Are tablets up to the task of accurate color testing?

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.

iPad mini color gamut and Gretag Macbeth colors against sRGB in CIE1976

To show what I mean by that, I measured how two different tablets show the colors found in the Gretag Macbeth color checker chart.Nexus 7 color gamut and Gretag Macbeth colors against sRGB in CIE1976

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.

Out of gamut colors at beach

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

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.

iPad Content Creation gets more Colorful with FiftyThree’s Paper app

App developer FiftyThree recently updated one of my favorite creativity apps for iOS, Paper, with an impressive new color-related feature. If you are not familiar with Paper, it’s a sketchbook app capable of making the work of even non-artists like me look gallery worthy with an intuitive and responsive interface.

The new feature, which FiftyThree calls “the biggest leap forward in color controls in the past 40 years,” is a color mixer that allows you to create a wide array of colors within the app just as you would in real life. They say they put a lot of time and effort into making the new mixer feel natural. The Paper color mixer works just like finger painting as a kid, mixing yellow and blue in the Paper app mixer produces green.

The new color mixer, shown at the bottom of this screenshot, lets you mix multiple colors to achieve a much wider palette in the new version of Paper.

This is a great feature that expands the content creation capabilities of an already exceptional app. But, as great as this app is, it’s still limited by the color capability of the device it’s installed on. Even the latest iPad, which can produce 100% of the sRGB color gamut, still only shows about 1/3 of the visible color spectrum.

The experience you will have mixing and creating colors on today’s tablets just will not be nearly as dynamic or visceral as making a physical painting. Not until better, wide color gamut technology is adopted in displays will the digital color experience match the stunning world of color we live in.

Time to Ditch the Diagonal?

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.

Source: apple.com

Updated: How does the iPhone 5’s color saturation measure up against Apple’s claims?

Commenter William thankfully double checked our math and we’ve corrected a small error in our % NTSC calculation.

We finally got our hands on an iPhone 5 yesterday. I tried asking Siri if she really has 44% more color saturation but she wouldn’t give up the goods, so I went with plan B and aimed our PR-655 spectroradiometer at the phone to find out just how impressive the screen really is. A lot has already been written about this display, but not much empirical evidence has been published about the color performance. How does the screen actually stack up to the marketing claims?

In short, Apple did an exceptional job improving color saturation and display quality in general, but the unit we measured just missed the 44% more color saturation claim.

Measuring Up

The iPhone 5 has significantly more color saturation than the 4S.

The 44% more color claim for the iPhone 5 is the same claim Apple made for the new iPad. As with the iPad, increasing the color performance of the iPhone 4S by 44% of NTSC 1953 gamut, measured using the CIE 1931 color space, would result in color saturation matching the sRGB color standard.  Using these standards as the goal posts, we measured the iPhone 5 at 70% of NTSC 1953 in CIE 1931, a 39% increase from the iPhone 4S, which measured at 50%. That’s 5% less of an improvement than Apple’s 44% claim and just 99% of sRGB (measured against the sRGB primaries).

While 5% less might seem like a big deal, getting to 99% of sRGB is a major feat and will result in tremendously noticeable color improvement in the phone. Additionally, color filters are notoriously difficult to manufacture. Slight variances in performance like this are common and most likely outside the range of a just noticeable difference for the average person.

If you want to know more about NTSC, CIE and sRGB, and why we are using standards from the 1930s, I have written extensively about this issue in the past.

How did they do it?

Much like they did with the new iPad, Apple significantly improved the color filter performance of the iPhone 5. Based on our experience, this type of improvement typically means that the display requires 20-30% more power to operate at the same brightness. Considering that the display is already a major source battery drain on the phone, this further underscores the engineering effort Apple made to keep battery life about the same as the 4S.

Let’s take a quick look at the changes in each of the red, green and blue color filters, starting with white, which is all three filters turned on:

Looking at the white spectrum of the iPhone 5, we see that the new color filters are very similar to those of the new iPad. Compared to the 4S, the peaks are slightly narrower, which improves color purity. In order to meet sRGB, they also moved to deeper reds and blues.

As with the new iPad, the biggest difference between the 4S and the 5 is in blue. Apple moved the peak to a deeper blue but, more importantly, they narrowed the filter so less green light leaks through. The green leakage causes blue to look a bit “aqua” on the 4S.

Retinal neuroscientist Bryan Jones looked at both displays under his stereo microscope earlier this week. His close-up shots really show off the difference in blue filters.

Apple again chose a slightly deeper wavelength of green which is less yellow and eliminated some of the blue leakage that had been muddying the green on the 4S.

The change here is subtle but as with the other filters, the peak is narrower, deeper in the red and leakage is reduced. One difference worth noting is that, while we are seeing less peak leakage in the red filter, there had been relatively broadband leakage across yellow, green and into blue that has been largely eliminated.

Conclusion

In all, it’s an exceptionally well-calibrated and accurate display for any kind of device, especially a smartphone. Apple has gone to great lengths to design a screen that brings the vibrancy of sRGB to the palm of your hand.
If you are not familiar with color filters or the inner-workings of LCDs in general this great live teardown by Bill Hammack is well worth watching: http://youtu.be/jiejNAUwcQ8

Beyond Retina: holiday releases see device makers move beyond PPI in display marketing efforts

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.