Presentation of the Secondary ImageMeasuring Instrument

Dr. B. Grubert

7.th International CTI-Forum ”Automotive Glazing“

October, 8th 2013

Overview

1 Introduction to MOLLER-WEDEL OPTICAL GmbH2 Causes and properties of the secondary image3 Presentation of the laser-based measurement method4 Comparison with the method according to ECE R43

Introduction to MOLLER-WEDEL OPTICAL GmbH

Established 2000 asindependent subsidiary ofMOLLER-WEDEL GmbHLocated: Wedel nearHamburgMid-size companyMember of the Swissbased HAAG-STREITHolding AGMain products: Opticalmeasuring and testinginstruments

Appearance of the Secondary Image

Double image with 6 arcmin secondary image angle separation.

Limits for Secondary Image Angle Separation

Investigations showed that a separation below 10 arcsecare not recognised as annoying.According to the regulation ECE R43 the largestpermissible value is 15 arcmin for area A and 25 arcmin forarea B.

Secondary Images in Windshields Caused by Wedge

The secondary images are caused by internal reflectionsin the windshield.

Secondary Images in Windshields Caused by Wedge

The secondary image angle is the angle between primaryand secondary image beam.

Calculation of the Secondary Image Angle Separationfor Wedges

ε ≈ 2α√

n2 − sin θ2

cos θ

The amount of secondary image angle εincreases with increasing internal wedge angle α of thewindshield,increases with increasing tilting angle θ of the windshield,increases with increasing refractive index n of thewindshield material.

Secondary Images in Windshields Caused byCurvature

Calculation of the Secondary Image Angle Separationfor Curvature

ε ≈ 2t tan arcsin sin θ

nR

3438 arcmin

The amount of secondary image angle εdecreases with increasing radius of curvature R of thewindshield,increases with increasing angle of incidence θ on thewindshield,increases with increasing thickness t of the windshield,decreases with increasing refractive index n of thewindshield material.

Measurement Principle of the Secondary Image AngleMeasuring Instrument

Image of Camera with Linear Sensitivity

Secondary image not visible.

Brightness Ratio Primary to Secondary Image

The primary image is usually 50 ∼ 100 times brighter than thesecondary for incidence angles 50◦ ∼70◦.→ for a camera with 8-bit resolution depth the brightness of

the secondary image is only 3 to 5, when the primaryimage has maximum brightness (255).

Image of Camera with Linear Sensitivity(Overmodulated)

Primary image is strongly overmodulated in order to makesecondary image visible

Image of Camera with Logarithmic Sensitivity

Primary and secondary image are visible.

Polarisation of the Secondary Image

The secondary image is predominantly linearly polarised inthe perpendicular plane for incidence angles ∼60◦ (∼Brewsterangle).→ The contrast between primary and secondary image can

be improved by a factor of 2 when using linearly polarisedillumination instead of non-polarised.

Illumination with linearly polarised light — paralleldirection

Secondary image not visible.

Secondary Image Angle Measuring Instrument

Features of the Secondary Image Angle Measurement(SIAM)-Instrument

Instrument is based on a proven set-up used at VW.No disturbance by local refractive index and powervariations due to small diameter of the test beam.Easy and fast set-up of the instrument.Laser is eyesafe (laser safety class 2).Measurement value is independent of the distancebetween laser, windshield and telescope.Measuring range: 68 arcmin × 50 arcmin.Measurement uncertainty: better than 0.5 arcminObjective measurement results due to PC-based imageevaluation software.

Features of the Evaluation Software

Real-time evaluation with 25 Hz frame rate.Automatic brightness control.Evaluation according to ECE R43.Display of the double image angle with magnitude (ε) anddirection (ϕ).Use of predefined customisable measurement templates.Generation of protocol (PDF or ODT).Export of measurement results to spreadsheet software(CSV-format).

Windshield View of the Software

Live-Image View of the Software

Protocol View of the Software

Sketch of the Set-Up for Conformity Check Accordingto ECE R43

Double3Collimator Telescope3with3Camera

Telescope3with3CameraFolding3MirrorLaser

120°

150°

210°

240° 300°

330°

30°

60°

10´

10´

20´

20´

30´

30´

10´

10´

20´

20´

30´

30´

12,6S33330°

Beam-SplitterCube

Beam-SplitterCube

TestWedge

Photograph of the Set-Up for Conformity CheckAccording to ECE R43

Camera Image from the Set-Up According to ECE R43

Results for Magnitude of Secondary Image Angle

Results for Direction of Secondary Image Angle

Summary

1 The secondary image is predominantly linearly polarised.2 By using a camera with logarithmic sensitivity and linear

polarised illumination the visibility of the secondary imageis strongly improved.

3 Automatic real-time evaluation allows objectivemeasurement of the secondary image angle separation.

4 Evaluation is according to ECE R43.