Designing a Light Shaper with Prism Cells¶

🎬 Overview¶

With VirtualLab Fusion's Light Shaping Package, you can control light using a Light Shaper [Prism Cell] element. This tutorial introduces the basic usage of this component.

📌 Required Package: Light Shaping

This light shaper design approach is based on deflecting light in different directions using many prism surfaces with different orientations, arranged like a chessboard. Each "cell" in this array has the same rectangular base area.

This cell-based design concept is a local light shaping approach. Each cell generates one light spot. Together, the many cells can address many positions in the target plane, creating different patterns. In simulation terms, this is a multi-channel concept: each cell is treated as one channel.

You can imagine that the light is first cut into many small cell apertures. Behind each aperture, a prism surface redirects the light. The basic redirection happens purely geometrically through these micro-prisms. However, light propagation is strongly influenced by diffraction at the micro-apertures. So, we have many diffracted beams that ultimately need to produce a desired light distribution in a target plane.

The advantage of this method is its geometric nature: Compared to other purely diffractive beam shaping approaches, it is often well-suited

• for a divergent light source without additional collimation. However, there are cases where some pre-collimation can still be useful.
• for light sources with wider bandwidths or even for white light, especially when the requirements regarding residual chromatic aberration are not too stringent.

The following images illustrate geometrically how a single light spot is created:

• Image (a) shows a divergent light source and the portion of light that passes through a cell aperture to the target plane.
• Image (b) emphasizes the incident and outgoing light direction of one channel for same ideal point source illumination. It also shows that the light wave hitting a single cell locally has an almost flat wavefront. This is one of the basic assumptions in our design: the geometry (distance from source to element and cell size) should allow the approximation of plane wave illumination per cell.
• Image (c) shows that an extended emitter area leads to different light incidences per cell which form the final spot size for once cell. With these spots the desired light pattern will be created.

To avoid running thousands of diffraction simulations, a simplification is made. Only one diffraction simulation is performed: the on-axis case, where a plane wave hits the central cell perpendicularly and travels straight to the target plane. This calculated spot is then copied to all positions, where a cell deflects to, weighted according to Fresnel effects depending on the actual incidence angle on the differently oriented prism surfaces.

Basic Facts About Using the Design Approach¶

The design expects a target pattern for the light distribution in the target plane: a sampled region that defines where the PCE should direct light portions. Since you rarely have as many target pixels as cells in the light shaper, and the efficiencies of the different deflected beams vary due to Fresnel effects, each target pixel should be hit multiple times to achieve averaging. A multi-hit factor > 10 is recommended. The assignment of prism cell to target position is random, providing additional averaging. (Our customers have also used this averaging effect for light homogenization.)

Based on the above, the following aspects should be considered in this design:

• Spot size: The cell size, together with the distance to the light source, the extension of the light source, and the distance to the target plane, defines the size of a light spot – the effective "light brush size". It is always useful to run test simulations to determine achievable spot sizes.
• Resolution: The pixel spacing of the specified target pattern, together with the spot size, defines the resolution of the achievable pattern.
• Cell and pixel count: The number of cells and the number of target pixels must be well matched.

Depending on your requirements (for example, a given light shaper size determines the possible number of cells), different parameters can be used as a starting point for subsequent decisions.

The actual design of a light shaper and the simulation with it is performed using a special Optical Setup (OS) designed for this purpose. This OS only allows the use of elements explicitly intended for this light shaper system. However, for some aspects during the design process, standard OS are also used. As mentioned, this light shaper design approach is particularly well-suited for shaping light from a divergent source. Therefore, the default light source in such a light-shaper-specific OS is the Far Field Source, which uses modulable spherical waves as a basis.

The following sections describe the different steps and present the dialogs.

🚀 Step-by-Step Tutorial¶

Step 1: Determine the Spot Size¶

First, check how large a single light spot is in your system. To do this, you only need to look at a single aperture of a light shaper cell.

For these simulations, use a standard OS with the default light source and default detector of the Cells Array Specific OS: the Far Field Source and the Camera Detector. Now specify the complete system geometry: light source, any lenses, aperture, and detector. If there is no pre-optics, we do not need an aperture element because our light sources always include an aperture setting.

For all simulations, it is recommended to proceed step by step. First, simulate how the light spot looks using only a single light mode of the Far Field Source.

💡 Note: The Far Field Source is one of VLF's light source models for representing an extended light source. On such an emitter surface, several light modes start. The light of each mode is coherent with itself, but different modes are considered incoherent with respect to each other. This way, partially coherent light sources like LEDs can be simulated very well. This reflects the real world accurately. It has been shown that you do not need to simulate thousands and thousands of light modes; settings from 3 × 3 to 17 × 17 lateral modes are often sufficient, making this approach numerically very efficient. The name of the light source comes from the fact that it is defined by its appearance in the far field, which is — if no programmable or databased modualtion is given — a spherical wave.

Then add the edge modes of the extended light source (or even more) so that you get a result from which we can clearly see the actual spot size.

If the resulting light spot is not small enough to represent your desired pattern with sufficient detail, consider which parameters can be changed to achieve a better result.

⬇️ Download here the OS used for evaluating above shown spot sizes.

Step 2: Design Target Pattern (DTP)¶

Now you know how fine your "light brush stroke" will be.

For the desired target pattern, called the Design Target Pattern (DTP), you often use a suitable graphics software to first create a black-and-white image with fine resolution. Import this bitmap image into VLF and specify the extent of the image. Then you'll need to resample the image. Choose a sampling distance that relates to your spot size – this will determine the degree of overlap between adjacent spots. Smaller spots are beneficial for fine details in your pattern, while larger spots will overlap more, which leads to additional averaging.

💡 Note: Although binary (black-and-white) DTPs are always used in these light shaper designs, this does not necessarily limit you to binary resulting light patterns. Different intensity gradations can also be achieved by using different densities of target pixels, with or without spot overlap. Graphics programs can be used for this as well. However, VLF also includes a Floyd-Steinberg quantization option that can be used for this purpose.

For generating the design target pattern (DTP) use VLF's image import and convert the imported data into a 2D region, which is expected in the design dialog later.

⬇️ Here you can download the bitmap file with the shape of an "X".

⬇️ Here you can download the converted region with the shape of an "X".

Step 3: Optical Setup Specification¶

Now that you know the spot size and have created your DTP accordingly, build your system. As mentioned above, a special OS is used for this. You can find it in the Start Ribbon › Setups: Light Shaping › Light Shaping Optical Setup with Prism Cells.

By default, this OS consists of:

• A Far Field Source
• The Refractive Light Shaper (Prism Cells) element
• A Camera Detector

This OS allows the use of structure-based optics in front of the light shaping element.

Now configure this system:

1. In the light shaper element, define the material and whether the structure should be placed on the first or second surface.
2. In the edit dialog for the light shaper element, define the number of prism cells and their size.
3. In the Far Field Source, set the wavelength and a sufficiently large "Field Size" so that the entire ligth shaper is illuminated.
4. Position the Camera Detector at the desired location.

Step 4: PCE Design¶

Based on the pre-configured OS from Step 3, you can now start the light shaper design via Profile Editing & Run › New Cells Array Design.

The design document, which can be saved, has two tabs:

• Lighting Setup: Gives an overview of the system and its basic settings.
• Design: Here you specify your Design Target Pattern (DTP). Additional options are available, which are described in detail in the manual.

🔔 Important: The DTP is defined in the coordinate system of the detector, as for a transmitted-light projection. If the image is to be projected onto an opaque plane, you must mirror it horizontally first. VLF provides the option Manipulations › Lateral Displacement › Mirror Horizontally for this purpose.

When everything is set, click Go! and you will then get our system, in which all prism cells have been assigned parameter values by the design. The detector in the resulting OS was given a new detector window size and resolution, which can be easily adjusted as needed.

💡 Note: If VLF cannot find permissible parameter values for a prism cell to redirect light to its assigned target position, it will try a different assignment. If this fails after 10 attempts, the design process will stop with an appropriate message. In this case, review your system parameters and consider possible modifications.

Simulation Approach: For a quick preview, ray tracing can provide geometric validation of the prism deflection angles, but only field tracing captures the crucial diffraction effects that determine final spot size and shape.

We wish you great success with your design!

Further Downloads¶

⬇️ An exercise DTP in the shape of the letter "P", which works well with the settings of the default OS for light shaping with a light shaper [Prism Cell] element.

⬇️ The default OS for light shaping with prism cells, which you can use as starting point for some tests.

⬇️ The OS with a ready designed prism cells element to generate the pattern "P"

With these data the following results can be generated:

Related Links¶

🔗 Use Case "Shaping of White Light by Using Prism / Grating / Mirror Cells Arrays" demonstrates the different variants of the light shaping approach using cell arrays

🔗 Use Case "Export of Fabrication Data" shows how to export a prism cells array surface or even solid

Tags: light shaper prism cell prism cell array prism cell element design light shaping divergent LED pattern homogenizing white light