STRAY LIGHT ANALYSIS AND DESIGN OF OPTICAL IMAGING SYSTEMS

Presented by :Lambda Research Corporation25 Porter Rd.Littleton, MA 01460www.lambdares.com

Topics

● Definition of stray light

● Quantifying stray light

● Causes of stray light

● Examples of stray light

● Predicting stray light (examples using TracePro®)

● Stray light aspects of optical designs

● Design methods for reducing stray light

What is stray light?

● Stray light is any unwanted light in an optical system

● Stray light may cause:

– Reduction in image contrast

– Uneven haziness in an image

– Artifacts like ghost images

– Other effects like narcissus in infrared systems

● Common stray light sources

– The sun

– Earth and moon (spaceborne optical systems)

– Any bright light source in or near the field of view

● Note: modern software (e.g. Photoshop ®) lets you add fake stray light to make images look more “real!”

Quantifying stray light

● Stray light is manifested by stray irradiance at the image plane of an optical imaging

system

● Stray light is characterized by the Point Source Transmittance (PST), a ratio of

irradiances:

● PST is unitless and is analogous to the Point Spread Function (PSF) for characterizing

imaging performance.

● PST is useful for comparing the stray light performance of disparate optical designs.

● Stray irradiance from a source is calculated from the PST by integrating over the

luminance of the source, weighted by PST.

i

d

E

EPST

Causes of stray light in imaging systems

● “Straight shots” or zeroth-order stray light

● Ghost images from lenses (and mirrors and sensors!)

● Scattering from optical surfaces

– Surface micro-roughness

– Particulate contamination

● Bulk or volume scatter from lenses and windows

● Scattering from mechanical surfaces

● Diffraction from apertures or other edges

● Combinations of the above

Stray light example

Photo courtesy of NASA

“Zeroth-order” paths or straight shots

Cassegrain-type designs

Mechanical light leaks Leaks around lens mounts

Ghosts – reflections from lens surfaces

Imaging ray

Ghost rays

Scattered stray light

Scatter from optical surfaces

Multiple scatter

Predicting stray light

● A first-order formula for estimating PST:

– Where fs = BSDF of primary optical surface, F= f-number, s = shading function

● For accurate predictions, ray tracing simulation with a Monte Carlo program is needed

– Build a stray light model

• Import optical design and mechanical design and combine

• Create and apply surface and material properties

• Define source(s)

• Define importance sampling

– Trace rays

– Calculate PST or other stray light metric

)(4 2

s

FfPST s

Estimating PST by reverse ray tracing

● Forward ray tracing is limited to estimating PST for one field point for each simulation

● Reverse ray tracing enables estimating PST for all angles with one long ray-trace.

– Make a Lambertian source emitting from a small region on the sensor or focal plane,

with total flux equal to π.

– The intensity pattern emerging into object space (units W/sr) is approximately the

PST vs angle (if you ignore the units).

● For more accurate analysis, and to determine causes of stray light for a particular point

in object space, requires a forward ray-trace.

● Reverse ray tracing is useful for locating “hot spots” where the PST is high, i.e. where

the optical system is sensitive to stray light sources.

Example of reverse ray tracing to estimate PST

● PST for the hemisphere of

object space can be

estimated with one massive

reverse ray-trace.

● This example had 100 million

rays starting from the

detector/sensor surface.

● Even more rays are needed if

a smoother distribution is

desired.

Forward ray tracing tools

● Irradiance Map and 3D Irradiance Map to measure PST at the sensor.

● Ray Sorting to separate stray light into categories.

● Path Sorting to summarize flux reaching a surface for each unique path.

● Path Sort Filtering to group together similar Path Sorting paths.

● Flux Report to summarize where light is incident and absorbed in the optical system.

● Incident Ray Table is a list of all rays incident on a selected surface.

● Ray History Table gives the history of every ray incident on a surface in complete detail.

● The following features interact with the display of rays and the Irradiance Maps:

– Ray Sorting

– Path Sorting and Path Sort Filtering

– Incident Ray Table

Ray tracing modes

● Analysis Mode saves all data for every ray. Consumes more memory, but allows

interactive display of analysis results for any surface or object in the model.

● Simulation Mode is used when accurate (reduced noise) results are needed for final

evaluation of an optical system, after all design and diagnostic tasks are completed. You

must choose which surface(s) for which you will get analysis results before beginning

the ray-trace.

Forward ray tracing – Analysis Mode, 18˚ off axis source

PST for single surface scatter (Ray Sorting example)

Incident Ray Table with “Display Selected Rays”

Path Sorting – ghost path selected for display

Flux report – incident and absorbed flux

Ray History Table for each ray

Optical design types vs stray light

Good Bad

Reflective Refractive (ghosts)

Unobscured Obscured (hard to baffle)

Unfolded Folded (extra stray light paths)

Reimaging with field and Lyot stops Simple imaging (more diffraction and scatter)

Narrow FOV Wide FOV (hard to baffle)

Reimaging design: field stop and Lyot stop

Aperture stop Field stop Lyot stop Final Image

B. Lyot, “A Study of the Solar Corona and Prominences without Eclipses,” Royal Astron. Soc. Month. Not. 99, pp. 580-594 (1939)

An optimal optical design for stray light suppression

● Off-axis optics

● All reflective

● Reimaging

with field and

Lyot stops

● No fold

mirrors

● Narrow FOV

Basic baffle design rules

● In-FOV sources

– Eliminate straight shots

– Beware of ghosts – change lens curvatures and/or AR coating

– Reduce or eliminate “critical surfaces”

● Out-of-FOV sources

– Eliminate straight shots

– Beware of ghosts – change lens curvatures and/or AR coating

– Reduce or eliminate “critical surfaces”

– Reimaging design virtually eliminates diffracted stray light

– Make sunshade as wide and long as possible.

• Goal: shield optics from direct illumination by stray light sources

Designing vanes for a sunshade: no first diffuse reflections

● Establish region for vanes to stay within.

● Trace a ray entering the sunshade at an extreme angle.

● Draw a ray (line) from the scatter point to the edge of the primary optic.

● Insert a baffle vane to block this path.

● Repeat this process until the reaching the primary optic.

● The deeper the vanes (wider the sunshade) the fewer vanes are needed.

● The vanes should have a chisel edge so they have small critical area and therefore do

not cause much stray light.