Diffraction

Diffraction Standard Expert

Aperture diffraction is light “bending around obstacles.” TracePro can model edge diffraction, also called aperture diffraction, which occurs when light is partially blocked by an edge. The method TracePro uses for modeling diffraction is asymptotically correct, i.e., it is correct for large-angle or wide-angle diffraction. A more complete discussion of the diffraction model is given in the Technical Reference section.

(See “Aperture Diffraction Example” on page 9.22.)

Defining Diffraction in TracePro

Diffraction results in interference downstream from the obstacle. When light passes an obstacle and part of it is blocked by the obstacle, diffraction occurs. This physical process places a limit on the resolution of a lens and may cause stray light.

Diffraction is an interference phenomenon. When light waves propagate unobstructed, they are continuously and constructively interfering as they propagate. In other words the interference pattern is in some state of equilibrium, providing a wavefront that describes the propagation from each point of the emitter. When a wave is interrupted by an obstacle, the wavefront equilibrium is upset. Thus, interference ripples occur upon further propagation. In the far field, an equilibrium is reestablished. When a wave is interrupted by an obstacle, the wavefront is suddenly incomplete. Interference ripples occur.

A two dimensional diffraction-interference phenomenon can be observed when water waves strike a breakwater. A long, straight wave striking a breakwater (with the wave parallel to the breakwater) results in approximately circular waves emanating from the end of the breakwater. Far away from the breakwater, the original straight wave continues uninterrupted. The circular wave interferes with the straight wave in the “slit” region, causing interference maxima and minima-- and causes waves to go into the “shadow” region where the casual observer might not expect waves to go at all. This effect is analogous to the diffraction of light.

A complete theoretical treatment of diffraction is a mathematical subdiscipline of physics, and is beyond the scope of this manual. For the interested reader, an introductory treatment of diffraction can be found in most any optics textbook. Some additional technical details can also be found in the Technical Reference section.

Do I need to Model Diffraction in TracePro?

TracePro’s implementation of diffraction modeling is designed for stray light analysis and for system transmission studies of specialized optical instruments where diffraction might have a significant effect on transmittance.

Generally, image quality diffraction studies are done using optical design software such as OSLO.

If you are modeling diffraction for stray light, it is necessary to define importance sampling for each diffracting aperture in the optical system. The importance

TracePro 5.0 User’s Manual

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