OLED Drive Current Correction for Voltage Droop in Optical Writing
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Solution Overview
Problem
In optical writing devices using OLEDs and LTPS-TFTs, voltage droop leads to a decrease in light emission over time, causing sub-scanning direction density unevenness in images, which existing solutions attempt to address with multiple sensors increasing device size and cost.
Innovation Solution
An optical writing device with a controller that measures continuous on-state duration of OLEDs and adjusts luminance signals to maintain desired light emission, compensating for voltage droop effects by using digital-to-analog converters and active drive methods to control drive currents, thereby preventing density unevenness without additional circuit components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If LTPS-TFTs are used to drive OLEDs in optical writing devices, then device size and cost are reduced by integrating light-emitting elements and drive circuits on the same substrate, but voltage droop occurs in the TFTs causing light amount decrease and sub scanning direction density unevenness
Solution Approach 1:
The controller performs preliminary measurement of continuous on-state durations for each OLED before image formation, and pre-calculates correction values based on these durations. By preparing correction data in advance according to the actual operating conditions, the system can compensate for voltage droop effects without adding physical correction hardware, thus maintaining device compactness while ensuring density uniformity.
2Manufacturing precision
If light amount sensors are provided for each OLED to measure light amounts and perform feedback control, then density unevenness caused by voltage droop is overcome, but device size and cost increase due to additional sensors
Solution Approach 1:
The invention replaces the physical light amount sensors with a computational measurement approach. The controller measures continuous on-state durations electronically and uses software algorithms to calculate correction values, substituting physical sensing hardware with electronic measurement and computational processing. This eliminates the need for additional sensor components while achieving the same density uniformity correction function.
Solution Approach 2:
The controller acts as an intermediary between the OLEDs and the image formation process. Instead of directly measuring light amounts with sensors, the controller measures continuous on-state durations and uses this intermediate data to calculate correction values that are then applied to the drive currents. This intermediary computational approach achieves correction without physical sensors.
3Reliability
If feedback control based on OLED light amounts is implemented, then voltage droop effects are compensated, but device complexity increases due to additional control circuits and sensors
Solution Approach 1:
The invention replaces complex physical feedback control circuits with software-based correction. Instead of implementing hardware feedback loops with sensors and additional control circuitry, the system uses electronic measurement of on-state durations and computational calculation of correction values. This software-based approach achieves light amount stability while minimizing additional hardware complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution effectively maintains consistent light emission across OLEDs, preventing density fluctuations and ensuring high-quality images without increasing device size or cost, as it compensates for voltage droop and temperature changes, and degradations over time.
Implementation Method 1
optical writing devices including OLEDs arranged in line(s) as light sources
Implementation Method 2
a light-emitting element and a drive circuit (TFT) that supplies a drive current to the light-emitting element
Data Source
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AI summary
An optical writing device including: a current-driven light-emitting element; a thin film transistor configured to supply the light-emitting element with a drive current based on a luminance signal to put the light-emitting element in an on state; and a controller configured to, for each line of an image page, correct a first value of the luminance signal to yield a second value of the luminance signal, and to supply the thin film transistor with the luminance signal at the second value. The second value compensates for a light amount fluctuation of the light-emitting element that is dependent upon an emission/non-emission history, from an initial line of the image page to the line, of a first continuous period where the light-emitting element is kept in the on state and a second continuous period where the light-emitting period is kept in an off state.