Measurement of the Optical Properties of Complex Surfaces James - - PowerPoint PPT Presentation

The Prediction and Measurement of the Optical Properties of Complex Surfaces

James Jafolla Surface Optics Corporation ITBMS 2018

Outline

• Introduction
• BRDF Definition
• BRDF Models

– Micro-Optic Surface Model – Sandford-Robertson Model

• BRDF Measurements
• BRDF Model-Measurement Comparison
• Implications for Signature Analysis

Introduction

• Development of advanced CGI rendering tools

requires better understanding of light scattering from surfaces

• Virtual prototyping for signature analysis and

system design requires accurate radiometric analysis

• Most current computer analysis relies on

simplified assumptions of surface optical properties

• Phenomenological ray tracing analysis provides

accurate prediction of optical properties

Bidirectional Reflectance Distribution Function (BRDF)

i = Incident Zenith Angle

r

= Reflected Zenith Angle



= Reflected Azimuth Angle BRDF: (

)

    , ,

r i



( ) ( ) ( )

i i r i i i r r

d N dN          cos , , ,  =

DHR:

( )

 

D i

( ) ( )

  =          d r d r r r i i i N r N sin cos , ,

( )

i D 

 =

(These apply to isotropic surfaces; also,

i r

   − 

here.)

D

   = 

For Lambertian Diffuse Surface

Modeling the BRDF of Complex Surfaces

• Many surfaces exhibit a complex surface

features

• Large scale features exhibit multiple

reflections and blocking and shadowing

• Small scale features based on

homogeneous material optical properties, surface roughness

• Non-homogeneous mixtures of materials

Micro-Optic BRDF Calculation

• Approach defines Micro-Optic

facet model of surface

• Ray-tracing includes reflections

and blocking between facets

• Sandford-Robertson model for

facet optical properties

Geometrical Surface Structures

• Structured Surfaces for Thermal Design
• Ref. Seigel R. and Howell J., Thermal

Radiation Heat Transfer, McGraw-Hill, 817pp, 1972

• Arbitrary Structure Surfaces

Represented as Faceted Wire-Frame Models

• Arbitrary Surface Coatings on Facets

Pyramid Surface Definition

Machined Surface

SOC-210 Bi-Directional Reflectometer

Unpolarized BRDF Bidirectional Transmittance Distribution Function (BTDF) Automated Full Hemispherical Mapping: i = 0 through +85 r = 0 through +85 i = 0 - 350 r = 0 - 345 Spectral Coverage: Sources 0.35 – 1.6 m quartz halogen 1.5 – 16.0 m SiC glower source Assorted bandpass filters Detectors 0.35 -1.0 m Silicon detector 1.0 – 1.6 m InGaAs detector 2.0 – 14 m dual InSb/MCT

BRDF Measurements

Sandford-Robertson Model

• Based on the separation of the spectral and

directional dependence of the total BRDF

• Fits four parameters to the BRDF

D(l) = Diffuse Spectral Reflectance e (l) = Spectral Emissivity b = Grazing Angle Reflectivity e = Width of Specular Lobe

) ( ) , ; , ( ) ; , ; , ( l      l     

r r i i r r r i i

f = 

) , ; , ( ) , ; , ( ) , ; , (

r r i i D r r i i S r r i i r

f f f             + =

Sandford-Robertson Model Fit to Measurements

Analytical SR BRDF Models

Good Specular Mid Specular Bad Specular

Model/Measurement Comparison

Prediction Measurement Theta Incident 40 deg

Study of Grooved Surface

• 30 degree right triangle surface wedge
• Movie generated of BRDF for high

resolution steps in Phi Incident for various Theta Incident angles

• Good specular surface BRDF

Incident Angle 35 Degrees

Incident Angle 55 Degrees

Incident Angle 65 Degrees

Implications For Signature Analysis

• Regular (structured) surface features

produce multiple BRDF lobes

• Lobes move predictably as a function of all

four incident and scattered angles

• Inclusion in signature models using high

resolution data tables and interpolation possible

• Parameterized models have been

attempted

Hilgers BRDF Model

• Developed to Model Materials with Multi-Lobe BRDF

Features

• Gaussian Lobe Shape
• Closed Form Solution - No Iterative Fitting
• Lobes “Tracked” to Define Significant Features

– i.e., Lobe Bifurcation or Coalescence

• Rapid Generation of BRDF for Rendering Applications





−

=  Ae k k

r i

) ˆ , ˆ (

2 2

) ( ) )( ( 2 ) (

r r r r r r r r

c b a          − + − − + − =

A determined from BRDF peak determined from first moment of BRDF a, b, c determined from second moment of BRDF

r r 

 ,

Multi-Lobe BRDF Comparison

MicroOptic BRDF Gaussian Model

Qi=10 i=0 Qi=50 i=40

Full BRDF Provides Realistic Rendering

Courtesy: George Borshukov, ESC Inc.

Computer rendered character from a “famous” movie.

photo rendered

Courtesy: George Borshukov, ESC Inc.

Conclusions

• Simple parameterized BRDF models not well suited for

complex surface treatments

• Micro-Optic BRDF model provides ability to represent

complex, non-homogeneous surfaces

– Geometric facet model of surface – Multiple bounce ray-tracing calculation – BRDF of homogeneous facets computed from Sandford- Robertson model

• Good agreement of BRDF calculations to

measurements for a manufactured structured surface

• More work needed for including these effects into

signature calculations