LED Light Measurement Using Stray Light Correction
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Solution Overview
Problem
Existing LED light measurement systems require hardware such as transimpedance amplifiers to accurately measure light output, which can be cumbersome and inefficient.
Innovation Solution
An LED light unit with a driver that includes a light sensor and a current source, where the driver provides an operating current to the LED assembly, interrupts the current to measure stray light, and then applies a measurement current to correct for stray light, allowing for accurate light measurement using minimal additional hardware.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a light sensor with limited measurement range is used, then the device complexity is reduced, but the measurement precision deteriorates when the LED operates at full brightness
Solution Approach 1:
The system uses periodic action by switching between measurement current mode and operating current mode. During measurement intervals, a reduced measurement current is applied to keep the LED output within the light sensor's linear range, allowing accurate measurements without requiring complex hardware. The control device alternates between these modes to collect measurement data while maintaining operational functionality.
Solution Approach 2:
The system applies parameter changes by dynamically adjusting the LED drive current between two states: a first current level for accurate measurement (within sensor linear range) and a second current level for normal operation. The control device measures light output at the first current level, then calculates the light output at the second current level using the measured data and scaling factors, thereby achieving accurate measurement capability without hardware that can handle full operating brightness directly.
2Illumination intensity
If the LED is operated at full current continuously, then the light output is maximized, but the measurement precision deteriorates due to sensor saturation
Solution Approach 1:
The system applies partial action by using a measurement current that is lower than the full operating current. This reduced current level ensures the LED output remains within the light sensor's linear measurement range, avoiding saturation. The control device then uses the measured data at this partial current level to calculate the equivalent light output at full operating current through scaling calculations, thereby achieving measurement precision without sacrificing maximum light output capability.
Solution Approach 2:
The system performs preliminary measurement at a reduced current level before determining the light output at full operating current. The control device first measures the light output at the first current level (within linear range), then uses this preliminary measurement data along with scaling factors to calculate the light output that would occur at the second current level, enabling accurate prediction of full-power performance without directly measuring it.
3Device complexity
If temperature effects are not compensated, then the device complexity is reduced, but the measurement precision deteriorates due to temperature-dependent LED efficiency variations
Solution Approach 1:
The system implements feedback by using the light sensor measurement as feedback to determine and adjust scaling factors. The control device measures the light output at the first current level, uses this feedback information to calculate scaling factors, and applies these scaling factors to determine the light output at the second current level. This feedback mechanism compensates for temperature effects and other variations, maintaining measurement precision without requiring additional temperature sensing hardware.
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
Enables accurate light output measurement with reduced hardware complexity, accounting for temperature effects and saturation levels, while maintaining operational efficiency.
Implementation Method 1
a light sensor configured to sense light emitted by the LED assembly
Data Source
Figure 1~3
Figure 4
AI summary
An LED light unit comprises an LED assembly and a light sensor to measure light emitted by the LED assembly and having a measurement range; a current source to drive the LED assembly at an LED current, A control device is configured to: - pre-heat the LED assembly by driving the current source to operate the LED assembly at an operating current; the LED assembly thereby illuminating the light sensor at a light level above the measurement range; - interrupt operating the LED assembly during a stray light measurement time; and read an output signal of the light sensor; - operate the LED assembly at a measurement current, to emit light at a measurement level; - subtract the output signal of the light sensor during the stray light measurement time from the output signal of the light sensor during the light measurement time to obtain a stray light corrected light measurement signal; - scale the stray light corrected light measurement signal by a scaling factor based on a ratio of the operating LED current and the measurement LED current to obtain a scaled operating current LED light output signal and derive an illumination of the light sensor therefrom.