by Jin Kim
This is Part II of a comprehensive multi-part article series with in-depth measurements and analysis for displays on the Google Nexus One and the Apple iPhone 3GS. It is produced as a collaboration between DisplayBlog and DisplayMate Technologies. We will show you the good, the bad, and also the ugly unfinished rough edges and problems lurking below the surface of each of these displays and display technologies. Each article will be introduced and discussed on DisplayBlog by me, Jin Kim, followed up with a detailed technical analysis and measurement data on the DisplayMate website by Dr. Raymond Soneira. There will be three parts to this series:
Parts I and II will be rolled out and expanded in several stages over a period of about a week, so be sure to check each day for updates.
What more can be said of the iPhone than what has already been said. The gist of it is: Apple completely revolutionized the smartphone market with the iPhone. No question about it. There is much going for the iPhone: Multitouch that actually works. An elegant and beautiful device. A legion of developers creating hundreds of thousands of apps that enhance the iPhone’s capabilities. And little things like visual voicemail, the iPod app, and mobile Safari that makes browsing on the little display as pain-free as can be, etc. But not everything is perfect. And in this article we will be looking very closely at the display: the 3.5-inch 480xRGBx320 pixel format TFT LCD.
Everyone seems to think all displays regardless of platform need to have a 16:9 aspect ratio. Hogwash! The most applicable is the TV but even then if you’re watching a lot of film you’re better off getting one of those incredibly wide Philips TVs with a 21:9 aspect ratio. Aspect ratio is just one small part of the entire experience of a display-based device. Take for instance the iPad: it has a 1024xRGBx768 pixel format and that’s an “old” 4:3 aspect ratio. But think about the usage scenario for the iPad. I haven’t used one so I can’t say for sure but I think I’ll be using it for more than watching the occasional video: blogging, checking email, reading e-books, flipping through photographs, etc. Books aren’t wide. Most photographs aren’t wide. I’d rather have a portrait display for blogging. I do think email might benefit from a wide display but if you’re the type of person who prefers writing dissertations to texting, the portrait mode will provide a better more focused experience. So clearly, the display on the iPad needs to fit in with all of its usage scenarios. The same logic applies to the iPhone: the 480xRGBx320 pixel format has a 3:2 aspect ratio and it works quite well.
I have often hoped that Apple would raise the bar and incorporate an OLED display on the iPhone. There are many benefits of using an OLED on the iPhone: superb contrast, deep blacks, a thinner display, better power management via UI tweeks, etc. There are some drawbacks too: limited lifetimes relative to LCDs, differential aging of the primary colors’ phosphors, more power draw if UI isn’t tuned for OLEDs, etc. Even though we have seen some imperfections on the OLED implementation in the Nexus One, I still hope Apple puts an OLED display into its next-generation iPhone. The Nexus One is the result of many companies working together and from what I have seen recently it seems that Apple’s approach of taking total control (sans the wireless carrier part) over bringing a smartphone to market results in a better experience. So it is just my guess that Apple would be the company to correctly implement an OLED display into a smartphone. I could also be completely wrong: Apple could use a 24-bit IPS TFT LCD instead.
COLOR & IMAGE QUALITY
The iPhone 3GS makes use of 18-bit color plus dithering to emulate 24-bit color. Each color (Red, Green, Blue) has 64 intensity levels. 24-bit emulation has been implemented well on the iPhone 3GS with artifact-free photographs and test patterns that show smooth intensity scales. In general the quality of images and UI elements are very good across the board. The weak link would be under-saturated colors and less-than-ideal contrast. Although scaling always takes away some quality the engineers at Apple have done a solid job, however, there were minor compression and/or scaling artifacts showing up on 480×320 images. These images were imported into iTunes, synced to the iPhone and then displayed using the Photos app.
The DisplayMate Multimedia Edition for Mobile Displays test patterns we were able to show some intensity scale irregularities in 256-level intensity color ramps due to the limitations of 18-bit plus dithering. At low intensities there were screen mottling and irregularities. And even with images at the display’s native 480×320 pixel format there were rescaling and/or compression artifacts.
The iPhone 3GS was compared side-by-side with a professional Sony high definition studio monitor using a large set of DisplayMate Calibration and Test Photographs. The results were less-than-ideal: images were under-saturated and exhibit low contrast. In other words, the images looked washed out. The culprit? A small color gamut and shallow intensity scales.
When a new product is imminent Apple engineers fly to Asia and live there for weeks to make sure everything is perfect. Overall factory calibration and quality control for the iPhone 3GS’ display is very good. White point, color and grayscale tracking is accurate and images are relatively free of artifacts. We will dig deeper into color gamut but even with a less-than-ideal color gamut colors on the iPhone 3GS are accurate. The shallow intensity scale resulted in low contrast and under-saturated colors. This could be corrected by changing the gamma curve to track closer to 2.2.
DISPLAY TEST RESULTS
Peak Brightness: 428 cd/m²
This level of brightness is excellent and is at the highest level for mobile displays. You will be able to see the display in all ambient light conditions except when the sun is hitting it straight on.
Black Level Brightness: 3.1 cd/m²
You want black to be black. On the iPhone 3GS black is dark gray and a lot grayer than you’d want. Overall image quality suffers because of a rather high black level brightness.
Contrast Ratio: 138
Because of the high black level brightness contrast ratio is low. Images look washed-out.
Screen Reflectance: 9.2%
Screen reflectance is the most important specification for a mobile display and the iPhone 3GS does exceptionally well and coupled with very high peak brightness allows you to view the display in bright ambient environments.
High Ambient Light Contrast Rating: 47
A rating of 47 is excellent and rates as one of the best mobile displays for viewing in bright ambient light environments. Clearly Apple was thinking about users in sunny environments, like Cupertino, CA. For mobile devices like smartphones, high ambient light contrast rating is a much more important specification than contrast ratio, unless of course you’re almost always in a relatively dark room.
Dynamic Color and Dynamic Contrast: n/a
The iPhone 3GS does not make use of dynamic color and contrast, a good thing since this feature often distorts images.
WHAT’S UP NEXT
In the next several days Dr. Raymond Soneira and i will be adding more interesting display test results for the iPhone 3GS:
For screen captures, more technical and in-depth explanations please visit DisplayMate.