Luminance (in)stability in OLED monitors
Topic
It is well known that OLED monitors down-regulate image luminance if, otherwise, the average image luminance would exceed a limit that cannot be handled by the monitor on the long run. However, this is not what shall be discussed here, which is the luminance stability when the monitor is not only set to the "uniform brightness" mode but is even operated well below the maximum luminance specified for the uniform brightness mode. One major difference between LCD and OLED technology is that, in LCD monitors, the light comes from a constant backlight and is just modulated by the pixel cells, whereas in OLED monitors, the light is directly generated by the pixels – on demand, so to speak. Assuming that the bulk of the monitor's energy consumption is spent on generating the light, the energy demand in an LCD is rather constant and much easier to control than in an OLED monitor, where energy demand is tightly coupled to the image content and can rapidly change from one image frame to the next. Moreover, this control of the overall OLED current not only has to be fast but also very accurate, given that there is no averaging out of errors across the OLED pixels – a 1% error in the overall current directly results in a 1% pixel luminance error for each single pixel. The practical relevance of potentially unstable luminance totally depends on the application, e.g. how fast and to what extent the (intended) average image luminance is changing, and how stable the luminance of a potential target stimulus has to be.
Luminance measurement method
A monochrome industry camera (IDS UI-3360CP-NIR, 2048x1088 pixels, 2/3" monochrome CMOS sensor) was used for luminance recording, which allowed to comfortably capture an ROI of 1024x1024 camera pixels with 10 bit gray-scale resolution at ~65 Hz. A 25 mm lens was used with a 20 mm C-mount distancer and a short rubber sleeve (diameter <35mm), which allowed the camera to be brought so close to the screen that a 5·5 mm target patch almost filled the ROI when slightly out of focus. The rubber sleeve was touching the screen and safely shadowed ambient light.
The target was de-focused to help blurring the subpixel structure and thereby equalizing the camera pixel levels. This allowed to choose a wider lens aperture (f≈4) without saturating camera pixels. The camera was operated in freerun mode with full exposure and a gain factor of 1. The camera's image frequency was set to 63.85 Hz, which is different from 60 Hz on purpose for avoiding phase-locking either to the monitor frequency, which was set to 120 Hz (i.e., k·60 Hz), or to the final sampling frequency of 4 Hz used for further analysis of the luminance signal. The stimulus presentation and final luminance sampling followed a 4 Hz clock. In essence, each 4 Hz sampling cycle started with synchronously updating the screen image buffer, whether the image content actually changed or not, and the camera images that happened to fall in this sampling cycle were averaged. Of course, the details were more complicated than that, because camera data arrived with a delay and because some camera images must be ignored which have either been recorded before the screen content has settled or which might be contaminated already by the subsequent screen image updates. These image exclusion criteria were interpreted independently of whether the screen content really changed, which made the 4 Hz samples more comparable in terms of how many camera images went into each luminance sample. The potential settling time after a screen image update, which might also include processing time uncertainties, was set differently depending on monitor technology (e.g., a generous 10 ms for OLED monitors, or 40 ms for the (fast) TN and IPS monitors).
In order to maximize the SNR, the camera image pixels were weighted according to their potential signal contribution. The weight mask was derived from a slightly blurred reference image recorded for a white target over a black background at the beginning of each measurement run. However, such pixel weighting is only beneficial if the image position is very stable over the entire measurement time. Therefore, the exact image position was continuously checked for potential image position shifts. This was also useful for detecting pixel shifts that some OLED monitors exhibit as part of their OLED care feature and which would invalidate measurements with small targets. The target was always surrounded by a black background with a 35 mm diameter to ensure, thanks to the lens rubber sleeve, that the camera image could not be affected by potential stray light coming from the room illumination and from the potentially changing background patterns.