1. FRED
Stray Light
FRED: A software tool
for modern engineering

2. What is FRED? FRED Overview
Comprehensive opto-mechanical systems design analysis tool
Stray light analysis (scatter and ghosting)
Coherent beam propagation
Imaging system design and photo-realistic rendering
Virtual prototyping
Opto-mechanical systems debugging (Forensic Optics)
Thermal self-emission analysis
Illumination design
Tolerancing, fabrication and testing
Both sequential and non-sequential raytracing modes
Multi-variable optimization capability

3. What is FRED? FRED Overview FRED
FRED can be used in virtually every step of the design process.
Initial proof of concept using catalog parts
Integration of optical and CAD designs
Full analysis of a virtual prototype including scatter, coatings, dispersion, absorption, volume scatter, diffraction, thermal emission, source characteristics…
Quickly manipulate model parameters for tolerance analyses
Integration with vendor catalogs for manufacturing or system debugging
Concept 1
FRED
Design 2
Analysis 3 4
Tolerancing 5
Manufacture

4. FRED Sources of Stray Light Common stray light sources include:
Diffraction
Specular ghosts
Grating effects from manufacturing
Scatter from structures
Particulate contamination
Thermal self-emission
Background sources
Bulk material effects

5. FRED Effects of Stray Light Stray light = performance reduction
Stray light should be accounted for during design
Correcting stray light requires knowledge of the source
FRED provides engineers a toolbox for stray light analysis

6. FRED Stray Light Tools Surface Scatter Models Lambertian Harvey-Shack
ABg
Phong
Diffuse Black Paint
Particulates (Mie)
Diffuse Polynomial
Diffuse Binomial
Tabulated BSDF
Tabulated PSD
K-correlation
User-scripted (BASIC)

7. FRED Stray Light Tools Scatter Importance Sampling
Scatter rays only into regions of interest
Saves time by raytracing efficiently
Correctly accounts for ray directions
Importance Sampling Options
Angular spread about specular
Angular spread about a given direction
Angular spread about a given point
Towards an entity
Through a closed curve
Through a volume

8. FRED Stray Light Tools Volume Scattering
Stray light effects can come from bulk material volume scatter
Implemented as “random walk” using a mean free scatter distance
Translucent materials, suspended particulates, etc.

9. FRED Stray Light Tools Surface Raytrace Controls
Allows specific interaction types: scatter, transmission, reflection etc.
Set ray power cutoff thresholds
Specify number of allowed ray splits due to scatter or specular
Precise control over model tracing
Applied to individual surfaces

10. FRED Stray Light Tools Raypaths and Stray Light Report
Keeps track of ray intersection paths
Sorts raypaths by scatter and specular
Reports scatter/ghost level
Provides information on ray count, path power, scatter/ghost surfaces
Paths can be marked for sequential tracing
Raypaths can be redrawn to the 3D view

11. FRED Stray Light Tools Diffraction Grating structures Linear
XY Polynomial with aspheric terms
Diffraction from apertures, edges, slits, etc. when tracing coherent rays

12. FRED Stray Light Tools BASIC Compiler and Scripting
FRED’s fully integrated BASIC scripting language allows for custom calculations such as thermal self-emission and PST

13. FRED Stray Light Example Ghosting in a Double Gauss Lens
10 degree off axis angle
Uncoated lenses
Allow only first order paths
straylight_ghostImage.frd

14. FRED Stray Light Example Ghosting in a Double Gauss Lens
Allow all raytrace paths
Irradiance distribution shows ghost image

15. FRED Stray Light Example Ghosting in a Double Gauss Lens
Use FRED’s Stray Light Report to identify the ghost path(s)
Ghost is caused by a reflection from surface 10

16. FRED Stray Light Example Ghosting in a Double Gauss Lens
Apply an AR coating to surface 10
Ghost image power reduced by an order of magnitude!

17. FRED Stray Light Summary
FRED provides engineers with a comprehensive toolbox for stray light analysis
Using FRED during system design can be invaluable
Models are easily constructed in the 3D cad environment with realistic opto-mechanical properties
FRED’s built in compiled scripting language adds flexibility for customizable analyses