Smart phone screens have always been the focus of buyers' discussion. At present, the mainstream screens are divided into two types: AMOLED and LCD screens. They have their own advantages and disadvantages in terms of performance, especially in the main indicator of brightness. Will using different methods to test brightness have a significant impact on the results? This is the question we are exploring today.
Before we begin, we still need to move the topic to Samsung's flagship mobile phone Galaxy Note 4 this year. In the evaluation of many websites, the maximum screen brightness of this mobile phone is different. For example, The maximum brightness obtained by the test of GSMArena website is 399 nits, the test result of PhoneArena is 468 nits, the test result of Anandtech is 462 nits, and the test result of Display Mate is 478~750 nits, of which (especially 750 nits) is inevitably controversial.
In addition, the screen brightness of the Nexus 6, which also uses AMOLED screens, is also controversial. The test results of various websites range from more than 250 nits to about 400 nits, which is easy to confuse people.
At this time, someone will definitely say that the test posture of a website is wrong, and even suspect that some websites cheat on the test results, especially the controversial AMOLED screen. Of course, after excluding the individual differences of the mobile phones used in the test, the specific test methods also have a significant impact on the test results, but it is not appropriate to say which is right or wrong.
Before discussing the maximum brightness, let's review the working principle of AMOLED and LCD screens. From the light emission principle, there are fundamental differences between LCD screen and OLED screen, but the problems and limitations they face are really similar. To put it simply, The LCD screen relies on the backlight panel, which can only emit white light. To make the screen display various colors, you need to filter the backlight through the LCD panel to control the amount of backlight passing through, which also involves the principle of polarization, so that the whole screen can work normally.
Because mobile devices now have high requirements for the thickness and power consumption of display modules, the current mainstream LCD screens use the form of edge lighting, that is, LED light sources are arranged in a row on the edge of the screen, and the emitted light passes through a transparent light guide material, so that the backlight can be evenly distributed to every corner of the screen, At present, the backlight of mainstream LCD screens is in the form of blue LED+yellow phosphor to obtain the best energy efficiency ratio, which means that the color temperature of LCD backlight is generally above 6504K, which requires the manufacturer to adjust later to calibrate white (the figure above shows the appearance of the iPod Touch screen after disassembly).
The backlight principle of LCD screen is so simple. The complex part is in the liquid crystal part. Here we take the current mainstream IPS liquid crystal panel as an example (the structure is not complex). As shown in the above figure, 1 and 5 are the upper and lower electrodes of the panel, which are parallel to each other, and can generate electric field between them to make the middle liquid crystal twist. The light emitted by the LED backlight can emit different colors through the twisted LCD array, and the color change is achieved by changing the voltage applied to the electrode by TFT, and finally each pixel can emit color.
As for the AMOLED screen, it is quite different, The foundation of AMOLED is organic light-emitting objects. Thousands of light sources that can only emit one of the three colors of red, green or blue are placed on the screen substrate in a specific form. These light-emitting objects will emit red, green or blue when the voltage is applied. The voltage conversion also depends on TFT. After adjusting the proportion of the three primary colors, Can emit various colors. In terms of structure, AMOLED screen is much simpler than LCD screen, but in fact, AMOLED will encounter more problems than LCD screen.
When testing the screen brightness, we must add the average picture level (APL) to the benchmark for consideration. This term mainly refers to the proportion of the number of lit pixels in the total number of pixels, that is, when a screen displays pure red, pure green or pure blue, the APL value is only 33%.
For the AMOLED screen, the higher the APL value under the same brightness, the more power consumed, because when a whole AMOLED screen displays white, all pixels must glow at the same time, while when it displays black, the corresponding pixels do not glow. On the mobile phone screen, the total maximum power consumption is certain, which means that when the energy consumption of the whole screen remains unchanged, When the AMOLED screen displays a picture with low APL value, the power allocated to a single pixel is relatively large, which means that the lighted pixels can obtain higher brightness.
That is to say, when the AMOLED on the mobile phone displays a non full white image, it will have a higher local brightness. For the common edge lit LCD screen on the mobile phone, because the light source comes from a whole rear light emitting component, there is no higher brightness of some part (assuming that the backlight is uniform), That is to say, the maximum brightness of the mobile phone LCD screen is basically fixed and will not change too much with the APL value.
Take Nexus 6 as an example. It is not difficult to see from the brightness APL value table that the brightness of the screen drops significantly after the APL value exceeds 40%, so it is "scientific" that various websites test the brightness of the AMOLED screen of the same mobile phone is different, because theoretically, It is indeed possible for the AMOLED screen to obtain the maximum brightness of 750 nits at a certain APL value, but it is not possible for us to enjoy the brightness of 750 nits only at 1% of the pixels on the screen in daily use.
In the Android 5.0 Lollipop system, the APL value of the whole system has improved a lot than before, and even the interface with APL value below 50% is quite rare. Again, take Motorola as an example. The average APL value of most bright UI is more than 80%, and this APL value is the most frequently contacted by users on a daily basis. After some tests on daily opened apps and web pages, their APL values are shown in the table above.
It is easy to see from the table that the Android interface, web pages and many third-party apps will have a fairly high APL. Of course, some apps will also provide the so-called "night mode". After this mode is enabled, the APL value will be very low. But in general, especially after upgrading to the 5.0 system and the prevalence of flattening, the APL value of the mobile interface is much higher than before.
Based on the above results, we often take 100% APL as the standard when conducting brightness tests. Indeed, in real life, the APL value will not always remain at 100%. Compare the brightness at 80% APL with that at 100% APL, The screen brightness of Nexus 6 has decreased by 44 nits, with a decrease of 18%. The 18% difference is visible in many cases, and the AMOLED screen also has aging phenomenon, and the brightness will also decrease after aging. In this view, it is also very scientific to set the APL value at 100% to measure the brightness of the screen It can also represent the brightness obtained by users when they actually use their mobile phones to some extent.
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