- Thorlabs LA 4725, plano-convex
- Thorlabs LD 1357, bi-concave
- Thorlabs LA 4052, plano-convex
- Thorlabs SM1L10 Lens Tube
- Light Source
- Plane Wave
- Circular, 5mm radius
- 633 nm wavelength
- Power, 1 W
- Unpolarized
- Detector: Input/Output
- Spot: Coherent Irradiance
- Analysis Rays: 1 million
- 500×500 pixels
For the analysis we have built up two identical Cooke Triplets with the lenses in the first without anti-reflective coatings and the second having the Thorlabs A- coating (RAVG<0.5% from 350-700 nm). The different systems will be denoted with BBAR or NC.
When we look at the light throughput before and after the optical components, we can get an idea of how much light is lost to Fresnel reflection since we are on-axis and not scattering on any mechanical components. This is our first step to determine stray light purely from the lenses.
The uncoated (NC) Cooke Triplet optical system loss due to Fresnel reflections amounts to ~19% which matches well with theory (~4% loss per surface).
Adding an anti-reflective coating is an obvious way to reduce unwanted stray light in the optical system by reducing Fresnel reflections. This is a simple but effective method to analyze how much light is being lost.
Next, we may want to determine how much light is reflected from a single surface, either a lens or a mechanical component. Instead of placing a detector after all optical components and measuring the light loss, we can place it at the surface from which we want to measure reflections. In this case the Fresnel reflections from the last lens.
As a reminder 3DOptix detectors have an “F” for the front face and a “B” for the back. No light will be analyzed if incident on the back surface. We want to measure the back reflected light so the detectors need to be flipped.
First, we need to turn on reflections to get the stray light to show on the analysis detectors. We will go into the optical settings for the uncoated and coated lens 3 and unselect the NO REFLECTIONS check box.
The MAX BOUNCES will stay at 1 since we only care about the first reflection at the surface.
There are now two surface contributions to the Fresnel reflections that the detectors will measure. Notice that the left detector above (BBAR) is now in log scale instead of linear. This is because the curved surface of lens 3 produces a large relative central reflection, but the actual power is still very small.
In addition to the Fresnel reflections, we also have scattering that can occur from the mechanical enclosure and optical surfaces. Although the lens tube is aluminum coated with a black powder, scattered light can still be a larger contributor.
For this application, we will turn on Lambertian scattering for surface 24, which is the top inner section of the lens tube.
The transmission was set to 0% since this is a metal component and 10% reflectance to simulate the scattering. The rest of the power will be absorbed. We are going to simulate some light source coming from an external location, so we will change the light source parameters:
- Plane Wave
- Circular, 5mm radius
- 633 nm wavelength
- Power, 1 W
- Unpolarized
