A new generation of mini 7-inch to 8-inch tablets from three of the major manufacturers has just completed with the belated launch of the Apple iPad mini with retina display. While the Kindle Fire HDX, Nexus 7, and new iPad mini all have improved displays over their previous year counterparts, only one truly stands out — as lagging significantly behind.
All of this year's smaller tablets should be, in principle, a notch down from the large size flagship tablets that we recently tested. But the mini tablets are growing rapidly in popularity and market share, so it's a fiercely competitive category. As a result, they have some of the very highest technology displays with quantum dots, low temperature poly silicon, IGZO and/or high efficiency backlight LEDs, all of which have a major impact on real image quality that we examine below.
The 7-inch tablet format was pioneered by the Barnes & Noble Nook Colour, Amazon Kindle Fire, and (original) Google Nexus 7. After dismissing the smaller 7-inch tablets, Apple subsequently introduced in 2012 its own iPad mini, with a 7.9-inch 1024×768 display with a (surprisingly) much smaller colour gamut and higher screen reflectance than the existing models of the Kindle Fire and Nexus 7. A lot has happened to displays and display technology over the past year so this is much more than a rematch.
These new mini tablets all have higher than full high definition displays that have about 325 PPI pixels per inch. At normal viewing distances a person with 20/20 vision can't resolve the individual pixels, so the displays appear to be perfectly sharp. With high resolution and sharpness taken care of (for now), there are many other equally important and even more challenging issues for mini tablets displays:
1) Picture quality as good or better than your HDTV (to entice you to watch downloaded content).
2) Excellent true colour accuracy and accurate image contrast for high fidelity images of all viewed content.
3) Improved screen performance in high ambient light since Tablets aren't used in the dark.
We'll cover these issues and much more, with in-depth comprehensive display tests, measurements and analysis that you will find nowhere else.
Amazon provided DisplayMate Technologies with a production unit of the Kindle Fire HDX 7 to test and analyse for this display technology shoot-out article.
To examine the display performance of the Amazon Kindle Fire HDX 7, the Apple iPad mini Retina Display, and the new Google Nexus 7 (2013) we ran our in-depth series of mobile display technology shoot-out lab tests. We take display quality very seriously and provide in-depth objective analysis and side-by-side comparisons based on detailed laboratory measurements and extensive viewing tests with both test patterns and test images. To see how far mobile displays have progressed in just three years see our 2010 smartphone display shoot-out and 2011 tablet display shoot-out, and for a real history lesson see our original 2006 smartphone display shoot-out.
In this results section we provide highlights of the comprehensive lab measurements and extensive side-by-side visual comparisons using test photos, test images and test patterns that are presented below. The comparison table section summarizes the Lab measurements in the following categories: Screen reflections, brightness and contrast, colours and intensities, viewing angles, display white spectrum, display power consumption, running time on battery. You can also skip the highlights and go directly to the conclusions.
Overview of the Kindle Fire HDX 7
The Kindle Fire HDX 7 is Amazon's third-generation LCD tablet, and their displays have been improving by leaps and bounds since we first tested them back in 2011. Their full-size flagship Kindle Fire HDX 8.9 is the best performing tablet display that we have ever tested, due in part to using the highest performance LCDs with low temperature poly silicon LTPS. But the mini Kindle Fire HDX 7 that we test here is also incredibly innovative because it is the first tablet display to use super high technology quantum dots, which produce highly saturated primary colours for LCDs that are similar to those produced by OLED displays. They not only significantly increase the colour gamut to 100 per cent but also improve the power efficiency at the same time. It's a very impressive display with very impressive technology. More on these issues below.
Overview of the new Google Nexus 7
The new Google Nexus 7 (manufactured for Google by Asus) has a very impressive display that uses the highest performance LCDs with low temperature poly silicon LTPS, the same technology used in the iPhone 4 and 5, but on the new Nexus 7 with more than 3 times the screen area — it's currently the second largest LTPS on a mobile display after the Kindle Fire HDX 8.9 mentioned above. The very high efficiency LTPS technology allows the new Nexus 7 display to provide a full 100 per cent colour gamut and at the same time produce the brightest Tablet display that we have measured so far in this shoot-out series. More on these issues below.
Overview of the iPad mini Retina Display
The iPad mini with retina display is Apple's second generation mini tablet. The first generation iPad mini was disappointing because not only did it have a low resolution low PPI display, but its small 62 per cent Colour Gamut was the same as the older iPad 2, instead of the 100 per cent Colour Gamut on the iPad 3 and iPad 4 (and the new iPad Air). The new iPad mini with retina display has a high resolution high PPI display like the other two mini tablets that we test here. But shockingly, it still has the same small 63 per cent colour gamut as the original iPad mini and even older iPad 2. As a result, the iPad mini with retina display comes in with a distant 3rd place finish behind the innovative displays on the Kindle Fire HDX 7 and new Nexus 7. More on these issues below.
IGZO and LTPS
For the last two years one of the most talked about developments in display technology has been the introduction of IGZO (Indium Gallium Zinc Oxide). For both LCD and OLED displays, IGZO can be used to make the electronic circuitry in their Backplanes, which controls all of the pixels and sub-pixels. IGZO's higher electron mobility allows the circuitry to be much smaller compared to traditional amorphous Silicon a-Si, which is currently what is used in most LCD displays. That allows the brightness and power efficiency of the display to significantly increase, which is very important. But the introduction of IGZO has been repeatedly delayed as the result of production and yield issues. Although all of the major display manufacturers are working on IGZO, Sharp has been the biggest advocate, and it is currently shipping some IGZO displays, including in the current iPads. LG is also shipping IGZO displays, including in its OLED TV, but not currently for the iPads.
This has created a production problem where Apple is using both IGZO and a-Si displays in the current iPads. The problem is that a-Si has much lower power efficiency than IGZO, so it uses much more power and also needs bigger batteries. So how can Apple use both display technologies in the same product? I've been told by using much higher performance (and cost) White LEDs in the Backlight of the a-Si panels, which equalizes the power efficiency for both types of displays in different ways. As a result, both types of displays can be engineered into the same tablet and can provide comparable results for the consumer.
All of this reliance on IGZO is really bad planning. Right now there is a readily available display technology that has much higher performance than IGZO. It's low temperature poly silicon LTPS, and it is used in all of the iPhones and in all of Samsung OLEDs (so it's available in large quantities). Two innovative tablet manufacturers, Amazon and Google, have significantly leapfrogged Apple by introducing Tablet displays using LTPS (in the Kindle Fire HDX 8.9 and the new Nexus 7), and they are significantly outperforming the IGZO and a-Si displays in the current iPads. Apple is now lagging in displays, an area where it was once the leader.
While IGZO has been getting most of the attention, a dark horse called quantum dots has emerged as a new and truly revolutionary super high-tech advancement for LCD displays — and it is showing up for the first time in the Kindle Fire HDX 7, which we test here.
Quantum dots are almost magical because they use quantum physics to produce highly saturated primary colours for LCDs that are similar to those produced by OLED displays. They not only significantly increase the size of the colour gamut by 40-50 per cent but also improve the power efficiency by an additional 15-20 per cent. Instead of using White LEDs (which have yellow phosphors) that produce a broad light spectrum that makes it hard to efficiently produce saturated colours, Quantum dots directly convert the light from blue LEDs into highly saturated primary colours for LCDs. You can see the remarkable difference in their light spectra in Figure 4. Quantum dots are going to revolutionise LCDs for the next 5+ years. To learn more about quantum dots read this from Nanosys. Congratulations to Amazon for leading the way and being the first to incorporate this revolutionary new display technology in Tablets! It will be interesting to see how rapidly other manufacturers adopt quantum dots. See Figure 1 and Figure 2 and the Colours and Intensities section for details.
These mini tablets all have almost exactly 326 pixels per inch PPI (the same as the Retina Display iPhones). For 20/20 Vision the pixels are not resolved for viewing distances of 10.5 inches or more, which is less than the typical viewing distance of 12 inches or more. As expected, all were incredibly and impressively razor sharp, especially noticeable with text and graphics (and significantly sharper than the previous models).
All of these mini tablets have fairly bright displays, with the Nexus 7 the brightest tablet that we have measured so far in this shoot-out series, with an impressive maximum brightness of 572 cd/m2 (sometimes called nits). Part of this is due to its high performance and high efficiency LTPS LCD display discussed above. The Kindle HDX 7 has a very bright 494 cd/m2, and the iPad mini a much lower but still very good 414 cd/m2 (but the Nexus 7 is 38 per cent brighter). Note that it is important to appropriately adjust the display brightness in order to preserve battery power and running time, and also to reduce eye strain from looking at too bright a display. See the brightness and contrast section for details.
Accurate Factory Display Calibration
The raw LCD panel hardware first needs to be adjusted and calibrated at the factory with specialised firmware and software data that are downloaded into the device in order for the display to produce a usable image — let alone an accurate and beautiful one. This is actually a science but most manufacturers seem to treat it as if it were a modern art form, so few tablets, smartphones, and even HDTVs produce accurate high quality images. The iPad mini actually has an excellent and accurate calibration considering its small colour gamut (below) because each unit receives individual display factory calibration. Each Kindle Fire HDX 7 also receives individual unit display calibration for the colour gamut and white point. For the Nexus 7 we don't have any specific calibration information.
Intensity Scale and Accurate Image Contrast
The intensity scale (sometimes called the grey scale) not only controls the contrast within all displayed images but it also controls how the red, green, and blue primary colours mix to produce all of the on-screen colours. So if the intensity scale doesn't accurately follow the standard that is used to produce virtually all consumer content then the colours and intensities will be wrong everywhere in all images. Unfortunately, many manufacturers are quite sloppy with the intensity scales on their displays.
The iPad mini has a virtually perfect intensity scale as the result of its detailed individual unit factory calibration. The intensity scale for the Kindle Fire 7 is a bit too steep, which increases the image contrast somewhat higher than it should be. That's not always bad, because high ambient lighting winds up reducing image contrast, so the extra steepness can be beneficial. On the other hand, the Nexus 7 like all Nexus tablets that we have tested, has a non-standard and too shallow intensity scale. That is always bad because it reduces precious image contrast, reduces colour saturation, and introduces additional colour errors. See Figure 3 and the colours and intensities section for details.
The colour gamut is the range of colours that a display can produce. In order to show accurate on-screen colours the display must match the standard sRGB/Rec.709 Colour gamut that is used to produce virtually all consumer content. Note that consumer content does not include colours outside of the standard gamut, so a display with a wider colour gamut cannot show colours that aren't in the original and will only produce inaccurate exaggerated on-screen colours — so in this instance, bigger than 100 per cent is not better. The measured colour gamuts for these mini tablets are shown in figure 1.
The Kindle Fire HDX 7 and Nexus 7 both have colour gamuts close to the sRGB/Rec.709 standard, in the range of 97 to 103 per cent, which is very good. However, the iPad mini retina display has a much smaller 63 per cent colour gamut, which is incredibly disappointing because it produces noticeably subdued image colours. In fact, it's almost identical to the gamuts on the much older iPad 2 and the original iPad mini. That is inexcusable for a current generation premium tablet. It's way below the Kindle Fire HDX 7 and 8.9, the iPad 3, iPad 4, iPad Air, and just about all current generation premium Tablets and Smartphones — see our Mobile Shoot-Out series. Compare the colour gamuts in figure 1 and in the colours and intensities section.
Absolute Colour Accuracy
Getting very accurate screen image colours is very important and also very difficult because the display and calibration all need to be done extremely well at the factory. We have performed a set of detailed Lab spectroradiometer measurements of the Tablet displays to see how accurately they reproduce a set of 21 reference colours within the standard sRGB/Rec.709 colour gamut. The reference colours and the colours actually reproduced by the mini tablets are shown in Figure 2. The iPad mini is shown separately because its small colour gamut results in very large errors within the plot.
The colour accuracy errors are examined in terms of JNCD (Just Noticeable Colour Difference). The Kindle Fire has the best overall accuracy with an average colour error of 3.0 JNCD, which is very good. The Nexus 7 came in a close second at 3.1 JNCD, and the iPad mini came in a distant third with 6.6 JNCD. The peak colour accuracy errors are much higher, particularly for the iPad mini, with 23.4 JNCD. The iPad mini does have some colour management that improves the colour accuracy for low saturation colours, but it can't fix the higher saturated colours. See figure 2 for a discussion of JNCD with plots of the reference colours and the actually reproduced colours, and the colours and intensities section for the numerical results.
Screen Reflectance and Performance in High Ambient Lighting
The screens on almost all tablets and smartphones are mirrors good enough to use for personal grooming. Even in moderate ambient lighting the sharpness and colours can noticeably degrade from light reflected by the screen, especially objects like your face and any bright lighting behind you. Screen reflectance has been steadily decreasing. These mini tablets have around 6.5 per cent reflectance, ranging from 24 to 36 per cent higher than the flagship Kindle Fire HDX 8.9, which has the lowest reflectance screen that we've measured for a tablet. The iPad mini is the highest mini reflectance at 6.8 per cent — considerably lower than the original iPad mini, which had 9.0 per cent Reflectance, so this is a large improvement. This article has screen shots that show how screen images degrade from reflections in bright ambient light. See the screen reflections section for details.
Viewing Angle Performance
While tablets are primarily single viewer devices, the variation in display performance with viewing angle is still very important because single viewers frequently hold the display at a variety of viewing angles, plus they are large enough for sharing the screen with others. All of these tablets have displays with high performance IPS or FFS LCD technology, so they were expected to show very little colour shift with viewing angle, and our lab measurements confirmed their excellent viewing angle performance, with no visually noticeable colour shifts. However, all LCDs, do have a strong decrease in brightness (luminance) with viewing angle, and these mini displays all showed, as expected, more than a 50 per cent decrease in brightness at a modest 30 degree viewing angle. See the viewing angles section for details.
The big differences in colour gamut between the Kindle Fire HDX 7 and Nexus 7, and the much smaller gamut in the iPad mini retina display were quite obvious and easy to see in the side-by-side viewing tests. The Kindle Fire had the best colour accuracy and overall picture quality, with slightly too much colour saturation and image contrast due too a slightly too steep Intensity Scale. The Nexus 7 was a close second, primarily as the result of too shallow an intensity scale. The iPad mini retina display came in a very distant 3rd place finish with significantly undersaturated colours — particularly noticeable are reds that appear too orange, together with greens and blues that appear weak and washed out. See figure 1 and figure 2 and the colours and intensities section for quantitative details.
Display Power Efficiency
We measured the power consumption of all three displays. The relative power efficiency (for the same luminance and screen area) is highest for the Nexus 7, which has the highest performance and most efficient LTPS low temperature poly silicon LCD. Second is the Kindle Fire HDX 7, which has a backlight using quantum dots that increases the power efficiency by 20 per cent while at the same time increasing the colour gamut by up to 50 per cent as discussed above.
The iPad mini retina display has the lowest power efficiency of the tested mini tablets. It uses 30 per cent more display power than the original (Non-Retina) iPad mini. But that means there has been a significant enhancement in its power efficiency (from either IGZO or high performance White LEDs as discussed above), because the jump up to retina display from the iPad 2 to iPad 3 resulted in more than a 100 per cent display power increase. See the IGZO discussion above and our iPad 3 display shoot-out for more information on LTPS, IGZO and a-Si power efficiency. See the display power consumption section for details.
Conclusions: Two Very Impressive Tablet Displays and One Disappointment
These mini tablets include some of the most impressive and innovative displays and display technologies, which is perhaps not that surprising given how popular and competitive this mobile category has become.
First, they all have high resolution displays, with more pixels than your 50 inch HDTV, but on a 7-8 inch screen, which is certainly impressive. With about 325 pixels per inch, at normal viewing distances a person with 20/20 vision can't resolve the individual pixels, so the displays all appear to be perfectly sharp.
Even more impressive is that the Kindle Fire HDX 7 and new Google Nexus 7 displays also deliver a full 100 per cent colour gamut, with colour accuracy and picture quality that is probably better than most HDTVs, laptops, and monitors. They accomplish this in two very different ways.
The new Google Nexus 7 has a very impressive display that uses the highest performance LCDs with low temperature poly silicon LTPS. The very high efficiency LTPS technology allows the new Nexus 7 display to provide a full 100 per cent colour gamut and at the same time produce the brightest tablet display that we have measured so far in this shoot-out series.
Most impressive of all is the Kindle Fire HDX 7--the first tablet display to use super high technology quantum dots, which produce highly saturated primary colours that are similar to those produced by OLED displays. They not only significantly increase the colour gamut to 100 per cent but also improve the power efficiency at the same time. Instead of using white LEDs (which have yellow phosphors) that produce a broad light spectrum that makes it hard to efficiently produce saturated colours, quantum dots directly convert the light from blue LEDs into highly saturated primary colours for LCDs. You can see the remarkable difference in their light spectra in Figure 4. Quantum dots are going to revolutionise LCDs for the next 5+ years. To learn more about Quantum Dots read this from Nanosys. Congratulations to Amazon for leading the way and being the first to incorporate this revolutionary display technology in tablets! It will be interesting to see how rapidly other manufacturers adopt quantum dots. This level of display competition and excellence is great to see! Consumers will come to appreciate and then demand this new high level of display performance excellence, which will hopefully spur other manufacturers into improving their display performance in order to remain competitive.
And finally, the iPad mini with retina display unfortunately comes in with a distant 3rd place finish behind the innovative displays on the Kindle Fire HDX 7 and new Nexus 7 because it still has the same small 63 per cent colour gamut as the original iPad mini and even older iPad 2. That is inexcusable for a current generation premium tablet. The big differences in colour gamut between the Kindle Fire HDX 7 and Nexus 7 and the much smaller 63 per cent gamut in the iPad mini retina display were quite obvious and easy to see in the side-by-side viewing tests. See figure 1 to compare the widely disparate colour gamuts and figure 2 to see the very large colour errors that result. This all appears to be due to incredibly poor planning. Instead of moving up to the higher performance (and cost) low temperature poly silicon LCDs, Apple chose to continue gambling on IGZO, which has resulted in both production shortages and inferior products.
Two innovative tablet manufacturers, Amazon and Google, have significantly leapfrogged Apple by introducing tablet displays using LTPS (in the Kindle Fire HDX 8.9 and the new Nexus 7), and they are significantly outperforming the IGZO and a-Si displays in the current iPads. Apple was once the leader in mobile displays, unfortunately it has fallen way behind in both tablets and smartphones. This should be a wakeup call.
There is still tremendous room for improvement and innovation in display technology, which I have covered in recent articles on super high density 440+ PPI displays, curved and flexible displays, OLED mobile displays and OLED TV displays.
The most important developments for the upcoming generations of mobile displays will come from improvements in their image and picture quality in ambient light, which washes out screen images, resulting in reduced readability, image contrast, and colour saturation and accuracy. The key will be in dynamically changing the display's colour management and intensity scales in order to automatically compensate for reflected glare and image wash out from ambient light. See this article on display performance in ambient light. The displays and technologies that succeed in implementing this new strategy will take the lead in the next generations of mobile displays.
Display Shoot-Out Comparison Table
Below we compare the displays on the Amazon Kindle Fire HDX 7, the Apple iPad mini Retina Display, and the new Google Nexus 7 (2013) based on objective Lab measurement data and criteria. For additional background and information see our flagship Tablet Display Shoot-Out, our 2012 Mini Tablet Display Shoot-Out, and our SID Tablet Display Technology Shoot-Out.
Below is a partial excerpt of the table; you can see the full comparison at DisplayMate.
Dr Raymond Soneira is President of DisplayMate Technologies Corporation of Amherst, New Hampshire, which produces video calibration, evaluation and diagnostic products for consumers, technicians and manufacturers. See www.displaymate.com. He is a research scientist with a career that spans physics, computer science and television system design. Dr Soneira obtained his PhD in Theoretical Physics from Princeton University, spent five years as a Long-Term Member of the world famous Institute for Advanced Study in Princeton, another five years as a Principal Investigator in the Computer Systems Research Laboratory at AT&T Bell Laboratories, and has also designed, tested and installed colour television broadcast equipment for the CBS Television Network Engineering and Development Department. He has authored over 35 research articles in scientific journals in physics and computer science, including Scientific American. If you have any comments or questions about the article, you can contact him at [email protected].
DisplayMate Technologies specialises in advanced mathematical display technology optimisations and precision analytical scientific display diagnostics and calibrations to deliver outstanding image and picture quality and accuracy — while increasing the effective visual contrast ratio of the display and producing a higher calibrated brightness than is achievable with traditional calibration methods. This also decreases display power requirements and increases the battery run time in mobile displays. This article is a lite version of our intensive scientific analysis of smartphone and mobile displays — before the benefits of our advanced mathematical DisplayMate Display Optimisation Technology, which can correct or improve many of the deficiencies — including higher calibrated brightness, power efficiency, effective screen contrast, picture quality and colour and greyscale accuracy under both bright and dim ambient light, and much more. Our advanced scientific optimisations can make lower cost panels look as good or better than more expensive higher performance displays. For more information on our technology see the summary description of our Adaptive Variable Metric Display Optimiser AVDO. If you are a display or product manufacturer and want our expertise and technology to turn your display into a spectacular one to surpass your competition, contact DisplayMate Technologies to learn more.