Most HDTVs, tablets, smartphones, laptops and monitors prominently list their display as a sales and marketing tool. Unfortunately, many of these specs are misleading and misunderstood by both consumers and professionals. This makes it harder to figure out which displays really are better. Below are many of the specs you'll see together with brief explanations that will help you understand what they actually mean.
Just because a manufacturer lists "technical specifications" doesn't mean those numbers are true -- very often it's marketing dressed up as science. Dr Raymond Soneira of DisplayMate Technologies breaks down some commonly misleading specs so you don't get fooled by the lies.
Screen Size: This spec sounds straightforward -- it's just the length of the screen diagonal. True, but that doesn't actually tell you how big the screen really is. What really matters visually is the area of the screen (width times height) and that changes significantly faster than the diagonal size indicates, so you'll either over or under estimate the true visual screen size based on just its diagonal size. To get an idea of the screen area, square the diagonal size (multiply it by itself on a calculator) and use that for comparison. For example, a 7-inch tablet actually has less than half the area (49) of a 10-inch tablet (100). There's more -- the area also depends on the shape of the screen, which is another Spec called Aspect Ratio (below). Lower Aspect Ratios have larger screen areas for the same diagonal size. For example, a 10-inch 4:3 Aspect Ratio screen is 12 per cent larger in area than a 10-inch 16:9 screen. If you're not handy with this maths, you'll often see the actual screen width and height listed in the specs -- multiply them on your calculator to get the area.
16:9 and 4:3 Aspect Ratios: The Aspect Ratio is the screen width divided by its height and it's the Spec used to indicate the shape of the screen. It can be described as a ratio like 16 by 9, or numerically by dividing the two, 1.78. For all consumer displays you can also get the Aspect Ratio by dividing the screen's listed horizontal and vertical Pixel Resolution - dividing 1920 by 1080 is also 1.78. 16:9 is the standard Aspect Ratio for HDTV content, so it fits perfectly on 16:9 screens. While that's often called "Widescreen" most Widescreen movies have Aspect Ratios much larger than 16:9, so you'll see black "Letterbox" bars on the top and bottom of the screen, which reduces the effective viewable Screen Size and Resolution. Another common Aspect Ratio is 4:3 or 1.33, which is also the same Aspect Ratio as content from 8.5x11 inch documents. This Aspect Ratio is better for reading in either Landscape or Portrait modes, but not as good for viewing Widescreen content. The iPad has a 4:3 Aspect Ratio and the iPhone has 3:2 or 1.5. Other common Aspect Ratios are 5:3 or 1.67 and 16:10 or 1.6.
PPI Pixels Per Inch: As a result of Apple's Retina Displays, Pixels Per Inch is a very hot Spec - it's one of the most closely followed display Specs, but it's also one of the most misunderstood. True, the higher the PPI the sharper the image on the screen, but what really matters is the sharpness perceived by your eye and that depends on the Viewing Distance from the screen (and also how good your vision is compared to 20/20 Vision). So PPI cannot be used by itself, but must be used together with the Viewing Distance in order to draw any conclusions about visual sharpness, and whether or not it qualifies as a Retina Display. While the iPhone 4 has an impressive 326 PPI, it is typically held relatively close and viewed from around 12 inches. Larger displays like Tablets and Laptops are typically viewed from 16 inches or more and need only 215 PPI to appear perfectly sharp with 20/20 Vision (what Apple calls a Retina Display ). In fact, existing 1920x1080 HDTVs, which are viewed from much larger distances, are already what Apple calls Retina Displays as explained in this Display News article.
Colour Gamut: The Colour Gamut is the range of colours that a display can produce. A widely held and exploited misconception is that the bigger the Colour Gamut the better - but it isn't... If you want to see accurate colours in photos, videos, and all standard consumer content the display needs to match the Standard Colour Gamut that was used to produce the content, which is called sRGB / Rec.709. A display with a larger Colour Gamut cannot show colours that are not in the original content - it just exaggerates and distorts the colours. A smaller Colour Gamut produces subdued colours, and too large a Colour Gamut produces oversaturated and even Gaudy colours. That's why a smaller Colour Gamut is visually better than too large a Colour Gamut. Most LCDs have a colour Gamut smaller than the Standard and most OLEDs have a Colour Gamut larger than the Standard, as illustrated in this figure. We objectively measure the colour Gamut in all of our Mobile Shoot-Outs and HDTV Shoot-Outs.
NTSC Colour Gamut: You'll see the NTSC Colour Gamut Spec listed for some displays and reviews. It's an indication that the manufacturer or reviewer is way out of touch. The NTSC Colour Gamut was defined about 60 years ago in 1953 and has been obsolete for over 30 years. It was never actually a true Colour Gamut standard because consumer TVs never produced the NTSC Gamut even way back then. Specifying the ancient NTSC instead of the current sRGB / Rec.709 colour Gamut Spec is ridiculous...
16 Million Colours: You'll see this spec for most HDTVs, tablets, smartphones, laptops and monitors. 16 million colours is effectively the standard for most consumer content (including digital cameras). But it doesn't mean what most people think it does - a larger number of colours does NOT mean a larger Colour Gamut - it is merely the total number of possible combinations of the red, green and blue primary colour intensities. The primaries each have 256 possible intensity levels - that produces 256x256x256 = 16.7 million possible intensity combinations, which are not really colours in the intuitive sense. There is also duplication - for example, pure red counts as 256 colours. Not all displays can produce 16 million colours - you'll sometimes see 262,144 colours, and some that claim 16 million colours can't directly produce them - more on that under 18-bit and 24-bit colour, below.
Billions and Trillions of Colours: Some display specs list billions and even trillions of colours, which as explained above does not indicate a larger Colour Gamut, but rather is just the total number of intensity combinations produced by the primary colours. What happens is those displays process the image internally using 1024 or more intensity levels. When you multiply this out as above you get billions up to trillions of intensity combinations, which sounds very impressive. But this spec is misleading and visually useless for two reasons: most consumer content only has 256 intensity levels (and that's all there is) and very few displays can accurately produce even the standard 256 intensity levels on-screen, which is the only place where it counts (most displays have irregularities that result in fewer than 256 distinct intensity levels on-screen). If you see a spec listing billions or trillions of colours, consider it meaningless marketing puffery...
18-bit and 24-bit Colour and Dithering: As mentioned in 16 million colours above, the spec for the number of colours is actually just the total number of possible intensity combinations for the red, green and blue primaries. For 16 million colours, each primary colour needs 256 intensity levels, which is 8-bits in binary. Since there are three primary colours and each has 8-bits, they add up to 24-bits. That's called 24-bit colour and you'll sometimes see this Spec listed instead of 16 million colours. Some lower performance displays can only produce 64 intensity levels, which is 6-bits per primary and adds up to 18-bits. That's called 18-bit colour and it produces only 262,144 colours. But the real problem is actually the smaller number of 64 intensity levels, which often introduces visually noticeable discrete steps into images that have smooth variations in intensity - from faces to the sky - it's called False Contouring. This effect is often masked by using two methods of dithering: spatial dithering, which uses combinations of pixels to produce intermediate intensity levels, but that reduces image sharpness; and temporal dithering, which rapidly switches the intensities to produce intermediate intensity levels, but may produce noticeable flicker in some content. A word of caution: some 18-bit displays with 262,144 colours use dithering methods to claim full 24-bits and 16 million colours. It's possible to detect the dithering on images with close visual inspection.
170+ Degree Viewing Angles: Many HDTVs, tablets, smartphones, laptops and monitors list a Viewing Angle Spec, which is the full angle (compared to 180 degrees) within which the display can supposedly be watched with satisfactory picture quality. 170 degrees is 85 degrees out of a possible 90 degrees. The spec seems to imply that unless you are watching from a ridiculous five degrees from the edge of the screen you will see a perfectly fine image on the screen. This spec is nonsense and very misleading because it is defined for the angle where the Contrast Ratio (below) falls to an abysmal 10. That is generally less than 1 per cent of the Contrast Ratio seen when viewing the screen face on - so the picture quality will also be abysmal at that Viewing Angle. Viewing from other than the face-on centre sweet spot generally decreases image and picture quality for effectively all displays. For example, in high-end IPS LCDs the Brightness and Contrast Ratio both fall by roughly 50 per cent at 30 degrees. For other LCDs there are also noticeable colour shifts at 15 degrees as shown in this article. For OLED displays we measured a 30 per cent Brightness decrease and noticeable colour shifts at 30 degrees. So, the Standard Viewing Angle Spec is useless. We objectively measure Viewing Angle performance in all of our Mobile Shoot-Outs and HDTV Shoot-Outs. The best way to evaluate it yourself is to look at a fixed (frozen) moderately colourful image or photo and see how the image changes as you shift your viewing position.
Contrast Ratio: Contrast Ratio is the Spec that tells you how good the display is at reproducing relatively dark content, particularly at or near black. It is measured in an absolutely pitch black Lab. It is very important if you watch movies with dark content under low Ambient Lighting. It's not so important if you are watching in bright Ambient Lighting (like for many HDTVs and most mobile devices) or when watching ordinary television shows and sporting events (because they have almost no dark content). Mobile displays should have at least a true Contrast Ratio of 500 and home theatre HDTVs at least 1500 (a good LCD). Videophiles will want at least 4000 from Plasma HDTVs, and are waiting anxiously for the much higher values in the new OLED HDTVs, as explained in this Display News article.
Dynamic and Mega Contrast Ratios: You'll often see displays advertised with Contrast Ratios from 20,000 up into the Millions. Unless it's an OLED display (which can easily do that) it's a phony Dynamic Contrast Ratio. The word "dynamic" may or may not appear. This Contrast Ratio is calculated by using the maximum peak brightness from one image with the minimum brightness from a different image that has a darkened backlight - so it doesn't apply to any actual single image, which makes it very misleading, especially when the word "dynamic" is left out. The true Contrast Ratio is what you actually see on any single image. The best LCDs have true Contrast Ratios of around 2000 and the best Plasmas around 5000, so ignore values much larger than that as marketing puffery. OLEDs deliver true Contrast Ratios from 50,000 up to almost infinity.
Response Time: This is another overhyped Spec that is a prime example of marketing wars fought with misleading numbers. LCDs sometimes produce visible Motion Blur because the Liquid Crystal (the LC in LCD) can't respond fast enough from video frame to frame when images change quickly due to motion, resulting in a blurred image. Standard video content is updated 60 times per second, so a new frame is transmitted every 17 milliseconds. In principle, the response time measures how quickly the display responds in milliseconds. It's important to make that number a lot smaller than 17ms (milliseconds). A lot of advanced technology has gone into improving the hardware response times, but also a lot of advanced marketing has gone into improving the published response time specs, with the values falling precipitously from 8ms, to 4ms, to even 1ms. Using high speed screen shots in this article we demonstrated that the true objective response times are actually considerably longer than 30ms. So the response time spec is mostly meaningless marketing puffery...
LED TVs and Displays: This one really amazes me because there aren't any LED TVs or consumer LED displays - the only true LED displays are large commercial outdoor signs. What are being marketed as LED TVs and displays are actually LCD TVs and displays. The LEDs are just the backlight for the LCD - nothing more - very misleading...
Brightness: In principle, the higher the maximum Brightness Spec the better, but people often set the screen Brightness too high, which causes eye strain and wastes power (and reduces battery running time). The optimum screen Brightness varies with the current level of Ambient Lighting. Many displays have Automatic Brightness controls that should appropriately adjust the screen Brightness, but we have found them all to be functionally useless as explained in this article. High Screen Brightness is only useful when you need to look at the screen under high Ambient Lighting. But under these circumstances the Reflectance of the screen is actually more important than the screen Brightness because it washes out the image, and you generally see reflections of your face and the area behind you, which is distracting and causes eye strain from involuntarily focusing on them instead of the screen content, which is much closer. We objectively measure the Brightness and Screen Reflectance in all of our Mobile Shoot-Outs.
Contrast Rating for High Ambient Light: Contrast Ratios (above) are measured in the dark and are only relevant for displays viewed in low Ambient Lighting. As the Ambient Lighting increases the Screen Reflectance becomes a dominating factor and the Contrast Ratio becomes increasingly irrelevant. We have defined a new Spec called Contrast Rating for High Ambient Light. It is based on Brightness and Reflectance Lab measurements that accurately indicate how well a display performs in high Ambient Lighting. This article includes the Contrast Rating, Lab measurements, and screen shots for 9 Tablets and Smartphones for Ambient Lighting levels up to 40,000 lux.
Anti-Glare or Anti-Reflection Treatments: Many displays list within their Specs that they have Anti-Glare or Anti-Reflection properties, but most of the time it's a vacuous statement. Our Lab tests show that there is a tremendous 3 to1 range in Reflectance between many popular Tablets and Smartphones. We objectively measure the Screen Reflectance in all of our Mobile Shoot-Outs. This article has the Lab results plus Screen Shots showing how the screens of various Tablets and Smartphones degrade under high Ambient Lighting levels up to 40,000 lux.
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]
About DisplayMate Technologies DisplayMate Technologies specialises in proprietary sophisticated scientific display calibration and mathematical display optimisation to deliver unsurpassed objective performance, picture quality and accuracy for all types of displays including video and computer monitors, projectors, HDTVs, mobile displays such as smartphones and tablets, and all display technologies including LCD, OLED, 3D, LED, LCoS, Plasma, DLP and CRT. This article is a lite version of our intensive scientific analysis of Tablet and Smartphone mobile displays — before the benefits of our advanced mathematical DisplayMate Display Optimization Technology, which can correct or improve many of the display deficiencies. We offer DisplayMate display calibration software for consumers and advanced DisplayMate display diagnostic and calibration software for technicians and test labs.
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