Apple iPhone 4 LCD Display Shoot-Out

Dr. Raymond M. Soneira

President, DisplayMate Technologies Corporation

This article, or any part thereof, may not be copied, reproduced, mirrored, distributed or incorporated

into any other work without the prior written permission of DisplayMate Technologies Corporation

Article Links: Google Nexus One OLED Display

Article Links: Samsung Galaxy S Super OLED Display

Article Links: Apple iPhone 3GS LCD Display

Article Links: Motorola Droid LCD Display

Article Links: Apple iPhone 4 LCD Display

Article Links: Smartphone "Super" LCD-OLED Display Technology Shoot-Out

Article Links: Mobile Display Shoot-Out Article Series Overview and Home Page

Article Links: Display Technology Shoot-Out Article Series Overview and Home Page

Series Overview

This is part of a comprehensive article series with in-depth measurements and analysis for the OLED and LCD displays in the Google Nexus One, the Apple iPhone 3GS, the Motorola Droid, the Samsung Galaxy S, and the Apple iPhone 4. 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, and then demonstrate how the displays can be improved by using images that have been mathematically processed to correct color and imaging errors on each smartphone so you can compare them to the originals. The series begins with the Google Nexus One and Apple iPhone 3GS. It then continues with higher performance “Super” displays in the Motorola Droid, the Samsung Galaxy S, and the Apple iPhone 4. Finally, there is a five way Smartphone "Super" LCD-OLED Display Technology Shoot-Out that compares all of the units simultaneously.

Introduction

A key element in the success of all smartphones and mobile devices is the quality and performance of their display. There have been lots of articles comparing various smartphone LCD and OLED displays and technologies, but almost all simply deliver imprecise off-the-cuff remarks like “the display is gorgeous” with very little in the way of serious attempts at objective or accurate display performance evaluations and comparisons – and many just restate manufacturer claims and provide inaccurate information, performance evaluations and conclusions. This article objectively evaluates the display performance of the Apple iPhone 4 IPS LCD Retina Display based on extensive scientific lab measurements together with extensive side-by-side visual tests.

The Apple iPhone 4 has a high performance In Plane Switching IPS LCD display with a White LED backlight. The screen is 3.5 inches diagonally and has a high-resolution high-density 960x640 pixel display with a screen Aspect Ratio of 1.50, which is identical to iPhone 3GS, but a bit lower than the other smartphones, which have Aspect Ratios between 1.67 and 1.78, the same as a 16:9 HDTV.

The inner details of the display technologies are very interesting, but our concern here is to evaluate the actual image and picture quality that they deliver, so we don’t really care how they do it, as long as they do it well. None-the-less with the measurements and analytical test patterns we will learn quite a bit about how they work.

FIGURE 1

Figure 1. Revealing Screen Shots for the Google Nexus One and Apple iPhone 4.

Nexus One: NASA Photo - Sunset on Mars

Gallery Application: Lots of false contouring and image noise

Apple iPhone 4: NASA Photo - Sunset on Mars

The same as it looks on a studio monitor

Nexus One: Intensity Scale Ramps

Gallery and Browser Apps: Coarse steps and tinting on white

Apple iPhone 4: Intensity Scale Ramps

Fairly smooth and artifact free

Figure 1. Revealing Screen Shots for the Google Nexus One and Apple iPhone 4.

The test patterns are 24-bit bmp at the native resolution of each display.

Results and Conclusions

The display was evaluated by downloading 24-bit native resolution 960x640 test patterns and 24-bit HD resolution test photos to the phone. Note that we are testing and evaluating the display on the iPhone with whatever hardware, firmware, OS and software are provided by Apple.

Color Depth and Granularity: Excellent Artifact Free 24-bit Color

The iPhone 4 provides full on-screen 24-bit color, which has 256 possible intensity levels for each of the Red, Green and Blue sub-pixels that are used to mix and produce all of the on-screen image colors. It’s the same as what is found on most monitors and HDTVs. When done properly, as on the iPhone 4, it produces a nice color and intensity scale with few visible artifacts. Figure 1 shows the smooth intensity scale for both a photograph and test pattern that are visibly free of all but minor artifacts on the iPhone 4.

Display Image Quality, Colors and Artifacts: Very Good except for Color Saturation

The image and picture quality on the iPhone 4 are very good across the board, including text, icons, and menu graphics. In the important category of images, pictures and photographs from external sources, whether they be from digital cameras or web content, are rendered quite well, except that the LCD panel is weak in color saturation – much more on that below. The calibration is very good and the images and photos are rendered relatively artifact free, including the critical rescaling function that is needed to fit images, photos and web content onto the native 960x640 resolution of the display.

The Measurements with Explanations and Interpretations:

The Measurements section below has details of all of the lab measurements and tests with lots of additional background information and explanations including the display’s Maximum Brightness and Peak Luminance, Black Brightness, Contrast Ratio, Screen Reflectance, Bright Ambient Light Contrast Rating, Dynamic Color and Contrast, Color Temperature and White Chromaticity, Color Gamut, Intensity Scale and Gamma, the variation of Brightness, Contrast Ratio and Color Shift with Viewing Angle, the Power Consumption and Light Spectrum of the display.

The Viewing Tests: Too Much Image Contrast and Not Enough Color Saturation

We compared the iPhone side-by-side to a calibrated Professional Sony High Definition Studio Monitor using a large set of DisplayMate Calibration and Test Photographs. All of the photos on the iPhone looked good had too much image contrast and too little color saturation due to it’s reduced color gamut. Fortunately the extra image contrast does partially improve color saturation.

Factory Calibration and Quality Control: Very Good

The overall factory calibration and quality control for the iPhone 4 display is good. Like the iPhone 3GS it was reasonably well calibrated, with fairly smooth and artifact free intensity scales. The color and gray-scale tracking are also very good, which means that the Red, Green and Blue primaries have been carefully calibrated and balanced. The one major flaw in the factory calibration is the steep intensity scale, which produces too much image contrast. The display Look Up Tables should be changed to deliver a lower Gamma closer to the standard value of 2.2.

Retina Display Acuity:

“Retina Display” is a great marketing name, and it’s the sharpest smartphone display available, 23 percent sharper than the nearest competitor, but objectively it does not meet the quantitative criteria for being a true Retina Display – it’s about a factor of two lower than the acuity of the human Retina. Rather, the iPhone 4 has a “20/20 Vision Display” because when it is held more than 10.5 inches away, a person with 20/20 Vision will not be able to resolve the iPhone 4 screen pixels, which are at 326 ppi (1 arc-minute resolution). But 20/20 Vision is the legal definition of “Normal Vision,” which is at the lower end of true normal vision. There are in fact lots of people with much better than 20/20 Vision, and for most people visual acuity is limited by blurring from the lens in the eye. The best human vision is about 20/10 Vision, twice as good as 20/20 Vision, and that is what corresponds to the acuity of the Retina. So to be a “True Retina Display” a screen needs about 652 ppi at 10.5 inches, or 572 ppi at 12 inches. Unfortunately, a “20/20 Vision Display” doesn’t sound anywhere near as enticing as a “Retina Display” so marketing and science don’t see eye-to-eye on this…

Suggestions for Apple:

The iPhone 4 display is a tremendous step forward over the iPhone 3GS and earlier models. It’s an excellent display, but here are some suggestions on how to make it better: The major shortcoming is the reduced color gamut, due to weak Red and Blue primaries. It’s worth trading some brightness and/or power efficiency to get more accurate and saturated colors. The image contrast (Gamma) is set too high, turning it down will increase image brightness a bit in addition to improving color accuracy and picture quality. Part II will include some important suggestions for correcting the Automatic Brightness control, which is very important for screen readability, viewing comfort and preserving battery power.

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 Optimization Technology, which can correct or improve many of the deficiencies – including higher calibrated brightness, power efficiency, effective screen contrast, picture quality and color and gray scale accuracy under both bright and dim ambient light, and much more. If you are a manufacturer and want our expertise and technology to turn your display into a spectacular one to surpass your competition then Contact DisplayMate Technologies to learn more.

Apple iPhone 4 Conclusion: Excellent Mobile Display wins Best Mobile Display Award

The iPhone 4 finally has a display worthy of an iPhone… It’s an outstanding “Super” IPS LCD delivering top performance in many of our test categories – it has the brightest and sharpest display, but a somewhat weak color gamut. The very high sharpness is a major marketing feature, but it’s actually something of an overkill (primarily there for App compatibility) because anti-aliasing methods already successfully reduce noticeable pixelation at lower resolutions. None-the-less the iPhone 4 Retina Display performed better overall in our tests and so it earned the DisplayMate Best Mobile Display Award and therefore won this Smartphone Display Technology Shoot-Out. Apple still needs to keep pushing hard because the competition in smartphone displays is closing in rapidly…

The Measurements with Explanations and Interpretations

This section explains all of the measurements incorporated in the article. The display was evaluated by downloading 24-bit native resolution 960x640 test patterns and 24-bit HD resolution test photos to the Apple iPhone 4. Note that we are testing and evaluating the display on the iPhone 4 with whatever hardware, firmware, OS and software are provided by Apple. All measurements were made using DisplayMate Multimedia Edition for Mobile Displays to generate the analytical test patterns together with a Konica Minolta CS-200 ChromaMeter, which is a Spectroradiometer. All measurements were made in a perfectly dark lab to avoid light contamination. All devices were tested with their Backlight set for maximum brightness with the Automatic Brightness light sensor control turned off, and running on their AC power adapter with a fully charged battery, so that the battery performance and state was not a factor in the results. For further in-depth discussions and explanations of the tests, measurements, and their interpretation refer to earlier articles in the DisplayMate Multimedia Display Technology Shoot-Out article series and the DisplayMate Mobile Display Shoot-Out article series.

Konica Minolta CS-200

1. Peak Brightness: 541 cd/m² – Excellent brightness for a Mobile Display

This is the maximum brightness that the display can produce, called the Peak White Luminance. 541 cd/m² is about as bright as you’ll find on any current mobile display. It’s fine for just about everything except direct sunlight, although it may be too bright for comfortable viewing under dim ambient lighting. If you find that to be the case, turn on the iPhone 4’s Automatic Brightness, which uses a light sensor to adjust the Peak Brightness settings. Since that can be used to decrease the power used by the backlight it will also increase the battery run time.

2. Black Level Brightness: 0.48 cd/m² – Good for a Mobile Display

The Black Level is the closest approximation to true black that the display can produce. Almost all displays wind up producing a visible dark gray on-screen instead of true black. This is a major problem for LCDs. The glow reduces image contrast and screen readability and can be distracting or even annoying in dark environments. It ruins the dark end of the display’s intensity/gray scale and washes out colors in the image. But note that in bright ambient lighting the Black Level is irrelevant because reflections off the screen dominate the screen background brightness. The iPhone 4’s value of 0.48 cd/m² is reasonably dark for a mobile display in typical ambient lighting. Note that if you decrease the screen Brightness with the (Backlight) Brightness Control, the Black Brightness will also decrease proportionally by the same amount, so in dimmer ambient lighting the Black Brightness can be reduced significantly if desired.

3. Contrast Ratio – Only Relevant for Low Ambient Light: 1,117 – Very Good for Mobile

The Contrast Ratio is a measure of the full range of brightness that the display is capable of producing. It is the ratio of Peak Brightness to Black Level Brightness. The larger the Contrast Ratio the better, but it is only relevant for low ambient lighting because reflections off the screen dominate the display’s Black Level in bright ambient lighting. The very best LCDs now have (true) Contrast Ratios of 1,500 to 2,000 so the 1,117 value for the iPhone 4 is very impressive in a mobile device. Don’t confuse the true Contrast Ratio with the tremendously inflated values that are published by many manufacturers.

4. Screen Reflectance of Ambient Light: 7.0 Percent – Very Good, Relatively Low

The often overlooked Screen Reflectance is actually the most important parameter for a mobile display, even more important than Peak Brightness. The screen reflects a certain percentage of the surrounding ambient light, which adds to the screen background, washes out the image, and makes it harder to see what is on the screen. In high ambient lighting the Screen Reflectance can significantly reduce the visibility and readability of screen content. The lower the Screen Reflectance the better. The value for the iPhone 4 of 7.0 percent is near the low-end of the range of values we have measured for mobile devices. Lowering the Screen Reflectance increases the cost of a display, but it’s the easiest and best way to improve screen readability under bright ambient light. The Screen Reflectance measurements were done in accordance with VESA FPDM 308-1, Reflectance with Diffuse Illumination, using an integrating hemispherical dome and a calibrated diffuse white reflectance standard.

5. Bright Ambient Light Contrast Rating: 77 – Excellent

In the same way that the Contrast Ratio measures the screen contrast under low ambient lighting, the Bright Contrast Rating specifies the relative screen contrast under high ambient lighting. It is the ratio of Peak Brightness to Screen Reflectance. The higher the value the better you’ll be able to see what’s on the screen when you are in a bright location. 77 is relatively high, so the iPhone 4 is among the best mobile displays for high ambient lighting. For all mobile devices the High Ambient Light Contrast Rating is much more important than the Contrast Ratio.

6. Dynamic Color and Dynamic Contrast: No – Which is Good

Some displays dynamically adjust the color, gray scale and contrast on every image that is displayed using an internal automatic image processing algorithm. The goal is generally to jazz up and “enhance” the picture by stretching and exaggerating the colors and intensity scale. It is similar to the Vivid mode found in many digital cameras and HDTVs. Since it alters and frequently distorts the image it is better left as an option for people who aren’t concerned with picture accuracy and fidelity. Since the Dynamic modes are generally triggered by changes in Average Picture Level, a very simple test for Dynamic Contrast is to separately measure the brightness of full screen Red, Green and Blue images and then compare them to White, which should equal their sum. If they don’t agree then there is Dynamic Color and Contrast processing. For the iPhone 4, the measured Luminance for Red=104, Green=350 and Blue=87 cd/m². Their sum is 541 cd/m², which is identical to the measured White Luminance.

7. Color Temperature and Chromaticity: 7,781 degrees Kelvin – White is Slightly Too Blue

White is not a single color but rather falls within a range that is normally specified by a Color Temperature. For accurate color reproduction of most content, including photographs, images and web content it needs to be set to the industry standard D6500, which is how most professional photo and video content is color balanced. D6500 is the color of natural daylight and is similar to a Black Body at 6500 degrees Kelvin. The iPhone 4’s White Point is slightly too Blue compared to D6500 – see the White Points in Figure 2 below. The measured CIE Chromaticity Coordinates of the White Point are u’=0.1886 v’=0.4602.

8. Color Gamut:

Much Smaller than the Standard Color Gamut – Colors are Inaccurate and Under Saturated

The Color Gamut of a display is the range and set of colors that it can produce. The only way that a display will deliver good color and gray scale accuracy is if it is accurately calibrated to an industry standard specification, which for computers, digital cameras, and HDTVs is sRGB or Rec.709. It’s the standard for most content and necessary for accurate color reproduction. If the Color Gamut is smaller than the standard then the image colors will appear too weak and under-saturated. If the Color Gamut is greater than the standard then the image colors will appear too strong and over-saturated. The important point here is that a Color Gamut larger than the standard is also bad, not better. Wider gamuts will not show you any colors or content that are not in the original images, which are almost always color balanced for the sRGB / Rec.709 standard. Wider color gamuts simply distort and decrease color accuracy and should be avoided, except for some special applications.

Figure 2 shows the measured Color Gamut for the Galaxy S, Motorola Droid and the Apple iPhone 4 alongside the Standard sRGB / Rec.709 Color Gamut in a CIE 1976 Uniform Chromaticity Diagram. The dots in the center are the measured White Points for the phones along with the D6500 Standard, which is marked as a white circle. The outermost curve are the pure spectral colors and the diagonal line on the bottom right is the line of purples. A given display can only reproduce the colors that lie inside of the triangle formed by its primary colors. Highly saturated colors seldom occur in nature so the colors that are outside of the standard sRGB / Rec.709 triangle are seldom needed and are unlikely to be noticed or missed in the overwhelming majority of real images. When a camera or display can’t reproduce a given color it simply produces the closest most saturated color that it can.

FIGURE 2

Figure 2. CIE 1976 Uniform Chromaticity Diagram showing the Color Gamut and White Point for the Apple iPhone 4

Both the iPhone 4 and Galaxy S perform poorly with reference to the standard Color Gamut, which is the black triangle in Figure 2. The iPhone has much too small a color Gamut and the Galaxy S has much too large a color Gamut. As a result the iPhone produces images that have significantly too little color saturation and the Galaxy S produces images that have significantly too much color saturation. This applies to all external content viewed on the displays, including web content, such as images, photos and videos. This was easy to see in the viewing tests where we compared the displays side-by-side to a calibrated Professional Sony High Definition Studio Monitor using a large set of DisplayMate Calibration and Test Photographs. Galaxy S photos had too much color, to the point of appearing gaudy, particularly faces, and well known objects such as fruits, vegetables, flowers, grass, and even a Coca-Cola can. The iPhone had the reverse problem, all of the photos looked somewhat pale, flat, washed-out and under-saturated.

9. Intensity Scale, Image Contrast and Gamma: Too Steep, Too Much Image Contrast, and non-Standard

The display’s intensity scale not only controls the contrast within an image but it also controls how the Red, Green and Blue primary colors mix to produce all of the on-screen colors. So if it doesn’t obey the industry standard intensity scale then the colors and intensities will be wrong everywhere on-screen because virtually all professional content and all digital cameras use the sRGB / Rec.709 standard, so it’s necessary for accurate image, picture and color reproduction. The standard intensity scale is not linear but rather follows a mathematical power-law, so it is a straight line on a log-log graph. Its slope is called Gamma, which is 2.2 in the standards. In order to deliver accurate color and intensity scales a display must closely match the standard. Figure 3 shows the measured (Transfer Function) Intensity Scale for the Apple iPhone 4 and Galaxy S alongside the industry standard Gamma of 2.2, which is a straight line.

FIGURE 3

Figure 3. Intensity Scale for the Apple iPhone 4

The iPhone 4 is too steep with respect to the Standard intensity scale, which is needed in order to accurately reproduce images and pictures for most content. Gamma is the slope of the intensity scale, which should be a constant 2.2 like the straight line in Figure 3. The Gamma for the iPhone 4 is 2.68, which is too high compared to the standard.

10. Brightness Decrease with Viewing Angle: 57 percent Decrease in 30 degrees – Bad, Very Large

A major problem with many displays, especially LCDs, is that the image changes with the viewing angle, sometimes dramatically. The Peak Brightness, Black Luminance, Contrast Ratio and color generally change with viewing angle (see below). Some display technologies are much better than others. At a moderate 30 degree viewing angle the Peak Brightness of the iPhone 4 fell by 57 percent to 235 cd/m², which is an incredibly large decrease. This behavior is typical for LCDs.

11. Black Level and Contrast Ratio Shift with Viewing Angle:

At a moderate 30 degree viewing angle the Black Level Brightness decreased somewhat to 0.42 cd/m², but the Contrast Ratio still fell considerably to 556. This behavior is typical for LCDs.

12. Color Shift with Viewing Angle: Excellent, Barely Visible Shift

Colors generally shift with viewing angle whenever the brightness shifts with viewing angle because the Red, Green and Blue sub-pixels each shift independently and vary with intensity level. At a moderate 30 degree viewing angle Blue shifted the most, by Δ(u’v’) = 0.0096, which is 2 times the Just Noticeable Color Difference. Green and Red shifted the least, both by Δ(u’v’) = 0.0022. These values are so low that the Apple iPhone 4 barely shows any detectable color shift with angle.

13. RGB Display Power Consumption: Excellent, Relatively Low

The power consumed by LCD displays is independent of the brightness and color distribution of the images – it only depends on the Brightness setting of the backlight that illuminates the LCD from behind. The Automatic Brightness option allows the ambient light sensor on the Apple iPhone 4 to adjust the backlight brightness and power setting as the ambient light changes. This not only improves visual comfort but can also increase the battery run time. We turned off Automatic Brightness for the tests. It is possible to indirectly determine the power used by the display by measuring the AC power used by the iPhone 4 with different backlight settings.

Table 1 lists the Measured Relative Power, the Measured Luminance, and the Relative Luminous Efficiency, which is just the Measured Luminance divided by the Measured Relative Power, and normalized to 1.0 for White, which has the highest total efficiency.

Table 1. Apple iPhone 4 LCD Display Power Consumption

Maximum Backlight Full Screen	Black	Peak Red	Peak Green	Peak Blue	Peak White
Measured Relative Power	0.42 watts	0.42 watts	0.42 watts	0.42 watts	0.42 watts
Measured Luminance	0.48 cd/m²	104 cd/m²	350 cd/m²	87 cd/m²	541 cd/m²
Relative Luminous Efficiency	0.0009	0.19	0.65	0.16	1.00

14. OLED and LCD Spectra: Very Interesting

The spectra of an LCD display is just the spectrum of the backlight filtered through the individual Red, Green and Blue sub-pixel filters within the panel. OLEDs are emissive devices so the spectra of the Samsung Galaxy S is just the sum of the individual Red, Green and Blue OLED spectra, modified slightly by the touchscreen layer and anti-reflection absorption layer through which their light must pass. We thought it would be very useful and interesting to compare the spectra of the Galaxy S with the spectra of the Apple iPhone 4, so we asked Konica Minolta to loan us their flagship CS-2000 Spectroradiometer to perform the measurements. The spectra for White, which is the sum of the Red, Green and Blue primaries is shown in Figure 4 for both the Galaxy S and Apple iPhone 4.

FIGURE 4

Figure 4. RGB Spectra for the iPhone 4 and Samsung Galaxy S

As expected the OLED RGB spectra are relatively narrow because of their high color saturation. The Apple iPhone 4 LCD RGB spectra is a filtered broadband spectrum. The backlight for the iPhone 4 is a white LED, which consists of a Blue LED with a yellow phosphor.

About the Author

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 Ph.D. in Theoretical Physics from Princeton University, spent 5 years as a Long-Term Member of the world famous Institute for Advanced Study in Princeton, another 5 years as a Principal Investigator in the Computer Systems Research Laboratory at AT&T Bell Laboratories, and has also designed, tested, and installed color 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 dtso.info@displaymate.com.

About DisplayMate Technologies

DisplayMate Technologies specializes in advanced mathematical display technology optimizations 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 Optimization Technology, which can correct or improve many of the deficiencies – including higher calibrated brightness, power efficiency, effective screen contrast, picture quality and color and gray scale accuracy under both bright and dim ambient light, and much more. Our advanced scientific optimizations 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 Optimizer 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 then Contact DisplayMate Technologies to learn more.