2022128 — Here's an introductory guide on how to accurately read an MTF chart and how to take their readings into account for your purchase.

The exit pupil is the image of the aperture stop formed by the light rays after they have passed through the optical system, i.e. it is the image of the aperture stop as seen through all the optics beyond the aperture stop.  It can be a real or virtual image, depending on the location of the aperture stop.

The so-called "blooming" that plagued early local-dimming LED LCDs has been greatly reduced. With the better TVs, there are few artifacts like these images. Instead, the processing errs on the side of safety, not allowing adjacent LEDs to get too different in brightness, so as not to have issues like you see here. The flip side of that is less "punch" in the image, as bright objects on an otherwise dark background don't appear as bright. So in our moon example, the moon itself is as dim as the background, where on a full-array local-dimming LED LCD, or on a plasma or OLED, it would be noticeably brighter.

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Backlit with local dimming This is the ultimate LED LCD, offering performance that rivals the better plasmas. Like the "direct-lit" TVs, these have their LEDs behind the screen (the image above for direct-lit works as a visual aid for this type as well). The full local-dimming aspect means the TV is able to dim zones behind the dark areas of the screen in fairly specific areas to make the image really pop, drastically increasing the apparent contrast ratio.

Along the bottom This design has all the LEDs along the bottom of the TV. Though manufacturers don't like to reveal how many LEDs they use, this is likely the type with the least number of LEDs.

The angular field of view is the angle formed by the edges of the entrance window at the on-axis position of the entrance pupil.   For the telescopic system shown above, we find the location of the entrance window by finding the location of the image of lens 2 formed by lens 1.  (1/4 + 1/xi = 1/3,  xi = 12)  The entrance window is located 12 units to the left of lens 1.  Since the magnification is M = -3, the diameter of the entrance window is three times the diameter of lens 2.  The entrance pupil is lens 1 itself.  The half angle θ for the field of view is given by tanθ = 3*(radius of lens 2)/12.

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There are a few models that are have their LEDs arrayed on the back of the TV, facing you. These are less common, though are making a comeback in the form of cheaper, but thicker, mostly low-end LED LCDs. There are a handful of high-end TVs that use full-array LED backlighting in a slightly different way, which we'll discuss later.

The aperture stop (AS) is defined to be the stop or lens ring, which physically limits the solid angle of rays passing through the system from an on-axis object point. The aperture stop limits the brightness of an image.

With the object positioned at the focal length fmag of the magnifying glass, the tangent of the angle made by the chief ray is tanθi = h(o)/fmag.  (Rays coming from a point in the object focal plane of a lens emerge as parallel rays from the other side of the lens.  A ray entering parallel to the optic axis is deflected through the image focus.)

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Bottom line Edge-lit LED LCDs look cool and are energy efficient, but have wildly different picture quality potential. Since contrast ratio is the most important factor in picture quality, the better the local dimming, the better the apparent contrast ratio of the TV. Though backlit local dimming is theoretically the best, some edge-lit models (like last year's HX850) do such a good job with their less-than-ideal LED placement that they can look fantastic.

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The following is simply a recipe for finding the aperture stop, entrance pupil, and exit pupil, given an optical system.

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Because the light is brightest nearest the LEDs, it's common for edge-lit LED LCDs to have poor uniformity. This is especially noticeable on dark scenes, where areas of the screen will appear brighter than others. Corners or edges can have what looks like tiny flashlights shining on the screen. Check out Is LED LCD Uniformity a Problem? for more.

Myopia can be accommodated for through the use of a negative lens that will cause the light rays to diverge.  The power of the lens is chosen by matching the lens’ focal point with the eye’s far point.

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Though TVs of this style claim to have "local dimming" you can see how this is a pretty broad definition of "local." Even if each LED is dimmable independently (highly unlikely), you're still only able to dim columns that stretch from top to bottom. Something like this:

So what does this mean? Well, there's a limit to how aggressive manufactures can program the local dimming. Imagine a night landscape with a bright full moon. Ideally, the moon is bright, and the rest of the scene is dark. With plasmas, OLEDs, and full-array local-dimming LED LCDs (more on these later), the moon's brightness is independent of the rest of the image. With bottom-lit LED LCDs, however, in order to make the moon bright, the LEDs that are lighting that entire vertical section of screen have to be bright. So in most cases, everything below the moon (in our example) would be brighter than the surrounding image. Here's a Photoshopped (and exaggerated) demonstration of what this can look like.

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All sides This is a less common method now, as it requires more LEDs than any of the other edge-lighting methods. The local dimming can get a little more accurate, but is still limited to large-ish zones. If we used our moon example image, the result with an all-sides edge-lit would look just like top and bottom. But with regular video (that has more light sources than just the moon), it will have a more zones to work with, sort of like this:

Nearly all "backlit" LED LCDs use this method. The LEDs are arrayed on the back of the TV, facing you, but there is no processing to dim them individually. They work instead as a uniform backlight, like most CCFL LCDs. The least expensive LED LCDs use this method, as do most of Sharp's ultra-massive LED LCDs. Uniformity is generally better than edge-lit displays, but because there's no local dimming at all, the native contrast ratio is limited to the LCD panel itself (which is typically much lower than the native contrast of a plasma TV).

Most LED LCDs on the market today are edge-lit, which means the LEDs are in the sides of the TV, facing in toward the screen. In the image at the top, the LED strips are above and to the side of this exploded-view of an LCD panel. There's a close-up view here (full article with more images here).

The entrance pupil is the opening an observer would identify as the limitation on the solid angle of rays diverging from an on-axis object point, i.e. it is the image of the aperture stop in as seen through all the optics before the aperture stop.  Again, it can be a real or virtual image, depending on the location of the aperture stop.

In contrast to myopia, hyperopia occurs when the eye is too short for the power of its optical components.  In hyperopia, the cornea is not steep enough and light rays hit the retina before they come into focus.  In the case of hyperopia, light from distant objects is focused to a point behind the retina.

Got a question for Geoff? First, check out all the other articles he's written on topics like HDMI cables, LED LCD vs. plasma, Active vs Passive 3D, and more. Still have a question? Send him an e-mail! He won't tell you what TV to buy, but he might use your letter in a future article. You can also send him a message on Twitter: @TechWriterGeoff.

A telescope has the parameters listed in the table on the right: If the two lenses are separated by a distance of 105 cm, show that the objective serves as the entrance pupil and its image is the exit pupil.  Find the position and size of the exit pupil.

A magnifying glass enables you to enlarge the image on your retina even further.  If you position the object less than or equal to one focal length fmag away from the magnifying glass, and if the glass is positioned close to your eye, a magnified virtual image is formed (at least 25 cm from your eye).  The eye forms a magnified real image of this enlarged virtual image on the retina.

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For two thin lenses in contact, 1/f = 1/f1 + 1/f2, and therefore power is P = Pthin(1) + Pthin(2), i.e. the powers of thin lenses in contact add algebraically.

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Solution: (a)  For the near point we have 1/s = Pa(max) + Pc = 5D,  s = (1/5) m = 20 cm.For the far point we set Pa = 0,  1/s = 3D,  s = (1/3) m = 33 cm.(b)  With correction we have for the near point 1/s = Pa(max) = 2D,  s = 50 cm.For the far point we have s = infinity. For the near point to be at 25 cm We need Pa(max) + Pc = 4D, the myopia must be corrected with a -1D lens. The person needs bifocals

All Sides used to be the most common edge-lighting method. But as the light guides improved, and costs had to come down (to make cheaper LED LCDs), this method became fairly rare.

For an off-axis object, the chief ray (CR) is the ray that passes through the center of the aperture stop.  Rays that pass through the edge of the aperture stop are marginal rays (MR).

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However, they basically don't exist. The Elite by Sharp looks amazing, but came out in 2011 and hasn't been updated (with no rumors of a replacement on the horizon). The LG LM9600 wasn't great last year, and LG has yet to announce any full-array local-dimming TVs for 2013. The only other local-dimming LED LCD was the Sony HX950, which was excellent, and is still current. In his review David Katzmaier called it "the best, and maybe the last, local-dimming LED TV to bear the Sony name."

Assume that an eye can form an image of an object with object distance s on the retina (without glasses).  Show that the lens equation for this eye may be written as

Every ray entering the system through the objective passes through the eyepiece.  The image of the aperture stop seen through all the optical elements to the left of the aperture stop is the aperture stop itself, since there are no optical elements to the left of it.  The objective serves as the entrance pupil.  Its image when viewed through all the optical elements to the right of it is the exit pupil.  The exit pupil therefore is the image of the objective formed by the eyepiece.  Its location is found from 1/s' + 1/s = 1/f, 1/s' = 1/5 - 1/105 = 20/105.  s' = 5.25.  The position of the exit pupil is 5.25 cm to the right of the eyepiece.  Its diameter is d' = M (10 cm) = s'/s (10 cm) = 5 mm.

Top and bottomAs you can guess, this design has LEDs on the top and bottom edges of the screen. The local dimming here is a little better, where the zones can be slightly smaller areas of the screen, like this:

The biggest difference between all the LED back/edge-lighting methods is how effective their "local dimming" is, which as you'll see, has become a pretty broad term.

Each manufacturer has a preferred method for edge-lighting, but some models may feature one type, while other models feature another type. Generally speaking, the fewer LEDs the cheaper the TV is to produce. Fewer LEDs also mean better energy efficiency, but LED LCDs are already so efficient that this is a tiny improvement. Unfortunately, specific details about where a TV's LEDs are located (beyond "direct" or "edge"), the number of LEDs, and other useful information about the backlighting, are rarely listed on a TV's spec sheet.

The aperture stop determines the solid angle of the transmitted light cone for an on-axis object.  It limits the brightness of an image.  The field stop determines the solid angle formed by chief rays from off-axis objects.  It limits the field of view of an optical instrument.  The image of the field stop as seen through all the optics before the field stop is called the entrance window.  The image as seen through all the optics after the field stop is called the exit window.

In a system with vignetting, the angular field of view may also be defined as the largest angle of an input chief ray with the optical axis.

Placement of an object at the focal point of a magnifier or eyepiece is the ideal way to use these instruments, since the rays from the virtual image entering the eye are then parallel, and thus no accommodation is required to view the image on the retina.

where D = diopters, f = lens focal length (in meters), and a "+" sign indicates a converging lens and a "-" sign indicates a diverging lens.

Edge-lit LEDs have a light guide that helps reflect the light from the edges of the TV somewhat uniformly across the screen. They do this with varying success. To give you an idea of what I'm talking about, here's an incredibly well-drawn diagram from that article that shows how these light guides work.

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For system (a) in the figure on the right the aperture is the aperture stop, for system (b) the first lens is the aperture stop and for system (c) the second lens is the aperture stop.

A label such as "2X" or "10X" printed on a magnifying glass or an eyepiece belonging to a microscope or telescope indicates the angular magnification mα when the object is positioned at the focal length fmag of the magnifying glass, such that the virtual image is infinitely far away.  This is called the magnifying power MP.

C-ya CCFL, hellllo LED Traditional LCDs use CCFLs, or cold-cathode florescent lamps, as their backlight. While cheap, they're not as energy efficient as LEDs. More importantly, all contain mercury, and aren't able to do some of the fancy area-lighting of which some LED backlit models are capable. Because of these issues and the falling prices of LEDs, CCFL backlit LCD TVs will disappear entirely very soon. In 2013 LG and Sony proudly announced they'd stopped using CCFL backlights altogether, even in their cheapest TVs, in favor of LEDs. Vizio, Samsung, Sharp, Toshiba, and Panasonic weren't as vocal about it, but none has announced any non-LED-backlit TVs for 2013.

(b)  With correction we have for the near point 1/s = Pa(max) = 2D,  s = 50 cm.For the far point we have s = infinity. For the near point to be at 25 cm We need Pa(max) + Pc = 4D, the myopia must be corrected with a -1D lens. The person needs bifocals

The two biggest-selling TV makers in the U.S. are Samsung and Vizio, and neither has sold a full-array local-dimming LED TV for the last couple years. At CES 2013, Samsung's only such TV announced was the insanely-expensive Ultra HD showpiece UN85S9, while Vizio once again went with edge-lighting for its flagship. Vizio is claiming some of it's 2013 TVs have local dimming, but David Katzmaier recently tested one such model, the E420i-A1, saying "Sure, black levels get darker, but the trade-off in shadow detail is one I'm not willing to make," and concluded that its "local dimming does nothing to improve picture quality."

A certain eye has 3D of myopia (nearsightedness) and 2D of accommodation. (a)  Find the near point (the shortest object distance for which the eye can form an image) and the far point (the largest object distance for which the eye can form an image).  (b)  What are the near and far points if the myopia is corrected with a -3D lens.

When making and selling eyeglasses, people prefer to speak of the lens power P, measured in diopters D, instead of the focal length f.  If you want to buy eyeglasses, you need to know the power of the lenses.  Focal length and power of a lens are related to each other.

Solution: For any refracting system we have n1/S1 + n2/S2 = P,  f = n1/P,  f'= n2/P. For the eye S2 = s' = constant.  If the eye forms an image when S1 = s, then 1/s + n2/s'  = P = Pr + Pa + Pc, where Pr is the power of a normal, relaxed eye. But for the normal, relaxed eye we have n2/s'  = Pr, therefore 1/s = Pa + Pc.

The following is simply a recipe for finding the field stop, entrance window, and exit window, given a an optical system.

All so-called LED TVs are really just LCD TVs that use LEDs for their backlight. This backlight creates the light that allows the LCD to create an image. However, there are multiple ways these LED backlights can be arranged, and that arrangement can have a dramatic effect on picture quality.

where Pa is the power of accommodation and Pc is the refractive error, i.e. the amount by which the eye is too strong or too weak.  Pc is positive for an eye with myopia or near-sightedness and negative for an eye with hyperopia or far-sightedness.

We define the angular magnification mα as the angle subtended by the virtual image (αi) divided by the angle subtended by the object when viewed with the unaided eye (αu).

If the object is a distance of 25 cm from the eye, then the tangent of the angle made by the chief ray from the top of the object is tanθu= h(o)/25.

As you can see, there's still some areas lit that shouldn't be, but it's a lot better. The idea with local dimming is you want as small and precise an area as possible (ideally, per-pixel, but that's not feasible with current technology). Remember, it's hardly ever going to look this severe, I'm merely illustrating the point so you can see it.

To make the image of a small object on the retina as large as possible, you must move the object as close to your eye as accommodation allows.

As I mentioned at the top, there's no easy way to tell, just by looking at a spec sheet, what kind of backlight a TV has. By extension, there's no way to tell how good its local dimming will be. Bad local dimming can, at worst, just be marketing hyperbole. At best, it does little to improve the picture. Good local dimming, however, can make a punchy image, with lots of apparent depth and realism. Or to put it differently, the best LCDs on the market have the best local dimming, allowing them to rival plasmas on the picture quality front. The better TV reviews, like ahem those here on CNET, will talk about all this, so you're not duped into paying for a "feature" that's little more than a check mark on a spec sheet.

A cone of light rays from an off-axis object to the entrance pupil will not necessarily be transmitted in its entirety.  It can be partially cut off by field stops or lens rims in the system.  This is called vignetting.

In a nearsighted eye, the cornea is too steeply curved for the length of the eye, causing light rays from distant objects to focus in front of the retina.  Distant objects appear blurred or fuzzy because the light rays are not in focus by the time they reach the retina.  The eye is able to form images on the retina for objects that are closer than the eye’s far point.

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The near point of a human eye, defined to be s = 25 cm, is the shortest object distance that a typical eye is able to accommodate, or to image onto the retina.

Here αi and αu are the angles made by the chief rays from the edge of the object with the optic axis in the case of the aided and unaided eye, respectively.  The chief ray is the ray that passes through the center of the pupil.

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