As OLEDs push toward higher resolution, refresh rate and brightness, display uniformity has become the core bottleneck for panel yield and visual quality. This article explains how Mura arises, how Demura optical compensation works, and what matters most for products.
Mura refers to irregular patches of brightness or chroma non-uniformity on a display. The human eye can perceive brightness differences of about 2%, and the effect is far more visible at low brightness and low grey levels.
OLED Mura is not caused by a single defect or process step — it is the combined result of the entire OLED manufacturing chain.
OLED pixels are driven by thin-film transistors (TFT). When LTPS or LTPO backplanes are fabricated on large glass substrates, devices cannot be produced under perfectly identical conditions: uneven temperature distribution, liquid-flow variations, and inner/outer-zone film differences across the panel — a whole series of thermal, mechanical, optical, electrical and gas/liquid non-uniformities — cause spatial variation in electrical parameters such as threshold voltage, carrier mobility and leakage current of each TFT cell. This directly affects pixel current and ultimately appears as non-uniform display.
💡 At low brightness and low grey levels the pixel current is smaller, so the fluctuation is amplified and Mura becomes much more visible.
OLED emitting and hole-transport layers are deposited by vacuum evaporation. Mask accuracy, angular distribution of the evaporation source and substrate flatness all cause in-plane film-thickness variation. Because luminance depends non-linearly on film thickness, even nano-scale variations produce noticeable brightness differences.
In addition, uneven electrode resistance, micro-deformation stress from the encapsulation layer, and vacuum-chuck marks introduce differences fixed at specific equipment/design positions.
During use, pixels in different regions age at different rates depending on displayed content, leaving ghost-image-like Mura. This happens after the panel leaves the fab — Demura cannot fix it; improvement relies on longer-lifetime emitter materials.
Demura compensates manufacturing deviations algorithmically, following the route of optical inspection → modelling & fitting → data compensation → programming & solidification.
A CCD camera in a dark room captures the actual luminance of sub-pixels at different brightness/grey levels. To balance accuracy, storage and throughput, the system typically captures high/low-brightness frames of pure R, G and B, with grey-level choices adapted to the panel's capability.
The capture system must strictly control shooting distance, angle and ambient lighting to eliminate lens distortion and light-leak errors.
The industry mainly uses the Gamma function to describe the grey-to-luminance response of a pixel:
L = L₀ × (G/255)γ
Multi-grey-level captures fit each pixel's Gamma parameters, and the centre-of-screen values are used to derive the grey-level compensation required by other pixels: reduce grey where too bright, raise grey where too dark — achieving display uniformity.
In practice, to balance precision and storage, a piecewise Gamma fit + interpolation scheme is common: exact compensation values are captured and computed at key grey-level anchor points, while intermediate grey levels get fitted compensation values via lookup tables and algorithmic interpolation.
The compensation coefficient matrix is programmed into the DDIC memory or external Flash. During display, the DDIC reads the original grey-level data in real time, converts it through the Demura lookup module, and outputs the final driving values.
Apart from wearables — whose small size, limited brightness and simple content exempt them — every high-end OLED product line needs a Demura calibration step to raise yield and cut cost. Low-brightness/low-grey Demura is today's focal difficulty, and the challenges keep growing as specifications advance.
Once pixel density passes 500PPI, pixel size shrinks and optical capture becomes harder: the diffraction limit narrows depth-of-field for high-magnification lenses, requiring even higher resolution for sub-pixel-level analysis, while sub-pixel optical crosstalk distortion worsens. Meanwhile, the growing pixel count extends single-frame capture and processing time, squeezing takt time; the compensation LUT swells, hitting storage limits and forcing trade-offs between compensation precision and data compression.
High refresh rates change the pixel driving mode: shorter per-frame charging time amplifies process fluctuation caused by insufficient TFT charging. The same panel shows clearly different Mura distribution and Gamma curves at normal vs. high refresh rates — a single compensation LUT cannot cover multiple refresh-rate scenarios, and switching refresh rates can void the compensation and degrade uniformity. On the capture side, synchronisation between the high-refresh image and camera exposure must also improve; sync errors cause smearing and brightness errors.
Bending mechanically stresses the OLED substrate, shifting TFT threshold voltage and carrier mobility, and micro-deforms the functional layers, causing brightness/chroma shifts in the folded region. Conventional Demura cannot guarantee uniformity of foldable products during use.
High-end screens carry multiple Gamma/brightness modes — standard Gamma 2.2, cinema Gamma 2.6, HDR, low-brightness eye-care mode — each with a different grey-to-luminance response curve. Generating a separate LUT for every Gamma mode multiplies the stored data, raising DDIC cost and programming time.
New high-efficiency emitters are more sensitive to process deviations in film thickness and doping concentration, and more susceptible to backplane current fluctuation. Tandem OLED and similar structures drive multiple emissive units in series, so per-layer process deviations accumulate — Mura causes become more complex, and grey-level compensation alone can no longer calibrate all layers at once, demanding higher-dimensional and more precise compensation models.
📌 This article is industry popularisation only; figures and conclusions are subject to official mass-production data.