LED Retroreflector Recycles Light to Escape Étendue Limit

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Solution Overview

Problem

Conventional LED collimators face limitations in achieving narrow beamwidths due to étendue-invariance, leading to inefficiencies in light output and increased heat load when reducing aperture diameter, which is particularly challenging in compact applications like automotive headlights.

Innovation Solution

The use of retroreflectors that utilize specular and total internal reflection to recycle light back to the LED, enhancing luminance and allowing for a smaller exit aperture while maintaining uniform beamwidth, thereby increasing efficiency and reducing heat load.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Volume of moving object

If the aperture diameter is reduced to achieve compact device size, then the device compactness is improved, but the light output efficiency deteriorates due to étendue-invariance

Engineering Contradiction:

Improvedevice compactnessVSAvoidlight output efficiency

Core Design Contradiction:

Volume of moving objectVSProductivity

Solution Approach 1:

The retroreflector continuously recycles light that would otherwise be lost, maintaining useful light output action. Light reflected out of the aperture is returned to the LED and re-emitted, creating a continuous cycle that sustains light output efficiency even with reduced aperture size.

Inventive Principle:

Principle #20Continuity of useful action

Solution Approach 2:

The retroreflector creates a feedback loop by returning light from the aperture back to the LED source. This feedback mechanism allows the system to recover and reuse light that would otherwise be lost, compensating for the reduced aperture area and maintaining light output efficiency.

Inventive Principle:

Principle #23Feedback

2Volume of moving object

If the aperture diameter is reduced to achieve compact device size, then the device compactness is improved, but the heat load increases due to reduced light output efficiency

Engineering Contradiction:

Improvedevice compactnessVSAvoidheat load

Core Design Contradiction:

Volume of moving objectVSTemperature

Solution Approach 1:

The retroreflector ensures continuous useful action by constantly recycling light that would otherwise be lost as heat. This continuous light recovery process reduces the proportion of energy converted to heat, thereby reducing the heat load in compact devices.

Inventive Principle:

Principle #20Continuity of useful action

Solution Approach 2:

The feedback loop created by the retroreflector returns light to the source, preventing energy loss that would manifest as heat. This feedback mechanism reduces the overall heat generation by maintaining higher light output efficiency even in compact configurations.

Inventive Principle:

Principle #23Feedback

3Illumination intensity

If conventional collimators are used to achieve narrow beamwidth, then the beam collimation is improved, but the beamwidth uniformity deteriorates due to comatic aberration

Engineering Contradiction:

Improvebeam collimationVSAvoidbeamwidth uniformity

Core Design Contradiction:

Illumination intensityVSStability of the object's composition

Solution Approach 1:

The system segments the light path into two distinct functions: the collimator handles beam collimation while the retroreflector handles beamwidth uniformity correction. This segmentation allows each component to optimize its specific function without the compromises required in conventional single-component collimators.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The retroreflector acts as an intermediary element that corrects the beamwidth non-uniformity produced by the collimator. By positioning the retroreflector in the light path, it mediates between the collimator's output and the final beam characteristics, restoring uniformity.

Inventive Principle:

Principle #24Intermediary (Mediator)

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 approach achieves a significant increase in light output efficiency, with up to 71% of the original flux maintained at half the étendue-invariant area, while minimizing heat load and reducing system size, making it suitable for compact lighting applications.

Implementation Method 1

The present luminaires exploit the reflectivity of light emitting diodes (or other light sources) relative to external illumination. In particular, an LED will diffusely reflect illumination from the retroreflection of its own emission.

Methodology Applied

Scientific EffectSpecular reflection: Reflection

Implementation Method 2

The present luminaires, however, use retroreflectors, which use specular reflection or operate via total internal reflection (TIR), to return light only to the LED or other light source.

Methodology Applied

Scientific EffectTotal internal reflection: Total Internal Reflection

Data Source

PatentUS7874704B2LED luminance-augmentation via specular retroreflection, including collimators that escape the étendue limit

Publication Date: 2011.01.25 LIGHT ENGINE

AI summary

The diffuse reflectivity of an LED source is utilized to recycle some of its emission, thereby enabling a luminaire to escape the étendue limit. Retroreflectors intercept the rays destined for the outer part of the luminaire aperture, which can then be truncated. The resulting smaller aperture has the same beam-width as the full original, albeit with lesser flux due to recycling losses. A reduction to half the original area is possible.