Method to test and quantify 3D active shutter glasses
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Hoffmeister, Kurt
Liquid Crystal Shutter (LCS) 3D glasses (also known as 3D shutter glasses, active glasses, or just shutter glasses) are the selection device commonly used to view 3D stereoscopic content on time-sequential 3D displays. There are many available models of LCS glasses to choose from; although, unfortunately, there can be wide performance differences between these glasses. Ultimately the perceived quality of 3D image viewing depends on the display viewed as well as the performance of the shutter glasses used. The objective of this paper is to define key glasses performance parameters and to present a testing method to help quantify glasses and select between models available. Differences in shutter opening speed and open transparency (shuttering performance) are shown between tested models using the method presented here. Additional differences are noted for other performance parameters and features illustrating that there can be many product differences to consider that do not directly affect shuttering performance. 
 Okay, good morning. So I'm Kurt Hofmeister with Mectine and just to give you a little bit of motivation, if you're not familiar with my company, Mectine is a solutions provider. We go back, clear, to the late 80s providing stereoscopic displays as well as visualization software. And these solutions have been pretty much deployed worldwide. If I were to sum it up, we're a display systems integrator working on complex multi-projector displays. Oftentimes, these systems are 3D capable. We're probably best known for the cave system. In a cave, it's not like sitting in an audience in a fixed position. You have the ability to walk around. You're completely surrounded by graphics. In many times, there's even projection on the floor beneath you. So the glasses performance in this environment is probably the most demanding that I've found for the use of active stereo glasses. Just to do a little bit of recap, the liquid crystal shutter glasses that we often refer to as active glasses, they're commonly used to view this type of sequential display. Typically, they work by blocking one eye while you see the image intended for the open eye and then alternating at a pretty high frame rate. Typically rates that we see now on the order of 120 Hertz and higher so that there shouldn't be any perceived flicker. You fuse that together into a stereoscopic image. Another important aspect of these glasses is somehow they're synchronized or triggered by a signal that traditionally was an infrared signal, but certainly could be RF frequency, Bluetooth, or somehow embedded in the light of the projected image. So if we flash back a little bit, 10 years ago, my company, we were regularly using typically these crystallized glasses, sometimes new vision, third player e-dimension. There were other glasses available, but for the most part, these were really kind of the gold standard that were used in most systems. Now as an integrator, it was frequently our job to recommend among components available what we thought was the best option for the customer at the time. And to do that, we needed actual data to do that comparison. If we flash forward to today, the number of active stereo glasses on the market has certainly exploded. Not only are there more third party manufacturers, pretty much every major television manufacturer with a 3D TV is providing glasses. And we just seen this explosion of options that we really don't have the one or two choices that we did in the past. And the performance variation is actually quite wide as well. So just to recap here, not only are there many models available, but we find it important to understand both the objective and subjective performance. And based on that, it's going to affect our perceived image quality, stereoscopic experience. And for a customer who's playing on the user system for many years, there's also durability issues and comfort issues working with these products. So to test some of the objective measures, we use a pretty simple test rig that combines a light source, in this case an infrared emitter, to trigger the glasses. Some sort of a photodiode sensor and ability to mount glasses in front of that. Measurements done within the oscilloscope, in this case a simple signal generator, although that could certainly be a PC or whatever the other display system mechanism is to trigger the glasses being evaluated. So again, we use the photodiode sensor to become our electronic eye. This is a low latency, fast response photodiode circuit that's actually been covered and presented here in other talks, not by myself, but other authors. At the same time, we have this simple rig to hold glasses and adjust the viewing angle relative to that sensor. One thing that's nice about the sensor is it can see in the near IR range. So when we're working with glasses that are triggered by infrared pulse, we can see the pulse train. Andrew last year gave a presentation that oftentimes will see products with some encryption or information being conveyed in that pulse train, so it's more than just triggering. There may be some actual handshaking going on. When we can capture that, we can start looking in detail at a reference sync signal actually causes then this emitter to trigger. And although it's on the range of microseconds or nanoseconds, there is oftentimes some delay between the reference sync and when this trigger occurs. It's important to know that because in some cases, either the display system or the glasses themselves can be programmed to respond or adjust this delay between the reference sync and when the glasses trigger to swap right eye, left eye. So from the scope, if we have glasses in front of the photodiode, we now see where the scope detects more light as the glasses open, a closed cycle glasses open. It becomes very easy then to measure the glasses response. We take a traditional definition of the response being from 10% to 90% and quantify that as our response time for those glasses. Maybe it's a little problematic if the glasses continue to open throughout the refresh cycle, so this is a good example of a pair of glasses that opens quickly and then has its maximum transparency for the rest of that refresh cycle. So this is a really busy slide. I'm going to spend a little bit of time on it. What we're doing here is trying to illustrate the analysis of a pair of glasses that opens pretty quickly, remains transparent throughout the cycle. So it has an opening time on the order of one millisecond. At 120 hertz, your refresh rate is roughly 8.3 milliseconds duration, so that one millisecond is one eighth of your display interval is the time it takes for the glasses to fully open. Contrast that against an alternate pair of glasses that opens much slower. Pair from a quarter to 30% of your refresh cycle is spent with the glasses becoming their peak transparency. Typically, when we see this difference between a fast response and a much slower response, we're really looking at the difference in the lens technology. Pie cell lenses generally are going to be much faster response in terms of opening than the more common twist and pneumatic lenses. Unfortunately, a lot of the commercial televisions and glasses provided with those for 3D viewing tend to be this twisted pneumatic lower cost lens type that, in my opinion, also sacrifices some of the brightness and image quality you might otherwise have. At the same time, it may be not as clear here, but we're also looking at glasses that are open and then close. So in general, liquid crystal shutters close much faster than they open, and that close response isn't as critical as what we measure as a delay from when we have this triggering pulse to when the glasses really start to open, at least reach that 10% threshold. Same thing, we can have this triggering pulse and the glasses actually stay open a little bit longer. Maybe that's more visible here. They may stay open past that triggering pulse before they start to close. These are all sources of crosstalk in terms of when the glasses are inadvertently open and some light from the image for the alternate eye is getting through when you don't want that to happen. So in addition to the response for opening, we're also trying to measure this delay in time. Again, it's important to know that because some products allow an adjustability in that delay. A good example is a projector may actually have a phase delay parameter in the projector setup that can also delay this triggering pulse relative to the display rate of the projector. There's actually quite a few motivations for why we spend a lot of time on this delay. Another parameter you'll typically see in a projector setup is called a dark interval. That's actually a period of time between a right eye and a left eye frame when the projector is allowing no light on the screen so this transition or opening of the lens can occur. What we often see is a pretty objectionable artifact that if that dark time is too short or the glasses are too slow to open, on a DLP projector you essentially get a filtering artifact where you're losing some of your dynamic range information. When we look at an image here that fades from black to white and also fades from white to black across the top, we may see this very distinctive banding that's going to impact your ability to blend projectors and it's going to show in any smooth shaded graphical objects or might show as highlights around something like a cloud that really shouldn't be there because your dynamic range has now been compromised or the grayscale performance. So again, when we see this, if you were to look at this display with no glasses, it would look perfectly uniform. As soon as you put the glasses on, you see this pronounced banding, we would relate that back to your the dark interval being too small or conversely the glasses opening too slowly. In the paper, I go over a number of other subjective and objective measures we make. I guess for the sake of time I'd refer you to the projector, but I'll touch on a few of these kind of all relate to acceptability of the product for different application. So for example, polarization. We find many times customer wants to use a polarization modulator to convert an active stereo display to passive. Yet in general, the active stereo glasses provide higher quality stereo contrast so on. So they'll actually want to use it as a hybrid system where one or two people may still be wearing active shutter glasses and the rest of the viewing audience would be wearing passive glasses. In that case, it's very important to know the polarization behavior of the lenses in the glasses and possibly add core wave plate or add some sort of retarder to better optimize. The glasses selected to work with the passive stereo technology being used simultaneously. Other things we often look at is we'll have a variety of people work for us, try the glasses on, take pictures, get their comments on fit. These glasses that were originally made by stereographics were for many years the gold standard and they were kind of, there were things about them we didn't really appreciate till we had moved on to other styles. So for example, glasses today tend to be a lot narrower in terms of how they sit on the bridge of your nose and consequently when you're standing in a cave system where you're surrounded by graphics, the light on the floor now is readily visible between your cheek and the bottom edge of the glasses and it can be quite distracting. Whereas you see how these glasses come much further down on your cheek. That doesn't really contribute so much to the field of view as it certainly helps block out stray light or unintended image light from around the edges of the glasses. So we find that's an important thing and then conversely we use a lot of different motion tracking technologies and one of the most common today is optical motion tracking that typically use infrared light to trigger. So infrared light would reflect off these markers back to a camera system. Unfortunately the infrared light will often cause the glasses to trigger inadvertently causing image problems or actually not being able to use the glasses at all. So not only are we concerned about glass performance but also how well they play with other technologies in the system. Whether it's infrared tracking or maybe the presence of a plasma display in the same room, there are many sources of inadvertent triggering that need to be taken into consideration. Make sure the glasses are going to perform well with the rest of the system components. Now in the paper, let's see, in the paper I talk a little bit about color effects. When we work with automotive companies and some consumer product companies, they can be pretty demanding on color reproduction. And so in this case instead of a photodiode we're using a spectrometer to measure the response to the light source and then how that's filtered by the glasses. And if we look at the percentage of light loss, actually 35 to 25 percent, this is light transmission, not light loss, is pretty good when you have nonpolarized light source projecting. What's not so good is that we see a clear bias where more light in the green wavelength bands are getting through. It's pretty typical that these glasses might be more biased toward the color green because of the polarizers used. What that means is to get a true color reproduction, the color limetry settings in the projector or the display system need to depress the color green more than you might without a stereoscopic viewing. So in other words, for the display to look right to a person wearing glasses, it's going to look incorrect to someone without glasses. And if you have a display system that's not easy to switch back between color space settings, that can be a compromise customers are unhappy with. Also lastly, in the paper I mentioned, we do a little bit of work with field of view from the glasses as we have people wear them. Use a chin rest. Let's see, I jumped out. F5, that was pretty much the... Okay. So the paper talks a little bit about field of view. We try and take some rough measurements on your stereoscopic field of view as well as your total field of view when you're wearing these shutter glasses. At the same time, we also get comments that people typically perceive a lot more ghosting as they use the peripheral of the lens instead of looking straight out the center of the lens. So for future work, this is actually something we've been trying to quantify when you're in one of these surround screen projection systems, you are regularly using the full periphery of the lens as you're looking around and evaluating different things on the display. So there can be quite a bit of compromise in this crosstalk or the stereo contrast as you look at objects using more the limit of your field of view. So I'd almost rather have a pair of glasses that physically had less field of view but good quality stereo contrast throughout the lens if I were going to be in that type of display system. And that's all I had. Thank you. Questions? No voice. Ah, I'm here. Thank you, Kurt. Questions? Is there a price difference? No. I'm here. Thank you, Kurt. Questions? Yes. Okay. Okay. Questions? There's a big price difference but nowhere near what it was just eight years ago. Well from glasses in the several hundred dollars pair range to glasses down to twenty, thirty dollars a pair range. Pretty substantial variation. And are there more expensive ones consistently? Well there's market demands as well but there is definitely performance differences across that spectrum. Okay. Okay. Right. We'll go on. Just time, Chris. Yeah? Any questions? You got? We can't hear you. Sorry. Shout louder. Okay, what was the fastest speed in a pair of shutter glasses? Okay. For glasses commercially available, one millisecond is pretty good. If you went to a different technology like ferroelectric glasses that we had some experimental pairs made or have been used in research previously, those are definitely down in the millisecond or excuse me, fraction of a millisecond response time. But typically anything you can buy in the market today, one millisecond to fold open is pretty darn good. Okay. Let's say thank you. Okay. Thank you.
 Okay, good morning.  So I'm Kurt Hofmeister with Mectine and just to give you a little bit of motivation, if  you're not familiar with my company, Mectine is a solutions provider.  We go back, clear, to the late 80s providing stereoscopic displays as well as visualization  software.  And these solutions have been pretty much deployed worldwide.  If I were to sum it up, we're a display systems integrator working on complex multi-projector  displays.  Oftentimes, these systems are 3D capable.  We're probably best known for the cave system.  In a cave, it's not like sitting in an audience in a fixed position.  You have the ability to walk around.  You're completely surrounded by graphics.  In many times, there's even projection on the floor beneath you.  So the glasses performance in this environment is probably the most demanding that I've  found for the use of active stereo glasses.  Just to do a little bit of recap, the liquid crystal shutter glasses that we often refer  to as active glasses, they're commonly used to view this type of sequential display.  Typically, they work by blocking one eye while you see the image intended for the open eye  and then alternating at a pretty high frame rate.  Typically rates that we see now on the order of 120 Hertz and higher so that there shouldn't  be any perceived flicker.  You fuse that together into a stereoscopic image.  Another important aspect of these glasses is somehow they're synchronized or triggered  by a signal that traditionally was an infrared signal, but certainly could be RF frequency,  Bluetooth, or somehow embedded in the light of the projected image.  So if we flash back a little bit, 10 years ago, my company, we were regularly using typically  these crystallized glasses, sometimes new vision, third player e-dimension.  There were other glasses available, but for the most part, these were really kind of the  gold standard that were used in most systems.  Now as an integrator, it was frequently our job to recommend among components available  what we thought was the best option for the customer at the time.  And to do that, we needed actual data to do that comparison.  If we flash forward to today, the number of active stereo glasses on the market has certainly  exploded.  Not only are there more third party manufacturers, pretty much every major television manufacturer  with a 3D TV is providing glasses.  And we just seen this explosion of options that we really don't have the one or two choices  that we did in the past.  And the performance variation is actually quite wide as well.  So just to recap here, not only are there many models available, but we find it important  to understand both the objective and subjective performance.  And based on that, it's going to affect our perceived image quality, stereoscopic experience.  And for a customer who's playing on the user system for many years, there's also durability  issues and comfort issues working with these products.  So to test some of the objective measures, we use a pretty simple test rig that combines  a light source, in this case an infrared emitter, to trigger the glasses.  Some sort of a photodiode sensor and ability to mount glasses in front of that.  Measurements done within the oscilloscope, in this case a simple signal generator, although  that could certainly be a PC or whatever the other display system mechanism is to trigger  the glasses being evaluated.  So again, we use the photodiode sensor to become our electronic eye.  This is a low latency, fast response photodiode circuit that's actually been covered and presented  here in other talks, not by myself, but other authors.  At the same time, we have this simple rig to hold glasses and adjust the viewing angle  relative to that sensor.  One thing that's nice about the sensor is it can see in the near IR range.  So when we're working with glasses that are triggered by infrared pulse, we can see the  pulse train.  Andrew last year gave a presentation that oftentimes will see products with some encryption  or information being conveyed in that pulse train, so it's more than just triggering.  There may be some actual handshaking going on.  When we can capture that, we can start looking in detail at a reference sync signal actually  causes then this emitter to trigger.  And although it's on the range of microseconds or nanoseconds, there is oftentimes some delay  between the reference sync and when this trigger occurs.  It's important to know that because in some cases, either the display system or the glasses  themselves can be programmed to respond or adjust this delay between the reference sync  and when the glasses trigger to swap right eye, left eye.  So from the scope, if we have glasses in front of the photodiode, we now see where the scope  detects more light as the glasses open, a closed cycle glasses open.  It becomes very easy then to measure the glasses response.  We take a traditional definition of the response being from 10% to 90% and quantify that as  our response time for those glasses.  Maybe it's a little problematic if the glasses continue to open throughout the refresh cycle,  so this is a good example of a pair of glasses that opens quickly and then has its maximum  transparency for the rest of that refresh cycle.  So this is a really busy slide.  I'm going to spend a little bit of time on it.  What we're doing here is trying to illustrate the analysis of a pair of glasses that opens  pretty quickly, remains transparent throughout the cycle.  So it has an opening time on the order of one millisecond.  At 120 hertz, your refresh rate is roughly 8.3 milliseconds duration, so that one millisecond  is one eighth of your display interval is the time it takes for the glasses to fully open.  Contrast that against an alternate pair of glasses that opens much slower.  Pair from a quarter to 30% of your refresh cycle is spent with the glasses becoming their  peak transparency.  Typically, when we see this difference between a fast response and a much slower response,  we're really looking at the difference in the lens technology.  Pie cell lenses generally are going to be much faster response in terms of opening than  the more common twist and pneumatic lenses.  Unfortunately, a lot of the commercial televisions and glasses provided with those for 3D viewing  tend to be this twisted pneumatic lower cost lens type that, in my opinion, also sacrifices  some of the brightness and image quality you might otherwise have.  At the same time, it may be not as clear here, but we're also looking at glasses that are  open and then close.  So in general, liquid crystal shutters close much faster than they open, and that close  response isn't as critical as what we measure as a delay from when we have this triggering  pulse to when the glasses really start to open, at least reach that 10% threshold.  Same thing, we can have this triggering pulse and the glasses actually stay open a little  bit longer.  Maybe that's more visible here.  They may stay open past that triggering pulse before they start to close.  These are all sources of crosstalk in terms of when the glasses are inadvertently open  and some light from the image for the alternate eye is getting through when you don't want  that to happen.  So in addition to the response for opening, we're also trying to measure this delay in  time.  Again, it's important to know that because some products allow an adjustability in that  delay.  A good example is a projector may actually have a phase delay parameter in the projector  setup that can also delay this triggering pulse relative to the display rate of the  projector.  There's actually quite a few motivations for why we spend a lot of time on this delay.  Another parameter you'll typically see in a projector setup is called a dark interval.  That's actually a period of time between a right eye and a left eye frame when the projector  is allowing no light on the screen so this transition or opening of the lens can occur.  What we often see is a pretty objectionable artifact that if that dark time is too short  or the glasses are too slow to open, on a DLP projector you essentially get a filtering  artifact where you're losing some of your dynamic range information.  When we look at an image here that fades from black to white and also fades from white to  black across the top, we may see this very distinctive banding that's going to impact  your ability to blend projectors and it's going to show in any smooth shaded graphical  objects or might show as highlights around something like a cloud that really shouldn't  be there because your dynamic range has now been compromised or the grayscale performance.  So again, when we see this, if you were to look at this display with no glasses, it would  look perfectly uniform.  As soon as you put the glasses on, you see this pronounced banding, we would relate that  back to your the dark interval being too small or conversely the glasses opening too slowly.  In the paper, I go over a number of other subjective and objective measures we make.  I guess for the sake of time I'd refer you to the projector, but I'll touch on a few  of these kind of all relate to acceptability of the product for different application.  So for example, polarization.  We find many times customer wants to use a polarization modulator to convert an active  stereo display to passive.  Yet in general, the active stereo glasses provide higher quality stereo contrast so  on.  So they'll actually want to use it as a hybrid system where one or two people may still be  wearing active shutter glasses and the rest of the viewing audience would be wearing passive  glasses.  In that case, it's very important to know the polarization behavior of the lenses in  the glasses and possibly add core wave plate or add some sort of retarder to better optimize.  The glasses selected to work with the passive stereo technology being used simultaneously.  Other things we often look at is we'll have a variety of people work for us, try the glasses  on, take pictures, get their comments on fit.  These glasses that were originally made by stereographics were for many years the gold  standard and they were kind of, there were things about them we didn't really appreciate  till we had moved on to other styles.  So for example, glasses today tend to be a lot narrower in terms of how they sit on  the bridge of your nose and consequently when you're standing in a cave system where you're  surrounded by graphics, the light on the floor now is readily visible between your cheek  and the bottom edge of the glasses and it can be quite distracting.  Whereas you see how these glasses come much further down on your cheek.  That doesn't really contribute so much to the field of view as it certainly helps block  out stray light or unintended image light from around the edges of the glasses.  So we find that's an important thing and then conversely we use a lot of different motion  tracking technologies and one of the most common today is optical motion tracking that  typically use infrared light to trigger.  So infrared light would reflect off these markers back to a camera system.  Unfortunately the infrared light will often cause the glasses to trigger inadvertently  causing image problems or actually not being able to use the glasses at all.  So not only are we concerned about glass performance but also how well they play with other technologies  in the system.  Whether it's infrared tracking or maybe the presence of a plasma display in the same room,  there are many sources of inadvertent triggering that need to be taken into consideration.  Make sure the glasses are going to perform well with the rest of the system components.  Now in the paper, let's see, in the paper I talk a little bit about color effects.  When we work with automotive companies and some consumer product companies, they can  be pretty demanding on color reproduction.  And so in this case instead of a photodiode we're using a spectrometer to measure the  response to the light source and then how that's filtered by the glasses.  And if we look at the percentage of light loss, actually 35 to 25 percent, this is light  transmission, not light loss, is pretty good when you have nonpolarized light source projecting.  What's not so good is that we see a clear bias where more light in the green wavelength  bands are getting through.  It's pretty typical that these glasses might be more biased toward the color green because  of the polarizers used.  What that means is to get a true color reproduction, the color limetry settings in the projector  or the display system need to depress the color green more than you might without a  stereoscopic viewing.  So in other words, for the display to look right to a person wearing glasses, it's going  to look incorrect to someone without glasses.  And if you have a display system that's not easy to switch back between color space settings,  that can be a compromise customers are unhappy with.  Also lastly, in the paper I mentioned, we do a little bit of work with field of view  from the glasses as we have people wear them.  Use a chin rest.  Let's see, I jumped out.  F5, that was pretty much the...  Okay.  So the paper talks a little bit about field of view.  We try and take some rough measurements on your stereoscopic field of view as well as  your total field of view when you're wearing these shutter glasses.  At the same time, we also get comments that people typically perceive a lot more ghosting  as they use the peripheral of the lens instead of looking straight out the center of the  lens.  So for future work, this is actually something we've been trying to quantify when you're  in one of these surround screen projection systems, you are regularly using the full  periphery of the lens as you're looking around and evaluating different things on the display.  So there can be quite a bit of compromise in this crosstalk or the stereo contrast as  you look at objects using more the limit of your field of view.  So I'd almost rather have a pair of glasses that physically had less field of view but  good quality stereo contrast throughout the lens if I were going to be in that type of  display system.  And that's all I had.  Thank you.  Questions?  No voice.  Ah, I'm here.  Thank you, Kurt.  Questions?  Is there a price difference?  No.  I'm here.  Thank you, Kurt.  Questions?  Yes.  Okay.  Okay.  Questions?  There's a big price difference but nowhere near what it was just eight years ago.  Well from glasses in the several hundred dollars pair range to glasses down to twenty, thirty  dollars a pair range.  Pretty substantial variation.  And are there more expensive ones consistently?  Well there's market demands as well but there is definitely performance differences across  that spectrum.  Okay.  Okay.  Right.  We'll go on.  Just time, Chris.  Yeah?  Any questions?  You got?  We can't hear you.  Sorry.  Shout louder.  Okay, what was the fastest speed in a pair of shutter glasses?  Okay.  For glasses commercially available, one millisecond is pretty good.  If you went to a different technology like ferroelectric glasses that we had some experimental  pairs made or have been used in research previously, those are definitely down in the millisecond  or excuse me, fraction of a millisecond response time.  But typically anything you can buy in the market today, one millisecond to fold open  is pretty darn good.  Okay.  Let's say thank you.  Okay.  Thank you.
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