Micromirror Tilt Angle Optimization for Compact Projection Contrast

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Solution Overview

Problem

Conventional image projection systems face challenges in achieving higher resolution while maintaining a compact size, as higher resolution spatial light modulators require larger projection optical systems, leading to increased size and reduced contrast ratios due to stray light entry.

Innovation Solution

The image projection apparatus employs a configuration where the tilt angle of micromirrors and the backfocus distance are optimized to ensure that off-light is sufficiently separated from on-light, using conditional expressions to minimize stray light entry and enhance contrast ratio, while also miniaturizing the projection optical system.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If higher resolution spatial light modulators are used, then image resolution is improved, but the projection optical system size increases and contrast ratio deteriorates due to stray light entry

Engineering Contradiction:

Improveimage resolutionVSAvoidprojection optical system size

Core Design Contradiction:

Measurement precisionVSVolume of stationary object

Solution Approach 1:

The patent applies parameter changes by optimizing the tilt angle of micromirrors (θ1 ≥ 14 degrees) and the backfocus distance (BF/L ≥ 1.2) to achieve better light separation. By changing these geometric parameters, the system achieves higher resolution with a more compact projection optical system while maintaining contrast ratio through improved separation of on-light and off-light paths

Inventive Principle:

Principle #35Parameter changes

2Measurement precision

If higher resolution spatial light modulators are used, then image resolution is improved, but contrast ratio deteriorates due to stray light entry

Engineering Contradiction:

Improveimage resolutionVSAvoidcontrast ratio

Core Design Contradiction:

Measurement precisionVSManufacturing precision

Solution Approach 1:

The patent changes the tilt angle parameter of micromirrors to at least 14 degrees, which fundamentally alters the light reflection geometry. This parameter change ensures that off-light is reflected at angles that prevent it from entering the projection optical system, thereby maintaining high contrast ratio while supporting higher resolution displays

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent converts the potentially harmful stray light into a beneficial separation mechanism. By setting the micromirror tilt angle to θ1 ≥ 14 degrees, the off-light that would normally cause contrast degradation is redirected away from the projection optical system, effectively using the reflected light path to achieve both high resolution and high contrast ratio

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Manufacturing precision

If micromirror tilt angle is increased to separate off-light, then contrast ratio is improved, but device complexity increases

Engineering Contradiction:

Improvecontrast ratioVSAvoidoptical system configuration

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

The patent simplifies the optical system by establishing a clear parameter threshold (θ1 ≥ 14 degrees) for micromirror tilt angle. This single parameter change creates a straightforward geometric solution that separates on-light and off-light paths without requiring complex additional optical elements, thereby improving contrast ratio while maintaining relatively simple device structure

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

Instead of using complex optical elements to block or filter stray light, the patent inverts the approach by using the micromirror tilt to actively direct off-light away from the projection path. This inversion of the conventional stray light management approach simplifies the overall system configuration while achieving high contrast ratio

Inventive Principle:

Principle #13The other way round (Inversion)

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 increases the contrast ratio and reduces the size of the projection optical system, effectively addressing the challenge of achieving higher resolution with a compact form factor.

Implementation Method 1

each micromirror configured to change an angle of the reflecting surface with respect to the image display plane to change a direction of the light reflected by the reflecting surface

Methodology Applied

Scientific EffectReflection: Reflection

Implementation Method 2

a projection optical system configured to project the light reflected from the image display element onto a projection surface

Methodology Applied

Scientific EffectRefraction: Refraction

Data Source

PatentUS11630288B2Image projection apparatus

Publication Date: 2023.04.18 RICOH CO LTD

AI summary

An image projection apparatus includes: a light source; an image display element including multiple micromirrors arranged in two dimensions, the multiple micromirrors forming an image display plane, each micromirror having a reflecting surface; and a projection optical system. Conditional expressions (1) and (2) below are satisfied:θ1≥14(deg) (1)1.2<BF/L<2.2 (2)where θ1 is a maximum tilt angle of the reflecting surface of each micromirror with respect to the image display plane, L is a diagonal length of the image display plane, and BF is a distance between a vertex of a lens within the projection optical system and closest to the image display plane and the image display plane along an optical axis of the projection optical system.