Smartphone Displays

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Luminance

A display’s brightness (or luminance) is determined by the amount of light emitted, measured in candelas per square meter (cd/m²). The brighter the display the better.Bright displays are needed in sunlit environments (outdoors or well lit interiors) to create sufficient contrast for screen images to be seen clearly. Most Smartphone displays have a luminance that ranges from 200 to 550 cd/m2. By comparison, HDTVs range from 450 to about 1000 cd/m2. A luminance of at least 800 cd/m2 is required for a screen to be easily viewable in direct sunlight.


Contrast Ratio

Contrast Ratio measures the difference between the brightest white and the darkest black that a screen can display. Screens with high contrast ratios produce crisp images with subtle color variations and easy to read text. It is an excellent measure of a Smartphone screen’s readability.Based on the screen technology employed, contrast ratios on Smartphones range from several hundred to one to tens of thousands to one.

Basic Display: below 1000:1
Standard Display: above 1,000:1
Performance Display: above 10,000:1


Color Depth

Color depth indicates the number of colors a device is capable of displaying on the screen (aka its Color Gamut). The higher the number, the better.

For Smartphones, a “standard” device can be expected to display 65,536 colors (16 bit color). A “performance” device can be expected to display 16.7 million colors (24-bit color).


Screen Size

Determining which screen size works best for you requires that you find a balance between readability and portability. If you pick a phone that’s too small, it’s screen is unreadable. If you pick a screen that’s too big, the phone won’t fit comfortably in your hand, your pocket or your purse.

Smartphone screens are measured diagonally.(i.e. the distance between two diagonally opposing corners of the screen). The current crop of Smartphones have screens that measure between 2.2” (56mm) and 5” (127mm). The most popular Smartphones have screens that measure at least 3.5” (89mm). Screens below 2.5” (64mm) are hard to see and difficult to navigate. Too much panning and zooming is required. Most users find the experience unpleasant and fatiguing. This is especially true for older users and the visually impaired.

Smartphone manufacturers group their screens into the following categories:

Small size screens: below 3”.
Medium sized screens: between 3” to 4”
Large size screens: more than 4”.

Screen Size is combined with Pixel Count to determine a screen’s Pixel Density.


Resolution

Resolution (aka Pixel Count) is one of the ways that manufacturers have of describing screen quality. It’s simply a tally of the total number of pixels on a screen. In theory, the higher the number, the better the screen.

Resolution can be combined with Screen Size to determine a screen’s Pixel Density..


Pixel Density

Pixel Density is the single most important indicator of screen quality. It is measured in pixels per inch (ppi). The higher the number, the better the screen and the crisper the image.

Screens with a pixel density below 200 ppi are considered low resolution. Screens with pixel densities between 200 and 300 ppi are considered to be average quality. Screens with a pixel density above 300 ppi are considered high resolution.

Selection tip: if you compare two different Smartphones with the same size screen, and all other considerations are equal, pick the one that has the higher pixel density.


Display Technology

The Smartphone market is dominated by 3 different display technologies. The most popular is called Thin Film Transistor Liquid Crystal Displays*(TFT-LCD). TFT-LCD screens are relatively low cost and easy to produce. They work well in indoor light. Despite their popularity, they have some significant drawbacks. Most importantly, TFT-LCD screens can only be viewed from a narrow angle (typically about 35 °). In addition, TFT-LCD screens can only display a limited range of colors, and colors tend to shift as viewing angles change. TFT-LCD’s have a relatively slow screen refresh rate (aka image switching) which makes the technology unsuitable for watching HD videos.

In-Plane Switching Liquid Crystal Display*(IPS-LCD) screens are an advanced version of TFT-LCD technology. IPS-LCD screens can be viewed from much wider angles than TFT-LCD screens. The technology renders colors very accurately (i.e. it supports a wide color gamut) and has a high contrast ratio which means it can be viewed in direct sunlight and used very effectively as an eBook reader. All-in-all, IPS-LCD screens have better picture quality than a conventional LCD’s and they consume less power.

Super Active-Matrix Organic Light Emitting Diode*(Super AMOLED) display technology is currently considered the best on the market. They are light weight and durable. They can be viewed from a very wide angle with no image distortion of color shift. They have a very high screen refresh rate which makes them ideal for watching HD videos and animation. They are also more energy efficient than older display technologies. In addition, they also have very high contrast ratios so they can be viewed in direct sunlight and used as eBook readers. They render colors very accurately and have the highest pixel density of any Smartphone display technology. The result is superb image quality.

source: Smartphone Buying Guide > Technology Overview


thanks to the retina display on the iphone 4, the new generation of the high end phones has high pixel density(ppi) like the galaxy nexus and galaxy s3, htc one x... images are now clearer and sharper, once you see this high pixel density screen you will never see the older display the same way as before, and since apple has started this trend again in tablets and laptop, in the coming next generation of this devices, expect pixel density to play a factor again



iPad 2 on the left, the new iPad on the right

so next time you will buy your next smartphone, put into consideration the ppi or pixel density