incredibly rare and it will take a huge amount of time for the gravitational effect of a change in the earth's weather patterns to make a noticeable difference. Also, the result of a butterfly effect is the difference between one thing that doesn't seem out of the ordinary and something else that also doesn't seem out of the ordinary. A butterfly flapping its wings will change the weather forever, but the climate will remain the same. — Daniel 23:12, 28 October 2007 (UTC)

The Butterfly effect is an example of Chaos theory. Some physical phenomena can affect "initial conditions" in ways that are unpredictable even in theory. It is possible to construct a sequence wherein the fate of the universe depends on the indertimanite state of a subatomic particle that affects, via a cascade of events, the movement of a butterfly's wing. In practice, this will not happen in this universe. -Arch dude 02:49, 29 October 2007 (UTC)

Tartaric Acid

Do you know the boiling point of Tartaric acid? The melting point is 168°C - 170°C, but I cannot find the boiling point. All of the information that I could find is on this page: http://en.wikipedia.org/wiki/Tartaric_acid Thank-you for your help. 203.113.233.115 07:31, 28 October 2007 (UTC)

This site (bottom of page) lists the boiling point at 275°C: [1]. However, many other sites don't list a BP because most of the acid will decompose before it reaches that temp. StuRat 18:14, 28 October 2007 (UTC)

definition of orthologous , paralogous, analogous,homologous

definition of orthologous , paralogous, analogous,homologous —Preceding unsigned comment added by Sujbhaskar (talk • contribs) 11:35, 28 October 2007 (UTC)

See wiktionary:orthologous, wiktionary:paralogous, wiktionary:analogous and wiktionary:homologous. Algebraist 12:18, 28 October 2007 (UTC)

And Homology (biology) at wikipedia. Algebraist 12:20, 28 October 2007 (UTC)

Mushrooms

Is a mushrooms considered a decomposer or what? —Preceding unsigned comment added by 209.244.30.199 (talk) 17:12, 28 October 2007 (UTC)

The mushroom itself is not a decomposer... the mushroom is only the spore-producing fruit-like part of a fungus. The fungus decomposes organic matter. Sancho 20:34, 28 October 2007 (UTC)

If I understand right, not all fungi are decomposers. A decomposer, according to that page, consumes dead organisms. Certainly a lot of mushroom type fungi grow on dead plants, logs, etc, but aren't there also a huge number of mushroom type fungi that are symbiotic with plants? I'm thinking of Mycorrhiza type relationships. My (admittedly vague) understanding is that most land plants depend upon mycorrhizal fungi for their very survival. When you see mushrooms growing near a tree, are they getting their food from some dead organism underground or are they symbiotic with the tree, each helping the other? My guess is quite often it is the latter. Pfly 02:01, 29 October 2007 (UTC)

From the respective articles: "Decomposers are organisms that consume dead organisms, and, in doing so, carry out the natural process of decomposition." and "Decomposition (or spoilage) refers to the reduction of the body of a formerly living organism into simpler forms of matter." We eat dead things and reduce them to simpler forms of matter so we can use them for our own bodies. So aren't we (and all living things) decomposers? Actually, doesn't dead nature do that too? I'll ask that in a new thread. DirkvdM 09:40, 29 October 2007 (UTC)

fibrous and synovial joints

Are there two types of joints between the radius and ulna? I was lead to believe they are joined by a fibrous joint. Another article tells me they have a synovial joint to allow supination and pronation. Can anyone clarify please? kramnahtal —Preceding unsigned comment added by Kramnahtal (talk • contribs) 19:07, 28 October 2007 (UTC)

Pinhole cameras

How does a pinhole camera project an image? I thought you needed a lens of mirror to focus light. I read pinhole camera but couldn't find the answer. 72.155.207.79 19:47, 28 October 2007 (UTC)

The pinhole itself acts as the lens. The second sentence of the article states this clearly: "An extremely small hole in a very thin material can focus light by confining all rays from a scene through a single point." I assume you mean "glass" instead of "mirror"; there is no mirror needed in any camera. --24.147.86.187 20:25, 28 October 2007 (UTC)

I understand that. I'm asking how air focusses light. And a curved mirror can focus light, and mirrors are most certainly used in cameras. 72.155.207.79 20:37, 28 October 2007 (UTC)

A mirror is not needed in cameras. Yes, you can set up Newtonian-telescope style lenses but that's hardly standard. Maybe you meant "lens or mirror" up above, I now see. --24.147.86.187 21:13, 28 October 2007 (UTC)

There is no real focusing of an image in a pinhole camera. It would work even in a vacuum. This is because, if you pretend for a moment that the pinhole is only large enough for one photon to make it through, each position on the film could only have been illuminated by light coming from a single direction through the pinhole. Thus, if you place an illuminated 2D picture on the outside of the pinhole camera, each position on the film corresponds to only one position on the picture outside. So inside the camera you get a nice, clear image of what's outside. Someguy1221 20:56, 28 October 2007 (UTC)

The air doesn't focus anything. The pinhole acts as a sort of collimator, if you will. Take a pinhole camera with the front off, and aim it at the calendar page for October. Consider a point on the back of the camera where the image will be. Let's locate the point about halfway between the center and a side at three o'clock. Now shrink your eye down to microscopic size and put it on that point facing the front of the camera. You will see the whole calendar page. Now put a front on the camera that has a round hole in the middle about an inch across. You'll only be able to see a few dates off to the other side. Put a front on with a smaller hole, and you'll see only one day. Make the hole very small, and the only light ray that will be hitting your eye will be coming from a tiny spot. The same goes for all the other microscopic eyes stuck to the back of the camera, but each will see a different spot on the calendar page. Voila! You've got an image, upside down and backwards. --Milkbreath 21:09, 28 October 2007 (UTC)

Do this for me (don't just think about doing it - actually DO it!). Take a piece of paper and draw on the left a stick-figure to be the subject of our photograph. On the right of the page, draw a vertical line representing the photographic negative inside the camera.

Now (without the pinhole) consider light rays coming from the subject onto the negative. Light is emitted pretty much equally in all directions from every part of the subject. So you can draw straight lines representing rays of light radiating out from our stick-figure's head and going out in all directions - lots of them hit the photographic negative - and they hit it all over it's surface. Similarly, you can draw rays radiating out from the stick figure's feet reaching any point on the negative. In fact, light from everywhere in the scene can reach every place on the negative - so all you get when you develop the plate is the average of all of the light from the whole scene hitting every point on the negative. A big white blur in fact.

OK - so let's add a pinhole "lens" to our camera. Start again with another diagram) - put the subject on the left and the negative on the right just as before - but this time, draw a vertical line down the middle of the paper with a tiny gap halfway up - this represents the front of the camera with a pinprick-sized hole in it.

Now, lets draw those lightrays coming from the stick-figure's head. They still shoot out in all directions - but most of them hit the line going down the middle of the page and are stopped. Only a few rays make it through the gap in the middle of that centerline (the 'pinhole') and onto the negative. Notice that rays from the top of the stick-figure only hit the bottom of the photographic plate over to the right of your diagram. Now do the same thing with rays from the stick figure's feet - notice that ones that go through the pinhole only hit the top of the photographic plate. Do this with rays of light from all over the stick figure and you find that each point on the original subject emits light in all directions - but the pinhole shuts all of it out except for a teeny tiny amount - so light from each point on the subject ends up at a different place on the negative. When you develop the plate - you get a nice, sharp image because the light isn't all mixed up like it was without the pinhole.

You can deduce some other things with this kind of simple diagram: If the pinhole is too big, lightrays from several close-by points on the subject can end up on the negative in the exact same place. This results in a blurry (but perhaps still recognisable) image. But if you make the pinhole too small, almost all of the light rays from the subject will hit the line down the middle of your piece of paper - hardly any make it through that teeny-tiny gap in the middle. This means that the image on the photographic plate is rather dim (because not much light hits each point)...so you need either a more sensitive film or a much longer exposure (which in turn results in a blurry image if anything in the scene is moving or the camera is shaking).

The point is that the pinhole forces all of the light in the scene to pass through a single point - and blocks all of the light that was heading off in the wrong direction. A more conventional lens focusses light through a single point (the 'focal point') - which has the same effect as the pinhole. The benefit of the lens is that it gathers light over a larger area - so you can get more light onto the film without ending up with a blurry picture. The bigger the lens, the better the quality. This is why cellphone cameras are so crap compared to big SLR cameras. Pinhole cameras do have one HUGE advantage though - they keep the image in focus no matter the distance of the subject from the camera - lenses can't do that. Everything in photography is about these kinds of trade-off.

SteveBaker 21:13, 28 October 2007 (UTC)

Incidentally, this is the same mechanism by which squinting your eyes sharpens an image—you're effectively forcing light through a smaller pinhole. TenOfAllTrades(talk) 21:30, 28 October 2007 (UTC)

There's an episode of Home Improvement in which Wilson has Mark fashion a pair of glasses out of a piece of paper with two pinholes in it. Sancho 22:31, 28 October 2007 (UTC)

Thank you very much for your clear and helpful comments, and sorry for the misunderstanding, 24.147.86.187; I did mean or. 72.155.207.79 22:33, 28 October 2007 (UTC)

Incidentally there was a BBC show called "Genius of Photography" on the other night (Uk), it made a camera obscura out of a room. They went in covered it till it was pitch-black and cut a small hole. Projected onto the wall upside-down (and reversed?) was the view from out of the window. It was very impressive - possibly available on You-tube or that bbc-download thing. ny156uk 23:18, 28 October 2007 (UTC)

The very first response to this question began, "The pinhole itself acts as the lens." That's wrong.

For convenience let's talk about a specific use of a lens or pinhole: the use where you have an object and you're projecting an image of it onto a wall. In order to make the image, you must arrange the rays of light in such a way that each point of the object corresponds to one point of the wall.

By making the light pass through a pinhole, you achieve exactly that correspondence by simple geometry: from each point on the object, there is a line of sight through the pinhole to exactly one point of the wall. That's all there is to it! If the pinhole is point O, then point A of the object corresponds to point A' on the wall were AOA' is a straight line. Point B of the object corresponds to B' on the wall, where BOB' is a straight line.

But the downside is, the whole image is formed from the few rays of light that happen to be pointed at the pinhole. That's okay if the object is very bright, like the Sun, but otherwise you need the wall to be in darkness (a camera obscura), and even so the image is not all that bright.

The purpose of using a lens is to overcome that downside. With a lens, instead of using the few rays of light that come off the object and aim at a little pinhole, we can use all the rays that come off the object and land anywhere on the lens. By choosing the correct lens and placing it correctly, we can arrange to have light paths like APA', AQA', ARA', where P, Q, R are different points on the lens. And similarly BPB', BQB', and BRB'. And so on for all the other points on the object and all the points on the lens. These paths are not all straight lines, but the lens is curved in such a way that the light follows them anyway. That's what focusing means -- this bringing back together of the rays AP, AQ, and AR to a single point A'. And you need a lens (or a curved mirror) to do it. It gives you a much brighter image, the total brightness corresponding to the area of the lens.

But with a pinhole, you don't need to focus in the first place. The pinhole sets up the correspondence between points on the object and points on the wall by geometry alone. It doens't act as a lens because in this setup there is no need to focus.

(In all this I am pretending that the size of the pinhole is negligible. In real life it can't be zero size, and this limits how sharp an image it can form. This is another advantage of a lens.)

--Anonymous, 10:00 UTC, October 29, 2007.

Esophageal Temperature Monitoring During Surgery

It is fairly common practice to monitor a patient's body temperature during a surgical procedure with the use of an esophageal stethoscope with a temperature wire inside of it. Does the temperature wire hook up to a machine that reads the data and then displays a readout? Or how does that work? Also, is it possible to lose the esophageal stethoscope in the esophagus? Thanks, Lilly Upstairs —Preceding unsigned comment added by 24.19.72.81 (talk) 22:03, 28 October 2007 (UTC)

Effective Projected Luminous Lens Area

This term is found in Federal Motor Vehicle Safety Standards for testing of lights for motor vehicles. I am trying to understand this term (EPLLA) in the context of a motorcycle turn light. I want to know how to measure this EPLLA and determine whether or not my turn lights are over the required 3.5 square inches required by the regulation. Can someone explain to me in somewhat laymen's terms what Effective Project Luminous Lens Area is and how to measure it? Thank-you 154.20.86.223 22:50, 28 October 2007 (UTC) Ray Kwan

Ray, EPLLA means the area of the effective light-emitting surface of a lamp, measured by determining the area of the 2-dimensional graphic representation of the lamp's lit lens area on a plane perpendicular to the lamp's reference axis and touching the most exterior point of the lens. "Reference axis" means the H–V axis used for photometric requirements, i.e., the effective centre of the lamp's beam pattern as produced at the lamp.

To simplify this and bring it into the realm of practical application outside of a compliance laboratory setting: The reference axis of most automotive and motorcycle lighting devices is reasonably easy to determine; it is "straight back" from the device with the device oriented in space exactly as it is oriented when installed on the vehicle. if you will place a sheet of fine-grid graph paper such that it forms a vertical plane at 90° to the lamp's reference axis, move the paper such that it just barely touches whatever part(s) of the lens protrude closest to the paper, illuminate the lamp, trace the blob(s) of light on the side of the paper opposite the lamp, and then calculate the area enclosed by your trace, you will have a close enough approximation of the device's EPLLA for most purposes. The thing you will have to be most careful of is accurately tracing the blob(s) of light. If your device uses multiple light sources, such as a cluster or array of LED emitters, you must trace each individual spot of light, omitting the dark areas in between, then total up the areas of each trace to arrive at your EPLLA.

It sounds like you've modified your motorcycle's directional indicator system in some fashion and are trying to determine if the modified or handmade indicators meet the EPLLA requirements. Good for you, most people don't bother, but be aware there are other safety performance requirements for vehicle lighting devices, as well. Intensity through various vertical and horizontal angles, intensity ratio between bright and dim modes of a park/turn or brake/tail lamp, etc.

For additional explanation, you may want to read this technical bulletin as well as this one and this one, keeping in mind that they primarily make reference to the 7¾ in² and 11⅝ in² EPLLA requirements for passenger car and large-vehicle brake lamps, respectively. Also, you may want to read through this NHTSA rulemaking discussion which goes into detail on the meaning, intent, and methods behind EPLLA requirements. If you wish to discuss vehicular lighting with more specificity, feel free to contact me via my talk page. --Scheinwerfermann 03:35, 29 October 2007 (UTC)