Soon, we will launch a new and updated customer portal, which is an important step toward providing our customers with one place to learn, interact, and get help.
Learn more.

How to apply scattering in the presence of total internal reflection (TIR)

In OpticStudio, total internal reflection (TIR) is applied at a surfacebefore other surface properties, such as scattering. This can cause a problem when attempting to model light pipes or fibers that include optically rough surfaces. Such elements rely on TIR, but don’t achieve perfect TIR behavior due to the surface roughness.  To correctly model such systems, an embedded surface can be used so that the scatter function is applied before the TIR. 
 
Erin Elliott & Alissa Wilczynski
10/21/2015
OpticStudio

 

Summary

 
In OpticStudio, total internal reflection (TIR) is applied at a surfacebefore other surface properties, such as scattering. This can cause a problem when attempting to model light pipes or fibers that include optically rough surfaces. Such elements rely on TIR, but don’t achieve perfect TIR behavior due to the surface roughness.  To correctly model such systems, an embedded surface can be used so that the scatter function is applied before the TIR.
 

The problem

 
Consider the attached file, “ScatteringAndTIR_TIRAppliedBeforeScatterFunction.zar.” It contains a PMMA cylinder that is 10 mm in diameter and 50 mm in length. The Lens Data Editor is shown in Figure 1. Line 1 contains a source that launches rays at 15°. Lines 4 and 5 place detector rectangles just inside and just outside the surface of the cylinder.


Figure 1: The Lens Data Editor shows a light pipe created with the Cylinder Volume object.


Figure 2: The Lens Data Editor shows that the light pipe has a diameter of 10 mm and a length of 50 mm.


The Non-Sequential Shaded Model shows the rays incident on the bottom surface of the cylinder. The rays experience TIR and are reflected back into the cylinder.


Figure 3: A NSC Shaded Model plot shows that rays incident at 15° are reflected back out of the light pipe when no scattering function is applied.

Note that “Color Rays By:” has been set to “Segment #.” This changes the color of the rays each time they interact with an object. Note that “Scatter NSC Rays” is also checked.


Figure 4: Set “Color Rays By” to “Segment #” so that rays change color every time they interact with an object.

Now, suppose that the cylinder has a rough or ground surface that we want to model with a Scatter function. So a “Lambertian” scattering function is added to the side faces of the cylinder, like this:


Figure 5: Use the Object Properties to place a Scattering function onto the surfaces of the Cylinder Volume.

The new Shaded Model plot is shown below. The reflected rays (shown in green) are scattered. But the rays are all scattered into the cylinder. In reality, at a rough surface, rays would scatter out of the cylinder, too.
 
As a ray intersects the cylinder, there are two calculations that take place within OpticStudio. First, the software calculates the specular ray path according to Snell’s law. Next, the scatter function is applied to deviate the rays from the specular path. The result is that all rays are scattered internally at the interface, with no possibility for the rays to scatter outside the pipe.


Figure 6: The NSC Shaded Model now shows scattered rays, but the rays are only scattered into the Cylinder Volume. The TIR is applied before the scatter function.

The output on the detectors, one just inside and one just outside the cylinder, also show that none of the incident rays are scattered out of the cylinder on the first reflection. Rays only leave the cylinder after subsequent bounces, producing a uniform low-level illumination on the second detector, rather than the bright spot we would expect from a rough surface.


Figure 7: The detector just inside the light pipe shows a concentrated beam. The detector just outside the light pipe shows a diffuse beam, since rays only leave the light pipe after multiple bounces.

The solution

The solution to this problem is to embed a dummy surface just inside the Cylinder Volume, and apply the scattering function to that surface, instead. This forces the software to apply the scattering function before the TIR, because the rays encounter the scattering surface before they encounter the air-glass interface where Snell’s Law is computed.
 
In the attached example, “ScatteringAndTIR_EmbeddedScatterSurface.zar,” Line 3 of the Non-Sequential Component Editor is a Cylinder Pipe object. (See Figure 8.) The object has no material defined, so without a scattering function, Snell’s Law produces no deviation of the rays at the surface of the Cylinder Pipe. In this case, the radius of the Cylinder Pipe is set to 4.98 mm, or 20 mm smaller than the Cylinder Volume that defines the light pipe. The scattering function is placed on the Cylinder Pipe, instead of on the Cylinder Volume itself.


Figure 8: The Non-Sequential Component Editor, after adding a Cylinder Pipe surface.

The new 3D Layout plot is shown in Figure 9. Rays are now scattered out of the cylinder as well as back into the cylinder. And the detectors inside an outside the detector now show similar distributions, as shown in Figure 10.


Figure 9: The Shaded Model now shows rays scattering both into and out of the light pipe.

 

Figure 10:  Detectors just inside and just outside the light pipe now show similar distributions.

Conclusion

  • OpticStudio will always apply TIR before it applies a scattering function, if both are calculated at the same interface.
  • For light pipes or fibers with rough surfaces, that doesn’t produce realistic results.
  • To correct this, embed a dummy surface just inside the pipe’s surface, and apply the scattering function to the dummy surface.