Review Papers

Crosstalk in stereoscopic displays: a review

[+] Author Affiliations
Andrew J. Woods

Curtin University, Centre for Marine Science & Technology, GPO Box U1987, Perth 6845 Australia

J. Electron. Imaging. 21(4), 040902 (Dec 05, 2012). doi:10.1117/1.JEI.21.4.040902
History: Received June 4, 2012; Revised October 12, 2012; Accepted October 16, 2012
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Abstract.  Crosstalk, also known as ghosting or leakage, is a primary factor in determining the image quality of stereoscopic three dimensional (3D) displays. In a stereoscopic display, a separate perspective view is presented to each of the observer’s two eyes in order to experience a 3D image with depth sensation. When crosstalk is present in a stereoscopic display, each eye will see a combination of the image intended for that eye, and some of the image intended for the other eye—making the image look doubled or ghosted. High levels of crosstalk can make stereoscopic images hard to fuse and lack fidelity, so it is important to achieve low levels of crosstalk in the development of high-quality stereoscopic displays. Descriptive and mathematical definitions of these terms are formalized and summarized. The mechanisms by which crosstalk occurs in different stereoscopic display technologies are also reviewed, including micropol 3D liquid crystal displays (LCDs), autostereoscopic (lenticular and parallax barrier), polarized projection, anaglyph, and time-sequential 3D on LCDs, plasma display panels and cathode ray tubes. Crosstalk reduction and crosstalk cancellation are also discussed along with methods of measuring and simulating crosstalk.

Stereoscopic three dimensional (3D) displays present a 3D image to an observer by sending a slightly different perspective view to each of an observer’s two eyes. The visual system of most observers is able to process the two perspective images so as to interpret an image containing a perception of depth by invoking binocular stereopsis so they can see it in 3D.

There are a wide range of technologies available to present stereoscopic 3D images to an audience, and the discussion in this paper will be limited to so-called “plano-stereoscopic” displays1—i.e., displays that present both left and right perspective images on the same planar surface and then use a coding/decoding scheme (e.g., glasses) to present the correct image to each eye. Examples of such plano-stereoscopic displays include liquid crystal display (LCD) or plasma display panel (PDP) 3D TVs viewed using active shutter 3D glasses, 3D LCD monitors or 3D cinema systems viewed using passive polarized 3D glasses, or autostereoscopic displays utilizing either a parallax barrier or lenticular lens sheet to allow the 3D image to be viewed without 3D glasses. The aim of all of these displays is to send separate left- and right-eye views to each eye, but due to various inaccuracies, which will be described in detail later in the paper, the image intended only for one eye may be leaked to the other eye. This leakage of one image channel to the other in a stereoscopic display system is known as crosstalk or sometimes ghosting or leakage. Crosstalk is a primary factor affecting the image quality of stereoscopic 3D displays and is the focus of this review paper.

This paper starts by providing a summary of descriptive and mathematical definitions of crosstalk and related terms as they are now in common usage, along with a short summary of the perceptual effects of crosstalk. The bulk of the paper describes the various methods by which crosstalk can occur in various stereoscopic display technologies. This is followed by a description of the methods of measuring crosstalk, a discussion of ways in which crosstalk can be reduced, and last, some coverage of the role of simulation of crosstalk analysis.

In electronic engineering, the term “crosstalk” has been used as far back as the 1880s2 to describe the leakage of signals between parallel laid telephone cables. Crosstalk in stereoscopic displays has been a recognized term at least since the 1930s,3 if not earlier.

The use of the term “crosstalk” in the stereoscopic literature is very common—present in over 15% of all documents in a major stereoscopic literature collection.4,5 The term is also often written as “cross talk,”6 “cross-talk,”7 or “X-talk,”6 but “crosstalk” (without an intermediate space or hyphen) is the most commonly used variant, so that is the form that will be used in this paper.4 Other variants with the same meaning include “interocular crosstalk,”8,9 “crosstalk ratio,”10 and “3D crosstalk.”11

Despite the term’s long history of usage in the stereoscopic technical literature, many papers in the past have simply used the term without providing a descriptive or mathematical definition, nor citing a reference to such. The terms crosstalk and ghosting have been used interchangeably in some of the published literature, whereas modern usage provides separate definitions for these terms—this will be explained in the following sections. Unfortunately there are also some contradictory uses of the terminology in the literature.

The technical field of stereoscopic displays has grown considerably even in just the past five years and in order to foster the continued development of the field, it is important to have a common knowledge of the terminology and definitions of crosstalk and related terms. The following subsections provide a summary of definitions of the important terms in this field and identify ambiguities that still remain and could otherwise cause confusion for those reading the published literature.

Stereoscopic terminology can be used to describe a principle in general terms and can also be used to quantify a physical property—this paper will review both the descriptive and mathematical definitions where applicable.

Descriptive Definitions

A selection of descriptive definitions of crosstalk from the literature (1987 to 2009) were previously examined.4 It was found that despite some variations in wording, there was a common theme—i.e., light from one image channel leaking into another. The following descriptive definition will be used in this paper (based on Lipton12):

Crosstalk: the incomplete isolation of the left and right image channels so that the content from one channel is partly present in another channel.

There is also a mathematical definition of crosstalk, which will be provided in the following section. In the general stereoscopic literature and the lay media, the terms “crosstalk” and “ghosting” have often been used interchangeably,4 but in scientific discussion it is worthwhile to differentiate these terms. Crosstalk and ghosting appear to have been first documented as separate terms in 1987 by Lipton,13 which leads us to the following definition:

Ghosting: the perception of crosstalk.

The term “leakage” is also commonly used in discussions about crosstalk, however, a formal definition was not found in the stereoscopic literature.4 The following definition was developed based on dictionary definitions and current usage in the field:4

Leakage: the (amount of) light that leaks from one stereoscopic image channel to another.

Leakage is also known as “crosstalk luminance” and “unintended luminance.”14

Mathematical Definitions

Crosstalk can be used as a metric to express how much crosstalk occurs in a particular stereoscopic display system. There are several mathematical definitions of crosstalk in common usage as explained below.

Crosstalk definition 1

In its simplest form crosstalk can be mathematically defined15 as: Display Formula

where “leakage” is the luminance of light that leaks from the unintended channel to the intended channel, and “signal” is the luminance of the intended channel, as illustrated in Fig. 1(a).

Graphic Jump LocationF1 :

An illustration of the terms and luminance measurement variables used in this paper with respect to the left and right image channels and left and right eyes. The left and right image channels are shown separated here for illustrative purposes but would be visually overlaid on a stereoscopic display. (a) Illustration of the terms signal and leakage. (b) Illustration of the eight luminance variable L variants. The first subscript is the eye position (Left or Right) that the luminance is measured from. The second subscript is the value (blacK or White) of the desired image channel, and the third subscript is the value (blacK or White) of the undesired image channel. For example, LRWW specifies the luminance measured at the right eye position when the right image (desired) channel is set to white and the left image (undesired) channel is also set to white, which corresponds to the summation of light from the right channel plus a (hopefully) small amount of light from the left channel. (c) Illustration of the transfer function variables used in Huang’s definition of “system crosstalk” (see Sec. 2.2.3).16

In common practice, two luminance measurements are usually taken (from the intended eye position) with: (a) full-black in the intended channel and full-white in the unintended channel (this corresponds with “leakage” above) and (b) full-white in the intended channel and full-black in the unintended channel (this corresponds with “signal” above).

This can also be expressed as: Display Formula

and Display Formula
where CL and CR are crosstalk for the left and right eyes (which can be presented as a number or a percentage), and LLKW, LLWK, LRWK, LRWK are the luminance measured from the Left or Right eye position (first subscript) with White or blacK in the desired image channel (second subscript) and White or blacK in the undesired image channel (third subscript) as illustrated in Fig. 1(b).* The shortcoming of this definition is that it does not consider the effect of a non-zero black level of the display. Some displays are incapable of outputting zero luminance for full-black (e.g., LCDs)—this non-zero black level does not contribute to visible crosstalk (ghosting) and hence would bias the crosstalk calculation using this first definition. If the display black level is set at zero luminance, definition 1 is entirely valid, but definition 1 should only be used with displays which can have zero black level, and are set up that way.

Crosstalk definition 2

The second mathematical definition removes the effect of non-zero black level by subtracting the black level luminance: Display Formula

Crosstalk(%)=leakageblack levelsignalblack level×100.(4)

Several papers support this second formulation (but with different variable names).4,10,14,17,20

This equation can also be expressed as: Display Formula

and Display Formula
where the variables are as defined in Sec. 2.2.1 and LLKK and LRKK are the black level of the display.

Both of these definitions use what is commonly referred to as a black-white crosstalk test because full-black and full-white test signals are used.21 Full-white and full-black signals are used because maximum ghosting usually occurs when the pixels in the desired-eye channel are full-black and the same pixels in the opposite eye-channel are full-white.

The differences between these two mathematical definitions of crosstalk (definitions 1 and 2) create an ambiguity—therefore when quoting crosstalk values it is important to specify which definition is being used, and similarly if reading a report or technical paper, it is important to determine which definition has been used to calculate the results quoted.

System crosstalk and viewer crosstalk

In 2000, Huang et al.,16 defined two new terms in an attempt to disambiguate the terminology relating to crosstalk:

System crosstalk: the degree of the unexpected leaking image from the other eye.

Viewer crosstalk: the crosstalk perceived by the viewer.22

As defined, system crosstalk is independent of the image content (determined only by the display), whereas viewer crosstalk varies depending upon the content. These definitions are similar to the definitions of crosstalk and ghosting provided in Sec. 2.1 (based on Lipton12)—but are not exactly the same. The definition of viewer crosstalk includes the effect of image contrast (and indirectly the effect of parallax) but Lipton’s definition of ghosting includes any perception effect.

These are defined mathematically as:16Display Formula

System crosstalk(left eye)=β2/α1,(7)
Display Formula
Viewer crosstalk(left eye)=Bβ2/Aα1,(8)
where “α1 describes the percentage part of the left-eye image observed at the left eye position,” and “β2 describes the percentage part of the right-eye image leaked to the left-eye position”16 and vice versa for the other eye. A is the luminance of a particular point in the left-eye image, and B is the luminance of the same corresponding point (same x, y location on the screen) in the right-eye image, as illustrated in Fig. 1(c). It is worth noting that Eq. (7) does not include the effect of black level—as is also the case with crosstalk definition 1 in Sec. 2.2.1.

The philosophy upon which system crosstalk is defined is quite different to crosstalk definitions 1 and 2 provided earlier. Variables α1 and β2 are essentially transfer functions which characterize the optical performance of the entire system (from image display, through the glasses or image separation stage, to viewed luminance) and hence is probably the reason that the authors called it system crosstalk. In comparison, definitions 1 and 2 are observer-centric or output-luminance centric—based only on measurements of luminance at the viewer location. In order to calculate the system performance variables α1 and β2, both the source and output luminance need to be measured, but with some displays the source luminance cannot be directly measured (e.g., lenticular or parallax barrier displays). Fortunately, if some assumptions are made, the equation can be converted to an equation based on properties that can be easily measured, and hence can be expressed similarly to Eq. (1).

In 2009, Huang et al.22 provided a revised definition of system crosstalk that includes the effect of black level.§Display Formula

and Display Formula
where SCTL and SCTR are the system crosstalk for the left and right eyes, and LLKW, etc. are defined per Sec. 2.2.1.

As a result of this change of definition, it is important to establish which definition of system crosstalk (200016 or 200922) is being used when it appears in a publication. Equations (9) and (10) are equivalent to crosstalk definition 2 provided above [Eqs. (5) and (6)].

Gray-to-gray crosstalk

In most stereoscopic displays crosstalk is an additive process and roughly linear, so using the black-white test to measure crosstalk and expressing the result as a simple percentage is representative of the display’s overall crosstalk, but this is not true for all stereoscopic displays, particularly 3D LCDs or 3D PDPs using shutter glasses, and hence a more detailed definition is needed. For displays in which the crosstalk process is highly nonlinear, the gray-to-gray crosstalk measurement should be used.

In 2010, three papers21,23,24 all separately defined a new term: “gray-to-gray crosstalk.”

Shestak et al.,21 provided the following definition.§Display Formula

andDisplay Formula
where CLij is crosstalk for the Left eye (first subscript) calculated for the matrix of the desired image channel (second subscript) and the undesired image channel (third subscript) gray level combinations i and j,LLij is the luminance measured from the Left eye position (first subscript) with i gray level in the desired image channel (second subscript) and i gray level in the undesired image channel (third subscript), and so on.

Jung,23 Pan,24 ICDM,14 and Chen25 have also provided definitions for gray-to-gray crosstalk which vary from that of Shestak,21 so again, it is important to know which definition is used when gray-to-gray crosstalk values are published. Apart from variable notation differences, the main difference between definitions of gray-to-gray crosstalk is the choice of variables on the denominator and the use of absolute values. It would be useful to see a comparison between these definitions to know the pros and cons of each and help decide on the most useful definition—like Järvenpää et al., have done for autostereoscopic crosstalk definitions.26

There are some difficulties of these gray-to-gray crosstalk definitions—first, a singularity is present when i=j with some definitions, and secondly, the crosstalk values are not perceptually relevant. Teunissen et al.,27 and Shestak et al.,28 have described an extension of this work to provide a perceptually relevant measure of the visibility of crosstalk (ghosting) in relation to the gray-to-gray crosstalk measurement.

Multi-view autostereoscopic (inter-view) crosstalk

The crosstalk definitions described so far only apply to two-view stereoscopic displays, but the definition can be extended to apply to multiview autostereoscopic displays, where it can also be called inter-view, adjacent-view or inter-zone crosstalk.

Järvenpää et al.18,29 have provided the following definition of crosstalk for multi-view autostereoscopic displays.§Display Formula

Ci(θ)=j=1#of views[Lj(θ)LK(θ)][Li(θ)LK(θ)]Li(θ)LK(θ),(13)
where Ci(θ) is the calculated crosstalk curve for each view i as a function of the horizontal viewing angle θ, Lj(θ) is the measured luminance curve for view j when that view is white and the other views are black, Li(θ) is the measured luminance curve for view i (the view for which the crosstalk is being determined) when that view is white and the other views are black, and LK(θ) is the measured luminance curve when all display pixels (all views) are black.

Crosstalk can also vary with pixel position on the screen and vertical viewing angle of the observer, and the crosstalk equation can be extended to include these variables if needed.18

The above definition applies only to autostereoscopic displays with discrete views—a different formula would be needed for autostereoscopic displays with continuous views.18

Extinction and 3D contrast

Two other related terms are:

  • Extinction and extinction ratio: “The ratio of the luminance of the correct eye [view] to the luminance of the unwanted ‘ghost’ from the image intended for the opposite eye”9—usually expressed as a ratio, for example ‘501.’

  • 3D contrast: Unfortunately multiple definitions exist. Boher17 and ISO18 define 3D contrast as the inverse of (black-white) 3D crosstalk (definition 2 above). ISO18 also defines 3D contrast for multi-view autostereoscopic displays as the inverse of multi-view autostereoscopic crosstalk [Eq. (13) above]. However, ICDM14 defines 3D contrast as the arithmetic mean of the two (left and right) monocular contrasts, where monocular contrast is defined as the luminance ratio of both channels’ white level to both channels’ black level. ICDM14 defines system contrast as LLWK/LLKW (the inverse of crosstalk definition 1 above).

The perception of crosstalk in stereoscopic displays has been studied widely.10,22,3034 It is broadly acknowledged that the presence of high levels of crosstalk in a stereoscopic display is detrimental. Wilcox and Stewart35 reported that crosstalk was the most important attribute in determining image quality for 75% of their observers. The effects of crosstalk in a stereoscopic image include ghosting and loss of contrast, loss of 3D effect and depth resolution, viewer discomfort,36 reduced limits of fusion, reduced image quality and reduced visual comfort,9 and reduced perceived magnitude of depth.37

The perception of crosstalk (ghosting) increases with increasing image contrast and increasing binocular parallax of the image.21,30,33 This principle is illustrated in Fig. 2 which summarizes an experiment performed by Pastoor.30 One example of this principle is that a stereoscopic image with high contrast (lots of bright whites against a deep black background—e.g., a star field image) will exhibit more ghosting on a particular stereoscopic display than will an image with low contrast. Other image content aspects that can also affect perception of crosstalk include focus and motion blur (blur can disguise crosstalk)38 and the extent of objects (crosstalk is more visible on thin objects).39

Graphic Jump LocationF2 :

Visibility thresholds for crosstalk as a function of local image contrast and binocular parallax as conducted by Pastoor.30 The graph shows that “visibility of crosstalk increases (i.e., the threshold value is lowered) with increasing contrast and increasing binocular parallax (depth) of the stereoscopic image.”30 The four line segments on the graph show the threshold of visibility of crosstalk for four different values of stereoscopic image parallax (6, 12, 24, and 40 min of arc) and a selection of different image contrast levels (ranging from 21 to 1001). With the same image contrast (e.g., 201), it can be seen that the threshold of visibility of crosstalk decreases for increasing levels of parallax, meaning that ghosting is more visible with higher levels of stereoscopic image parallax. Keeping parallax constant (e.g., following the 12 minarc line), it can be seen that the threshold of visibility of crosstalk decreases with increasing image contrast, meaning that crosstalk is more visible with higher levels of image contrast. Image: © ITE and SID.30

The stereoscopic literature provides various advice on the amounts of crosstalk that are acceptable and unacceptable. Some examples include:

  • “Difference [change] in crosstalk between [from] 2% and [to] 6% significantly affected image quality and visual comfort” (40 paraphrasing 9)
  • “In order to reproduce a reasonable depth range (up to 40 minarc) on a high-contrast display (1001), crosstalk should be as low as 0.3%”30
  • Crosstalkvisibility threshold of about 1% to 2%” (40 paraphrasing 31)
  • “Crosstalk level of about 5% is sufficient to induce visual discomfort in half of the population”32
  • “Results show that a 1% increment in crosstalk is visible, while 5.8% crosstalk is perceptible, but not annoying”40
  • “For optimal image quality, crosstalk levels should be held below 1%. However, most of the depth percept is maintained at crosstalk levels of up to 4%”37
  • “A significant decrease in perceived depth was observed with as little as 2–4% crosstalk”41

As can be seen above, unfortunately there is considerable variability between the results and guidelines of different papers. This might just be a reflection of the nature of perception-based studies, but results can also be influenced by differences between stereoscopic display technologies, measurement methods, experimental conditions, and display content. There may also be different acceptability thresholds for different usage types—entertainment viewing may be more tolerant of crosstalk than an industrial fine tele-operation task. It is also important to understand that most of the current measures of crosstalk are not perceptually relevant—hence more research is needed in this area.27,28

The reason for determining the threshold of visibility of crosstalk is that it can be very difficult to totally eliminate crosstalk in a particular stereoscopic display technology, whereas if the level of crosstalk can be reduced to a point at which it is not noticeable to the observer, this may allow a more technically and economically viable solution. There is still a great deal to be learnt about the perception of crosstalk and there is considerable scope for more research in this area.27,28

Figure 3 shows the flow of images from the capture of the perspective images with a camera, through to the display of the images on a stereoscopic display, and subsequently viewing and perception by an observer. Crosstalk can occur in the capture, storage/transmission, display and separation stages—this paper focuses most of its attention on how crosstalk occurs in the display and separation stages.

Graphic Jump LocationF3 :

A flow diagram showing the transfer of stereoscopic images from image capture through to image viewing and perception by the observer. Crosstalk between the left and right image channels can occur in the capture (camera) stage, storage/editing/transmission stage, image display (light generation), and image separation (3D glasses or autostereoscopic optical layer) stages. Most crosstalk usually occurs in the display and image separation stages.

One of the fascinating things about crosstalk is that the mechanisms by which it occurs can vary considerably from one stereoscopic display technology to another.

The sections below summarize the important performance attributes for various stereoscopic display technologies and the mechanisms by which crosstalk occurs in each. This list of 3D displays is not intended to be exhaustive—people are incredibly inventive and there are literally hundreds of different stereoscopic display technologies, so it is not possible to discuss all possible stereoscopic display technologies in one short paper. This paper provides the reader with information about the factors which cause crosstalk in a selection of the most common stereoscopic displays and hopefully provide clues as to the crosstalk mechanisms in other displays not specifically discussed.

Time-Sequential 3D Using Active Shutter Glasses

The time-sequential 3D display method is a widely used technique to display stereoscopic images to an observer. It relies on the alternate presentation of left and right images on the display surface combined with a pair of active shutter 3D glasses to gate the appropriate image to each eye. In the past, mechanical shutters42 and lead-lanthanum-zirconate-titanate (PLZT) shutters43,44 have been used in the glasses, but current shutter glasses almost exclusively use a liquid crystal (LC) cell in front of each eye to sequentially occlude the images.45 The optical transmission properties of the liquid crystal shutter are a key determinant in the amount of crosstalk present with the time-sequential 3D displays which use shutter glasses.

The optical transmission performance of an example pair of shutter glasses is shown in Fig. 4. In this figure it can be seen that:

  • the LC shutters have non-zero transmission in the opaque state, which means that some light still leaks through when the shutter is nominally in the blocking condition,
  • the rise-time and fall-time are not instantaneous—sometimes taking several milliseconds to change from one state to another, and
  • the performance at different optical wavelengths is not all the same.

Graphic Jump LocationF4 :

The transmission versus time response of an example pair of active shutter glasses at red, green and blue wavelengths (measured using red, green and blue light emitting diode (LED) continuous light sources).46

In addition to the attributes listed above, the optical performance of the LC cell also varies with viewing angle through the cell. The best performance is usually achieved when the visual angle is perpendicular to the cell and drops off as the viewing angle varies from perpendicular.

There can also be considerable variability in the optical performance of the LC shutter between various makes of shutter glasses. Figure 5 provides an example of the performance of eight different pairs of shutter glasses and highlights the large differences possible. These optical differences can also affect crosstalk performance.

Graphic Jump LocationF5 :

The transmission versus time response of a selection of different LCS glasses at green wavelengths (measured using a green LED continuous light source). There can be a wide variability of performance between different shutter glasses.46

Next it is necessary to consider how the shutters operate in coordination with the sequence of the displayed left and right images. Figure 6 provides an illustration of how a pair of shutter glasses interacts with the image output sequence of a theoretical time-sequential stereoscopic display. Figure 6(a) provides an illustration of the light output of the left-right image sequence, with around 1 millisecond of blanking time between images. Figure 6(b) shows the transmission response of the left-hand LC shutter (the green response from Fig. 4). Figure 6(c) is an illustration of the image intensity that the left-eye will see when viewing the display through the shutter glasses. The intensity of the desired image (signal) is indicated in green and it can be seen that the intensity of the beginning of the left image is reduced because of the long rise-time of the shutter. The intensity of the undesired image (leakage) is indicated in red—in this case this represents the intensity of the right image as seen by the left eye caused by the shutter not fully switching to 0% transmission in the opaque state. The amount of crosstalk illustrated in Fig. 6(c) is approximately 7% (calculated by dividing the red area by the green area—assuming a zero black level display).

Graphic Jump LocationF6 :

An illustration of how a pair of shutter glasses interacts with the left/right image sequence of a theoretical time-sequential stereoscopic display. (a) The sequence of left and right images output by a theoretical display with instantaneous pixel response. (b) The transmission versus time of the left-eye LC shutter. (c) The image intensity as viewed through the left-eye of the LC glasses.

Another aspect to consider in reference to Fig. 6 is that if the shutters switch too early or too late relative to the sequence of displayed images, the incorrect image will be gated to each eye, hence causing crosstalk.

Another item to note in the example of Fig. 6 is that the transition of the left LC shutter from open to closed occurs within the blanking interval between the display of the left and right images. The presence of a blanking interval is useful in helping to hide the transition of the LC shutters. Some displays don’t have a blanking interval, which can compromise crosstalk performance.

Very few stereoscopic displays are able to achieve the theoretical time-sequential display output illustrated in Fig. 6(a)—Digital light projection (DLP) or organic light emitting diode (OLED) displays come close to this performance, but there will typically be three deviations from this ideal performance:

  • Image persistence. In cathode ray tube (CRT) and PDP displays, the phosphors which emit light have an exponential decay in light output from when they are first energized, meaning that the image on the display persists for a nominal period of time.46,47 Displays which exhibit long image persistence will typically exhibit more crosstalk because light from one frame is still being output during the period of the following frame.
  • Pixel response rate. In LCDs it takes a measurable period of time for a pixel to change from one gray level to another and this is referred to as the pixel response rate.48 A display with a slow pixel response rate will typically exhibit more crosstalk than a display with a fast pixel response rate.
  • Image update method. This term describes the way in which the screen is updated from one image to another. In some displays, new images are scanned or addressed from the top to bottom (e.g., CRTs46 and LCDs48), whereas some displays update all pixels on the screen at the same time (e.g., DLPs49 and PDPs47). In simple terms, it will be easier to synchronize a shutter to a display whose pixels all update at the same moment. When shutter glasses are used with a scanned display, the amount of crosstalk present will usually vary with screen position due to the different phase of the switching of the shutter relative to the time the pixels change at different screen coordinates.

These display performance attributes will affect crosstalk performance by varying amounts as will be discussed in more detail in Secs. 4.1.1 through 4.1.4 in relation to specific display technologies.

In summary, the methods by which crosstalk can occur in systems using shutter glasses are:

  • The optical performance of the liquid crystal cells—the amount of transmission in the opaque state, the rise-time, the fall-time, and the amount of transmission in the clear state.
  • The relative timing (synchronization) of the glasses with respect to the displayed images.
  • The angle of view through the liquid crystal cells—the optical performance of the cells usually falls off with viewing angles which are off perpendicular.
  • The temporal performance of the particular display being used and how this interacts with the temporal performance of the shutters.

The display-particular aspects will now be discussed in Secs. 4.1.1 through 4.1.4.

Time-sequential 3D on CRTs

CRT displays were the first display technology to be used with liquid crystal shutter glasses when they were introduced in the 1980s so that is where we will start our discussion. CRTs generate an image by scanning an electron beam over a phosphor-coated surface on the inside the screen. As the electron beam is scanned across the display surface from top to bottom, the phosphors emit light as they are hit by the electron beam and exponentially decay over time, as illustrated in Fig. 7. In this figure it can be seen that the red phosphor has a longer decay (persistence) than the green and blue phosphors. CRT displays are considered to be an impulse-type display because the displayed image is generated by a series of pulses of light.50

Graphic Jump LocationF7 :

Phosphor intensity versus time response for the three phosphors of a typical cathode ray tube (CRT) display.46

The interaction of shutter glasses with the light output of a CRT is illustrated in Fig. 8. As the electron beam energizes the phosphor it outputs a peak of light which then decays exponentially (exaggerated here for illustrative purposes). This figure considers the leakage from the left-image channel into the right-eye view, so the phosphor is shown energized during the left-eye period when the right-eye shutter is closed. When the right-eye shutter opens during the second vertical blanking interval (VBI2), the phosphor is still outputting some light from the previous image period—particularly for pixel positions at the bottom of the screen, which are energized shortly before VBI2. The bottom of Fig. 8 illustrates the amount of light leakage from the left image channel into the right-eye view—the area under the solid red curve from end of the first vertical blacking interval (VBI1) to the start of VBI2 represents leakage due to the incomplete extinction of the shutter, and the area under the solid red curve from start of VBI2 onwards represents leakage due to long phosphor persistence.

Graphic Jump LocationF8 :

Illustration of crosstalk on a cathode ray tube (CRT) (with exaggerated phosphor response for illustrative purposes).46 Top: phosphor response and shutter response. The phosphor is energized during the first frame (L-eye) period, when the shutter is closed, and exponentially decays. Bottom: multiplication of phosphor response by the shutter response to give the amount of leakage. The area under the solid red curve from end of VBI1 (vertical blanking interval) to the start of VBI2 represents crosstalk due to the incomplete extinction of the shutter, and the area under the solid red curve from start of VBI2 onwards represents crosstalk due to long phosphor persistence.

Figure 9 illustrates the spatial variation of crosstalk on a time-sequential CRT display. CRTs will exhibit more crosstalk at the bottom of the screen because phosphors at the bottom of the screen will be energized soon before the shutter is opened for the other eye and therefore more of that phosphor’s decay tail will be visible to the other eye.

Graphic Jump LocationF9 :

Illustration of spatial variation of crosstalk on a cathode ray tube (CRT), with increased crosstalk at the bottom of the screen: (a) actual screen photograph of CRT crosstalk through a pair of active shutter glasses, and (b) histogram of measured CRT crosstalk.46

With time-sequential 3D on a CRT, the important factors which cause crosstalk13,46,51 are therefore:

  • the performance of the liquid crystal cells in the shutter glasses (see Sec. 4.1),
  • the amount of phosphor persistence—the time that it takes for the phosphors to stop glowing after they have been energized (see Fig. 7) (Long phosphor persistence will cause more crosstalk because the light from the first frame is still being output during the period of the following frame),
  • the timing of the shuttering of the glasses with respect to the display of images on the screen—it is important that the switching of the shutters occurs during the vertical blanking interval (VBI) to minimize crosstalk (see Fig. 8), and
  • the x-y coordinates on the screen—the bottom of the screen will exhibit more crosstalk than the top of the screen due to the way that the electron beam scans the display from top to bottom (see Fig. 9).

Time-sequential 3D on PDPs

PDPs with time-sequential 3D display capability were first experimentally demonstrated in 199852,53 and first commercially released in 2008 by Samsung.54 PDPs generate light using phosphors which are energized up to 10 times per frame (see Fig. 10). These 10 pulses (subframes) per frame have different durations (sustain time) and hence luminance, in a binary sequence from longest duration to shortest duration. Different gray levels are achieved for each pixel by firing or not firing the phosphors for each pixel in none, some, or all of the 10 subframes per frame. This is quite different from the way that gray-levels are produced on a CRT which has analog control over the intensity of the pulse of light from the phosphors, whereas with a PDP each individual pulse of light per pixel per subframe can only be on or off—there is no in-between. Therefore, ten individual pulses of pre-determined intensity are fired selectively to collectively produce different gray levels.47

Graphic Jump LocationF10 :

The time-domain light output of an example plasma display (showing alternating frames of 100% white and black). The vertical axis is the normalized phosphor intensity.47 This graph illustrates the 10 pulses per frame used to construct images with various gray levels and the long phosphor persistence of the red and green channels (of this particular display).

With further reference to Fig. 10, it can be seen that the phosphors in PDPs also (like CRTs) exhibit an exponential decay in light output after they have been energized—this is particularly visible in the period between 16 ms and 33 ms with the red and green color channels. Figure 11 illustrates the interaction of shutter glasses with the light output of another conventional PDP display (different than Fig. 10). In Fig. 11(a) it can be seen that the long phosphor persistence from 17 ms onwards causes there to be light output from the previous frame when the right shutter opens which will in turn cause crosstalk. Figure 11(b) illustrates the relative intensity of the signal (left image channel into the left-eye view) and leakage (left image channel into the right-eye view) components. Additionally, the area under the red leakage curve from 0 to 17 ms represents leakage due to the incomplete extinction of the shutter, and the area under the red leakage curve from 17 ms onwards represents leakage due to long phosphor persistence.

Graphic Jump LocationF11 :

Timing diagram showing the relative timing of a pair of shutter glasses being used to view a time-sequential 3D image on an example conventional PDP display (a different display than Fig. 10). Part (a) shows the time-domain transmission of the left and right shutters along with the time-domain light output of the display (showing alternating frames of 100% red and black). Part (b) shows the intensity of light through the shutters as will be viewed by the left and right eyes. The desired signal to the left eye through the shutter glasses is shown in hatched green, and the leakage to the right eye through the shutter glasses is shown in solid red.47 This figure shows severe crosstalk for illustrative purposes and is not intended to be representative of all 3D PDPs.

With time-sequential 3D on a PDP, the important contributors to crosstalk47 are therefore:

  • the performance of the liquid crystal cells in the shutter glasses (see Sec. 4.1),
  • the amount of phosphor persistence—the time that it takes for the phosphors to stop glowing after they have been energized (see Fig. 10),
  • the timing of the shuttering of the glasses relative to the display of images on the screen (see Fig. 11), and
  • the particular gray level value of a displayed pixel and therefore which subframes are fired—a subframe fired immediately before the transition point will dump more light into the following frame due to phosphor persistence than for a subframe which is fired earlier whose phosphor persistence will have had more time to decay before the next frame (see Fig. 11).

Crosstalk does not vary with screen position on PDPs except where the visual angle through the shutter glasses might be non-perpendicular for viewing the corners of the screen.

It should be noted that the examples of Figs. 10 and 11 are derived from older conventional non-3D-Ready PDPs—newer 3D-Ready PDPs will typically exhibit less phosphor persistence and use better shutter glasses than shown in these figures, and also operate at 120 fps with a resultant fewer subframes per frame.

Time-sequential 3D on LCDs

Liquid-crystal displays (LCDs) generate an image by backlighting an LCD panel containing an array of individually addressable cells (usually three cells for each pixel—one for each of red, green and blue color primaries). Each LC cell gates the light from the backlight, either passing light, blocking light or somewhere in between for different gray levels. Traditionally, the backlight in LCDs has been based on a cold-cathode fluorescent lamp (CCFL) but light emitting diode (LED) backlights are now increasingly being used. The light source for an LCD projector may be a metal-halide arc lamp, LED, or laser. Conventional LCDs are known as a hold-type display because they output light for the entire frame period.50

Figure 12 illustrates the light output of a conventional (non-3D-Ready) LCD monitor driven with a video signal alternating between white and black frames—a common time-sequential 3D test signal. The green line indicates the row of pixels of the display that is being addressed (updated) as time progresses—starting at the top of the screen and scanning down to the bottom in the period of one frame. Looking horizontally from a point on the green line, it can be seen that as each pixel is addressed to change (either from black-to-white, or white-to-black) the pixels at that row take a finite period of time to change from one state to another—this is known as the pixel response time, as discussed in Sec. 4.1 in relation to LC shutters. The scanned image update method of a conventional LCD presents some problems for the use of the time-sequential stereoscopic display method, namely there is no time period available when one frame is visible exclusively across the entire display—this can be seen by referring to Fig. 12 and considering a vertical sector of the graph at a particular time. For example, it can be seen that at 8 ms, the top of the screen will be one frame (white), the bottom of the screen will be the previous frame (black) and a horizontal band in the middle of the screen will be a mix of both frames—this is obviously an unsuitable time to open the shutter. The closest moment to having a single frame visible across the entire screen is at 15 ms, however, there is still some darkening of the display at the very top and bottom (indicating some crosstalk), and additionally this is only for a very short instant (a much longer time period is necessary).48

Graphic Jump LocationF12 :

Time domain response of a conventional LCD monitor with a 4% vertical blanking interval between alternating black and white frames at 85 fps. The vertical axis represents the vertical position on the screen with 100% being the top of the screen and 0% being the bottom of the screen. The green line represents the time at which a particular row of pixels is addressed (updated). It can be seen that there is no time period when a white frame is visible across the entire display (by considering a vertical sector of the graph at a particular time).48

Starting in 2009, a new class of 3D LCD monitors was commercially released which successfully supported the time-sequential 3D method.55 This was achieved primarily by modifying (increasing the speed of) the image update method—either by increasing the frame rate, or increasing the vertical blanking interval, or both.48,5659

Figure 13(a) illustrates the light output of an example time-sequential 3D LCD monitor or TV using a modified image update method—driven with a video signal alternating between white and black frames. In this figure, the green line (indicating the row of pixels on the display which is being addressed at one point in time) can be seen to complete the full screen update in a much shorter time period, leaving part of the frame-period for the image to stabilize and show a full image across the entire display. For example, in Fig. 13(b), the highlighted period indicates the period when the shutters of a pair of active shutter glasses could be timed to open to present a stereoscopic image, however the gray tinting at the bottom of this area indicates that some crosstalk will still be present. Technologies such as black frame insertion (BFI) and modulated (or scanned) backlight can also be used with LCDs to improve 3D performance.56

Graphic Jump LocationF13 :

(a) Time domain response of a simulated time-sequential 3D LCD monitor with a fast addressing rate and fast pixel response rate. Note that the entire screen is updated in only 4.2 ms (the time period of the green line) versus 13 ms with a conventional LCD (Fig. 12). (b) The same monitor as (a) being viewed through shutter glasses with reduced duty cycle switching (the response rate of shutters are not shown).48 The highlighted period between 6.7 ms and 8.8 ms is almost exclusively white, which means one of the views will dominate, but there is a bit of gray tinting at the bottom of this area, which suggests some crosstalk will be evident at the bottom of the screen.

With time-sequential 3D on an LCD, the important contributors to crosstalk are therefore:

  • the performance of the liquid crystal cells in the shutter glasses (see Sec. 4.1);
  • the specific timing of the image update method on the screen (see Figs. 12 and 13) including the effects of BFI, increased frame rate, and/or modulated backlight;
  • the pixel response rate of the LCD (black-to-white, white-to-black, and gray-to-gray);
  • the timing of the shuttering of the glasses with respect to the display of images on the screen (see Fig. 13) including the duty cycle of the shutters;
  • the particular gray level value of a displayed pixel (pixel response rate varies with the input and output pixel gray level—small changes in gray level often take the longest to complete);28 and
  • the x-y position on the screen—depending upon shutter timing, the top and bottom of the screen may exhibit more crosstalk than the middle of the screen (see Fig. 13).48

Time-sequential 3D on DLPs

DLP projectors and DLP rear-projection TVs work by shining a light source (e.g., a metal halide arc lamp or LEDs) onto a DMD (digital micro-mirror device—an array of tiny mirrors that can each be individually commanded to tilt ±12° at very fast speeds). The reflection off the DMD is sent through a lens and focused on a screen and each mirror on the DMD corresponds to one pixel on the screen. In single-chip DLP projectors, a color-sequential technique is used to achieve a full-color image49 as illustrated in Fig. 14. DLPs operate most like a hold-type display—except that gray levels are achieved by a duty cycle modulation process and it is also possible to introduce a blanking interval between frames.60

Graphic Jump LocationF14 :

Illustration of the time-domain performance of an example 120 Hz 3D single-chip digital light projection (DLP) projector. In this figure, a stereoscopic image pair is being presented at 120 frames per second (60 frames for the left and 60 for the right in alternating sequence) and viewed using a pair of shutter glasses. The top of the figure shows the sequence of left and right images built up by a red, blue, green color sequence to construct a full-color image. The bottom half of the figure shows the optical transmission of the shuttering eyewear which must synchronize correctly with the sequence of left and right images. This particular projector is operating with a 6× color cycle speed [6 RGB color cycles per 60 fps frame period (16.7 ms)] and in this case one color cycle per left/right frame period is extinguished to create a blanking interval.