STRAY LIGHT ANALYSIS AND DESIGN OF OPTICAL IMAGING SYSTEMS Presented by : Lambda Research Corporation 25 Porter Rd. Littleton, MA 01460 www.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: E PST d Ei ● 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. 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: PST f s( ) s 4F 2 – 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 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..