a pragmatic speckle measurement method

Dr. Yanning ZhaoVisteon Electronics Germany GmbH, Germany

24.05.2016

A Pragmatic Speckle Measurement Method

Outline

1. Objective

2. Measurement setup

3. Critical parameters

4. Measurement procedure

5. Discussion

6. Conclusions

of 14

of 14

Driver information display for ADAS

Advantages of laser projection display

1. Bigger color range

2. Better contrast ratio

3. Tolerance to surface variation

4. Narrow spectral linewidth suitable for holographic application

of 14

of 14

Laser speckle

Laser

Camera

Subjective speckles Speckle contrast Image degradation

Measurement setup

Camera

Laser driver

Laser head

Opt. Spectrometer

Screen

Opt. Power meter

Light source: OSRAM laser Piccasso laser module with direct green and blue laser diode.

Camera: OSCAR F-810, AlliedVisionTechnology. (P=2.7μm, Resolution=3272*2469)Objective lens, Pentax TV Lens 75mm C7528-M, f/# = (2.8,32)

Optical spectrum analyzation:Thorlabs SA201 Spectrum Analyzer ControllerThorlabs CCS100 Spectrometer

Optical power meter:Coherent OP-2 VIS mit Coherent FieldMax II

of 14

Critical parameters

Laser source:

Camera:– Pixel size: 2.7µm– Focus length: f = 500mm (constant) – Integration time: depending on the light source intensity, need to be optimized

for individual measurement– Camera position: The position and angle between the paths from laser to screen

and from screen to camera kept constant and smaller than 5°

of 14

Color Wavelength (nm) Optical power (mW)red 639.81 3.77

green 514.30 4.21

blue 451.09 2.15

Critical parameters- f/# number

For a reliable measurement: D > 5 camera pixel length (2.7μm)

Speckle diameter in diffractive limited imaging systems:D = 1.22∙λ∙(f/#) [1][2][3]

Theoretically calculated (f/#): (f/#)Red > 9 (f/#)green > 11

(f/#)blue > 13

Chosen value: f/#=f/#max=32

of 14

f/#=32

[1] J. Keprt, et al., physica 38, 115-125, (1999).[2] E. Archbold, et al., Opt. Acta 17, 883-898, (1970).[3] K. A. Stetson, Opt. Eng. 14, 482-489, (1975).

f/#=16

f/#=6

Measurement procedure

of 14

No

Measurement procedure

of 14

No Saturation point of the camera

Dark current contrast dominating Speckle contrast

resolution increasingSpeckle contrast limited by camera saturation

Dig

ital i

nten

sity

(0-2

25)

Spec

kle

cont

rast

Integration time (µs)

Green laser speckle contrast with integration time

Max IntensitySpeckle Contrast

x 105Optimized integration time = 150 ms

Data processing

Reduce the background illumination inhomogeneity Calculated the intensity histogram of the speckle pattern image

of 14

Original image speckle contrast 0.554

Background removed image speckle contrast: 0.270

Gaussian smoothed background

8-bit Intensity (0-255)

Pixe

l num

bers

Mean value = 49.0Standard deviation = 23.8Speckle contrast = 0.485

Data processing-Speckle contrast measurement result

Calculate the speckle contrast for the whole image and local areas:

of 14

Color With inhomogeneous background Homogenized speckle pattern imagered 97.5% 48.0%

green 50.2% 29.2%

blue 61.3% 45.5%

Original image Dark background Speckle contrast over nxn, Contrastmean=0.274

Discussion - Theoretical laser linewidth calculation

Theoretical speckle contrast C(λ,δλ,cσt ) is determined by:

For normal copy paper as screen σt ≈ 1.681 ps

of 14

[4]

[4] F. Riechert, „Speckle Reduction in Projection Systems“, Universitätsverlag Karlsruhe (2009)

Color Inhomogen-eously illuminated Csp

Homogenized Csp

Calculated linewidthmin (nm)

Calculated linewidthmax

(nm)

Typical datasheet linewidth (nm)

red 97.5% 48.0% 0.06 0.76 0.04green 50.2% 29.2% 0.46 1.38 0.5-2 blue 61.3% 45.5% 0.22 0.43 0.5-2

Conclusions

1. Presented a pragmatic approach to quantify speckle behavior for laser projectors with commercially-available equipment

2. Standardized steps make the comparison of laser projectors’ image quality regarding to speckle fast and convenient

3. Provides the foundation of future work of laser speckle reduction in projection display applications

of 14

Q&A

Thank you for your attention.

Dr. Yanning Zhao (赵燕宁 )Visteon Electronics Germany GmbHVisteonstr. 4-10, 50170 Kerpen, GermanyPhone: +49 2273 595 4085 Mobile: +49 162 109 1050 E-Mail: [email protected]

www.visteon.com

a pragmatic speckle measurement method

Automotive