Finally getting around to posting a follow-up to a follow-up to John The Math Guy’s recent series on color gamut size, colorblindness and tablet displays. I thought I might be able to at least shed a little more light on his question about the differences in color accuracy between some of these devices.
In his testing, John found no statistically significant difference in scores among different people taking the EnChroma colorblindness test on different devices. I found this somewhat surprising since, in my experience, even tablets with similar color gamuts tend to show colors with very different levels of accuracy.
As you can see, the iPad mini and Nexus 7 each produce very different colors, even for those colors that are actually inside their gamuts.
For example, even though the iPad mini has enough gamut coverage to accurately display the Gretag chart’s deepest blue, it cannot do so without distorting the image in another way. This is because of data in the underlying image standard- most content today is encoded in the sRGB standard. If the iPad were to show that Gretag blue correctly, it would not have enough color saturation headroom left over to show you a different color if a deeper blue, say right at the bottom of the sRGB triangle, were called for.
A good real world example of this can be found in the picture below of my bloodhound, Louisa, racing down the beach at Carmel, CA. The middle of the sky in this image is right on the edge of the iPad’s color gamut, very similar to the Gretag blue in the charts above, while the deepest blues found in the ocean fall outside the iPad’s gamut.
If the iPad were striving for accuracy at all costs, it might map both colors right on top of each other at the edge of the gamut. There’d be no visible difference between the two in this case and the quality of the image would suffer but at least the sky would be accurate. In order to avoid this scenario, the designers of these devices have decided to compromise on accuracy so they can show a full range of color differences to the user.
They do this by remapping colors inward, away from the edges of the gamut, effectively compressing the gamut even further so that otherwise out-of-gamut colors can be seen. This is a good solution given the gamut limitations of the device since it results in more pleasing, if less accurate images.
As newer devices trend towards wider color gamuts this kind of compromise should become a thing of the past. In fact, tablet designers may be working on the reverse issue- how to avoid oversaturating images that were encoded for smaller gamuts.
Great, how does this relate to colorblindness again?
iPad mini vs Nexus 7 color accuracy comparison in CIE 1976
Taking another look at the Gretag results from the two devices plotted on top of each other, there clearly are major differences. But, in the reds and greens, two colors associated with a common form of color blindness, the devices are relatively close. So, the simple answer may just be that colorblindness tests do not require pinpoint accuracy to be effective, at least as basic screening tools.
Adrian Covert of Gizmodo has an interesting piece looking at the gadget industry’s recent obsession with high PPI displays. With devices like the HTC DNA pushing resolution well past 300 PPI, electronics makers may be turning PPI into the next overhyped marketing stat, just like contrast ratio is for the TV industry and megapixel is for the digital camera.
Adrian gets to the heart of the problem:
There are plenty of ways to make a better-looking display. But we’ve reached the point in the pixel density wars where higher figures have stopped automatically equating to improved performance for users. Any grandstanding about pixel density, from here on out, now is mostly just marketing fluff.
We tend to agree, and color performance is probably the display feature with the most room to improve. The best LCD smartphones on the shelves right now can show you more pixels than your eye can detect, but can only show you about a third of the colors you can see. If electronics makers want impactful feature improvements for new devices, color performance is where it’s at.
Size is a critical dimension for consumers to consider when buying a product with a display. Will this TV fit on my wall? Would this tablet fit in my jacket pocket? How much picture am I getting? To guage displays today, we take a diagonal measurement of a 16:9 rectangle. This leaves value on the table. Not just because consumers are notoriously bad at math, it fails to capture the full value of the increase. As display industry analyst Bob Raikes said:
A display that has twice the diagonal (and the same aspect ratio) has four times the screen area. Would Intel describe the clock speed of its CPUs by giving them a number that is the square root of the clock speed? If Intel went from 1GHz to 2GHz, would the company really give customers a number that is just 40% bigger? Ah, we’ve gone from 1 IntelMark to 1.4 IntelMarks. No chance!
Why would we say “twice” when the real value increase is “four times”? This is especially relevant as consumers shop more online. Although size may be apparent in a brick and mortar showroom, it is not easily conveyed online. Take a look at this image- which tablet is bigger? By how much?
Apple’s Phil Schiller demonstrated this yesterday at the iPad mini announcement. The new iPad mini is only 0.9 inches or 12% bigger than a Nexus 7 on the diagonal, he says, but it is actually 35% larger by area. This is another example of display marketing efforts starting to move beyond PPI comparisons. Product and display marketers: let’s get real about the value we’re adding – whether it’s surface area or color. Let’s stop leaving value on the table.
This is a great, exhaustive tutorial on managing color gamut for photographers by color expert Andrew Rodney. He does a great job making the case for working in wide gamut color spaces like Pro Photo, especially when capturing in RAW. Using smaller gamuts like sRGB throws away useful color data that printers and more and more displays can recreate.
We typically focus on color as it relates to displays here at dot-color, but I came across a fascinating post about color in the print industry from John the Math Guy that I had to share. In this post, John takes a close look at how ink looks at different thicknesses and uncovers the reasons for some seemingly unconventional color-naming habits in the print industry.
What happens when we double the amount of ink on the paper? …it would seem that the thick layer of magenta is a lot closer to red. The plot below shows the actual spectra of two magenta patches, one at a larger ink film thickness than the other. The plot leads one to the same impression – that a thick layer of magenta is closer to red in hue than a thin layer.
Chart shows different spectrums of thick (red line) and thin (blue line) layers of magenta ink.
Ken Werner of Display Central has a post comparing the benefits of quantum dots to OLEDs in consumer TV applications. Being the authority on quantum dot displays that we are here at Nanosys, Ken contacted us for an analysis. Here is the explanation our Ph.Ds gave Ken:
OLEDs use organometalic compounds to emit light. They typically have a central metal atom surrounded by organic ligands. The decay issues are the same as with typical organic fluorophores. In the excited state these molecules are very reactive to H2O and O2, as well as other small molecules that may be around. Once they react they become a different molecule and they will no longer fluoresce or phosphoresce and give off light. The more blue the light emission, the higher the energy of the excited state, and the more reactive the excited molecule will be. So your blue organic phosphores will have a much shorter lifetime than will red phosphores. The burn-in problem seen in OLED displays, that can be seen after just several weeks of operation with static content, is a manifestation of early blue degradation compared to green and red.
Conventional phosphores like YAG are doped materials. YAG used in white LEDs is actually cerium doped YAG. The cerium atom emits the yellow light and is surrounded by a vast amount of YAG. Quantum dots are similar in that a central core crystalline semiconductor material is used to confine the holes and electrons of the exciton (analogous to the cerium in YAG), and in our material this is surrounded by a thick shell of a different, lattice-matched semiconductor material (analogous to the YAG.) We call this a core-shell Quantum Dot structure. If the lifetime of our materials is less than that of conventional phosphors, it is typically because we have not made a perfectly lattice-matched shell, which may distort the core and cause defects at the core/shell interface that reduces the quantum yield.
The big difference here is that a perfectly made core-shell quantum dot does not have an intrinsic lifetime failure mechanism, whereas the organometallic compounds are intrinsically reactive to their environment, which makes them prone to shorter lifetimes especially at higher energies such as blue.
Over the past couple weeks we’ve seen device manufacturers start to gear up for the holiday season, highlighted by big product announcements from Nokia, Motorola and Amazon. It’s been especially interesting for me to follow how these companies market the most important part of the device – the screen. While pixel per inch still seems important, device makers have moved into more nuanced territory, highlighting deeper features like reduced reflectivity, improved touch sensitivity and color saturation.
Here’s a roundup the most interesting new display features in this holiday’s hottest devices:
Nokia was first up this week with a new crop of Lumia handsets, the 920 and 820. They introduced a slightly larger display for the flagship 920 (now 4.5 inches compared to last year’s 4.3” Lumia 900), touted a new level of touch sensitivity that even works with gloves and claimed 25% more brightness than rival phones. Also of note, they switched from AMOLED to IPS LCD. It’s not yet clear if cost/supply issues or performance drove this switch. It may be that they preferred the brightness and power efficiency of LCD.
Right on the heels of Nokia, Motorola and Google announced a group of new smartphones, led by the Droid Razr Maxx HD. The company described the new Super AMOLED display as having “85% more color saturation than the iPhone 4S, so everything is in lifelike detail.” It’s great to hear them talking about the value of color performance. Hopefully they’ve included some color rendering optimization to artfully take advantage of that extra saturation without overdoing it.
Amazon followed up yesterday with several new devices across their entire Kindle line-up and a surprisingly technical presentation that took a deep dive into the LCD film stack. They showed how a reduced air gap between the touch screen and LCD surface can reduce screen glare, suggesting the new Fire HD has reduced glare by 25%. Also, in a move that’s sure to please LCD film manufacturers like 3M, they discussed the value of better polarizing filters for achieving wider viewing angles without color distortion.
Of course, everyone still compared their products to the now year old iPhone 4S, so it will be interesting to see how these features stack up to whatever Apple introduces next week. We’ll be sure to pick up a few of these devices and run them through their paces to see how the marketing-speak stacks up to real world performance.
For many who are new to the world of display measurement, the prevalence of two distinct, but often-interchanged color spaces can be a source of confusion. Since my recent post about the color performance of Apple’s new iPad, a number of people have asked about this topic, so I thought it would be worth a closer look.
In the world of displays and color images, there exists a variety of separate standards for mapping color, CIE 1931 and CIE 1976 being the most popular among them. Despite its age, CIE 1931, named for the year of its adoption, remains a well-worn and familiar shorthand throughout the display industry. As a marketer of high color gamut display components, I can tell you from firsthand experience that CIE 1931 is the primary language of our customers. When a customer tells me that their current display “can do 72% of NTSC,” they implicitly mean 72% of NTSC 1953 color gamut as mapped against CIE 1931.
“…we strongly encourage people to abandon the use of the 1931 CIE color diagram for determining the color gamut… The 1976 CIE (u’,v’) color diagram should be used instead. Unfortunately, many continue to use the (x,y) chromaticity values and the 1931 diagram for gamut areas.”
So why are there two standards, and why are we trying to declare one of them obsolete? Let me explain.
What is a color space?
First, a little background on color spaces and how they work.
While there are a number of different types of color spaces, we are specifically interested in chromaticity diagrams, which only measure color quality, independent of other factors like luminance. A color space is a uniform representation of visible light. It maps the all of the colors visible to the human eye onto an x-y grid and assigns them measureable values. This allows us to make uniform measurements and comparisons between colors, and offers certainty that images look the same from display to display when used to create color gamut standards.
In 1931, the Commission internationale de l’éclairage or CIE (International Commission on Illumination in English) defined the most commonly used color space. Here’s a look at the anatomy of the CIE 1931 color space:
What makes a good color space?
An effective color space should map with reasonable accuracy and consistancy to the human perception of color. Content creators want to be sure that the color they see on their display is the same color you see on your display.
This is where the CIE 1931 standard falls apart. Based on the work of David MacAdam in the 1940’s, we learn that the variance in percieved color, when mapped in the CIE 1931 color space, is not linear from color to color. In other words, if you show a group of people the same green, then map what they see against the CIE 1931 color space, they will report seeing a wide decprepancy of different hues of green. However, if you show the same group a blue image, there will be much more agreement on what color blue they are seeing. This uneveness creates problems when trying to make uniform measurements with CIE 1931.
The result of MacAdam’s work is visualized by the MacAdam Elipses. Each elipse represents the range of colors respondents reported seeing when shown a single color, which was the dot in the center of each elipse:
A better standard
It was not until 1976 that the CIE was able to settle on a significantly more linear color space. If we reproduce MacAdam’s work using the new standard, variations in percieve color are minimalized and the MacAdam’s Elipses mapped on a 1976 CIE diagram appear much more evenly sized and circular, as opposed to oblong. This makes color comparisons using CIE 1976 significantly more meaningful.
The difference of the CIE 1976 color space, particularly in blue and green, is immediately apparent. As an example, lets look at the color gamut measurements of the iPad 2 and new iPad we used in an earlier article. Both charts do a reasonably good job of conveying the new iPad’s increased gamut coverage at all three primaries. But, the 1976 chart captures the dramatic perceptual difference in blue (from aqua to deep blue) that you actually see when looking at the displays side by side:
The increased gamut of the new iPad is worth testing. Next time you find yourself in an Apple store, grab an iPad 2, hold it alongside a new iPad, Google up a color bar image and see the difference for yourself.
So, why do we still use CIE 1931 at all? The only real answer is that old habits die hard. The industry has relied on CIE 1931 since its inception, and change is coming slowly.
Fortunately, CIE 1931’s grip is loosening over time. The ICDM’s new measurement standard should eventually force all remaining stragglers to switch over to the more accurate 1976 standard. Until then, you can familiarize yourself with a decent color space conversion calculator, such as the handy converter we built just for this purpose:
This week I am featuring a guest blogger here– Jason Hartlove, CEO of Nanosys, the Palo Alto, CA-based maker of high color gamut-enabling display products. The following is a fairly in-depth, technical look at how and why Nanosys’ quantum dot phosphor technology improves LED LCDs just by changing the color content of the backlight. Note: a shorter version of this article originally appeared in the October 2011 print edition of LEDs Magazine.
LCD technology has made great progress in the past few years with innovations such as high resolution and 3D, yet color performance continues to lag. Displays on popular tablets can only express about 20 percent of the color a human eye can see; HDTV’s, only 35 percent. Surprisingly, color performance in displays has actually gone backwards since the days of CRTs.
Still, LED backlit LCDs have become the standard for the mobile devices and are fast becoming so for televisions due to their high resolution, low cost and thin form factors. According to Paul Semenza, Senior Vice President of DisplaySearch, a leading display market research firm, LED backlights will be used in 47.5% of LCD televisions and 98.4% of notebook PC displays shipped in 2011.
While new technologies with better color capabilities have emerged in recent years, such as discrete RGB LED, YAG with red phosphor and OLED, they face critical hurdles to mass adoption; primarily cost, scale and lower brightness. Until now, consumers have chosen cheaper, thinner and more efficient displays over a truly cinema-quality experience- but could they have it all?
What’s wrong with my current display?
To better understand the limitations faced by current TV and display makers, let’s take a look inside an LCD. For those who are not familiar, a typical LCD is made up of essentially two major parts: a light source, called the back light unit (or BLU) and a Liquid Crystal Module (or LCM). (See FIG. 1)
FIG. 1. LED LCD exploded diagram
Usually, when a display is operating, the BLU is on, providing a uniform, white sheet of light behind the LCM. The LCM contains millions of pixels, each of which is split into sub pixels, typically with two green sub-pixels, one red and one blue. By controlling the amount of time each sub-pixel is “open” or allowing light to pass through it, and making use of the human eye’s persistence of vision, any color that can be rendered from a combination of red, green and blue can be displayed at each pixel location. Since the quality or fidelity of those colors is a direct function of the sub-pixel color quality, how good is the quality of red, green and blue light coming from each sub-pixel?
The color of each sub-pixel is a function of two things; the quality of the light in the BLU, and the color filter at the sub-pixel. The color filter will separate its component color from the white light of the BLU, for example, the red color filter on the red sub-pixels will cut off the green and blue light. However, to make a high quality color of red, either the filter function needs to be very narrow, which results in substantial attenuation and loss of brightness, or the red spectra in the BLU white light should be narrow and well matched to the desired peak red color. The same is true for the green and blue sub-pixels as well.
Since making perfect color filters is not practical from either a cost or brightness perspective, why not make a better white light?
The problem is, the LED light source at the heart of the BLU is starving those filters of the colors that they really need to shine. Today, white LEDs are very good at producing some of the spectrum of light that we see as ‘white’ but not all. While there are a variety of approaches for making white light from LEDs, the conventional approaches all suffer some drawback for LCD displays.
A YAG based white LED (i.e. Yttrium-Aluminum-Garnet phosphor pumped by an GaN blue source), produces a spectrum rich in blue with a broad yellow component. This light has very weak green and red content, and the spectra is widely distributed from aqua-marine through green, yellow, orange and red (see FIG. 2 below). When this light is filtered into the component RGB colors by the sub-pixels, the result is not accurate enough to produce the quality of color we see when we look at the natural world as illuminated by daylight.
FIG. 2. Spectrum of a conventional white LED (GaN + YAG) backlight, which does not provide a good match with red, green and blue color filters in the liquid-crystal module (LCM).
So, an ideal light source for an LED LCD BLU would therefore be something in between daylight and two-color white. For vibrant colors, it would need to generate lots of energy across all of the red, green and blue wavelengths used by the filters. But, for efficiency’s sake, it should also not spend energy producing light between R, G and B because we just won’t see that light after its passed through the filters.
So how do we do THAT?
To solve the problems described above, what we need is a new class of material, not found naturally occurring anywhere on Earth, that can be tuned to emit light at just the right wavelengths for our displays and do so very efficiently. Fortunately, nanotechnology researchers have been working on designing just such a material for decades, building it literally one atom at a time, and Nanosys, a company in Palo Alto, California has perfected the art. Called “Quantum Dots”, the tiny, nanocrystal phosphors they make are a bit bigger than a water molecule but smaller than a virus in size.
Unlike conventional phosphor technologies such as YAG that emit with a fixed spectrum, quantum dots can be fabricated to convert light to nearly any color in the visible spectrum. Pumped with a blue source, such as the GaN LED, they can be made to emit at any wavelength beyond the pump source wavelength with very high efficiency (over 90% quantum yield) and with very narrow spectral distribution of only 30 – 40nm full width at half maximum (FWHM).
The real magic of quantum dots is in the ability to tune (at the fabrication stage) the color output of the dots, by carefully controlling the size of the crystals as they are synthesized so that their spectral peak output can be controlled within 2 nanometers to nearly any visible wavelength.
This capability makes quantum dots stand out against emerging iterations of YAG phosphor technology such as red phosphor doped YAG, which adds some red-emitting phosphor to the green-yellow emitting YAG to boost color performance. This idea is similar to quantum dot technology in that it attempts to engineer a spectrum of white light by combining materials with different emission spectra. However, these crystalline phosphor materials are still fundamentally limited by their atomic structure and therefore cannot be precisely tuned to match either existing color filter or manufacturers desired specifications. This leaves display manufacturers with a system that still results in light and efficiency losses due to the relatively wide FWHM output of the phosphors and poor conversion efficiencies and stabilities of red phosphors.
With quantum dot technology, display designers will have the ability to tune and match the backlight spectrum to the color filters. This means displays that are brighter, more efficient, and produce truly vibrant colors.
FIG. 3. Spectrum from a Nanosys quantum-dot enhancement film (QDEF). The film contains green- and red- emitting phosphors and is stimulated by a blue GaN LED.
How does it all come together?
Engineering the quantum dots to precise display industry specifications isn’t enough on its own to revolutionize the way LCDs are experienced. The dots need to be easily integrated into current manufacturing operations with minimal impact on display system design if they are to be widely adopted. To do this, Nanosys spent a lot of time working with major display manufacturers to get the packaging just right so that it would be a simple, drop-in product that did not require any line retooling or process changes. The end result is called Quantum Dot Enhancement Film or QDEF.
FIG. 4. The quantum-dot enhancement film (QDEF) is designed to replace the diffuser in an LCD backlight unit (BLU) and is placed between the BLUE and the liquid-crystal module (LCM). The QDEF contains red- and green-emitting quantum dots and is illuminated by blue LEDs in the BLU.
Designed as a replacement for the an existing film in LCD backlights called the diffuser, QDEF combines red and green emitting quantum dots in a thin, optically clear sheet that emits white light when stimulated by blue light. (Of coursesome of that blue is allowed to pass through to make the B in RGB at the LCM). So manufacturers who’ve invested billions in plant and equipment for LCD production can simply slip this sheet into their process, change their ‘white’ LEDs to blue (the same LEDs but without the phosphor) and start producing LCD panels with the colors and efficiencies of the best OLEDs, at a fraction of the cost.
Nanosys is currently shipping production samples to display manufacturers and is on track to begin producing at commercial volumes by the end of 2011.
What does it look like?
The result is stunning color. A QDEF-enabled display can express over 60% of the spectrum a human eye can detect, compared with 20% for todays LED backlit LCD’s. This means that browsing through photos on your tablet will be more like holding a stack of high quality, professional prints in your hand and watching a movie on the big screen in your living room is more akin to attending a private screening at a Hollywood studio.
Comparison shot showing two 47" TVs with QDEF on the right and a YAG based display at left. While your display can't display the true difference in color it is possible to see that the frog on the left is just not as green (looks kind of yellow-ish by comparison)
LED backlit LCD TVs have established market dominance and tablet computers –which predominantly use LCDs– sales are expected to eclipse 100 million units over the next few years. Color is likely to be the next big differentiator in what is an increasingly cutthroat consumer display market as more players enter the market and alternative technologies are further developed.
Higher color performance displays will allow developers and content creators to create a stunning new visual experience for consumers. Display makers who can bring user experience closer to reality without sacrificing efficiency or cost will be able to establish a dominant market share.
You’d be surprised at how often these two terms are used interchangeably in describing display performance. In fact, their recent misuse by a top display executive at a major consumer electronics company was one of the chief inspirations for this blog.
Basically, it boils down to this:
More bits means more colors can be displayed
More gamut means more colors can be displayed
Got it yet?
While factually accurate, the statement above clearly illustrates the source of many a gamut vs bit depth misconception. Let’s dig a little deeper and define these terms.
First you should understand the simplicity with which modern displays create the color picture you are currently looking at. Digital displays create all the colors you see by mixing just three primary colors, much like you did while finger painting in grade school. So, red plus green equals yellow, red plus blue is magenta and all three primaries combined creates white.
So, what is bit depth? Bit depth refers to the number of bits that your computer uses to describe a specific color to your screen. A typical modern display has “8-bit” color depth, which is a shorthand way of saying “8 bits of data per primary color.” Since 8 bits translates into 256 distinct values, your computer can call for 256 distinct hues of red, green and blue.
While ‘256’ does not sound like a lot of colors – it’s actually quite impressive. Mixing 256 reds with 256 greens and 256 blues (256 x 256 x 256), in a massive expansion of the finger painting analogy from above, creates nearly 16.8 million possible colors. Not too bad, right?
Well, you might ask, how many different colors can I actually see? Turns out a number of studies have been conducted on this, and most researchers agree that humans can detect anywhere between 7-10 million unique colors (see: http://physics.info/color/). This means that even a measly 8-bit display should have plenty of color accuracy headroom above and beyond the performance of your eyes.
So, a more accurate way to describe bit depth would be: more bits mean that a higher number of distinct colors can be displayed.
If bit-depth refers to the number of distinct colors that can be displayed, where does that leave gamut? Perhaps the easiest way to think about gamut is as a measure of the range of colors that a display can show. Your 8-bit display may be able to show 16.8 million colors but that doesn’t tell you much about which colors. You see, not all reds, greens or blues are created equal. When your computer calls for a specific color of deeply saturated red, for example, what you actually see is limited by the physical capabilities of the systems in your display.
How is gamut measured?
Color gamut performance is measured by a variety of standards, typically defined by groups like the National Television System Committee (NTSC) as a way of maintaining consistent color, from capture, to broadcast to end viewer. Creative professionals like graphic designers and Hollywood cinematographers rely on these standards to make sure that their work looks how they intended it to across a variety of screens and print media. The most common gamut standard, developed by the NTSC in 1953 in anticipation of color television, which is typically referred to as simply “NTSC,” covers about 50% of what your eye can see. 58 years and a couple generations of IPads later, we must be able to do better than that, right?
The above diagram, called a “CIE Diagram” (CIE stands for Commission Internationale de l'Éclairage or International Commission on Illumination) is an abstract, two-dimensional (missing luminance) mathematical chromaticity model. In layman’s terms, what you are looking at is the full range of colors that humans can perceive, represented by the horseshoe shape and the subset of colors that HDTVs can display, inside the triangle. Note that the deepest reds, greens and blues are out of reach for HDTV.
Putting it all together
Now we can put the two definitions together to clarify the oversimplification from the introduction:
More bits means more distinct colors can be displayed, within a range of colors that is defined by the display’s gamut
For a really in depth look at how color is displayed on your screen check out Steve Patterson’s very informative, much more in depth post on this topic over at photoshopessentials.com.