Over the last few years we’ve been repeatedly reporting how Samsung was able to improve AMOLED power efficiency by employing better emitter materials on each new device generation. This went largely unverified as nobody really tried to accurately measure the actual power consumed by the screen. To rectify this situation, I went ahead to try to measure the screen power consumption of the last 4 generations of Galaxy S devices to get a better understanding just how much emitter improvements have contributed to screen and device battery efficiency.

AMOLED screens are emissive displays, meaning they emit light on their own without having to resort to backlights in transmissive technologies such as LCD screens. This causes white to be the most power-intensive color for an OLED screen to display because it requires all sub-pixels to operate at high emission. The advantage of OLED screen though is that for dynamic content, the technology is able to save power in comparison to transmissive displays because only the pixels which are actually used are powered up.

Because white is a worst-case power scenario, it serves well as good representation of general efficiency gains in OLED devices. I've noticed that the power composition of the display consists of the sum of individual component’s power, and was able to verify this on the Galaxy S6: the sum of pure red, green and blue (in sRGB) was largely correlating with the power consumption of pure white when you take into account APL (Average Picture Level) brightness adjustments of the display.

To get a meaningful representation of efficiency of the displays, we chart the power curve of the Galaxy S4 (E5410), Galaxy S5 (S801), Galaxy S5 LTEA and finally the newest generation Galaxy S6 at different screen luminance points:

Immediately the Galaxy S4 sticks out as the most power consuming device of the last few generations. One has to keep in mind that most of this power is not due to the display itself, but due to the lack of Panel Self Refresh on past generation devices. PSR is able to achieve better power efficient in static screen scenarios by not re-sending frame-buffer information to the screen’s display controller IC. The DDIC instead buffers the screen contents on-chip and refreshes the screen matrix that way, enabling the SoC’s display controller pipeline to shut down and save on power.

Due to inconsistencies in SoCs, DDICs and general component efficiency, each device starts at a different power value at minimum brightness. For example the Galaxy S5 is able to post by far the best value in this metric as it will consume down to 258mW while idling at minimum brightness. The Galaxy S5 LTEA and Galaxy S6 will use a tad more at around 360mW each. This is likely due to the fact because of the heightened resolution of their 1440p screens compared to the 1080p screen on the Galaxy S5.

To be able to determine the efficiency of the OLEDs themselves though, we have to look at how the luminance curve behaves for each device. To do this, one has to adjust the power numbers to compensate for this base-power consumption: