🔬 optics

Towards digital phantoms: emulating scattering with a spatial light modulator

This paper introduces a versatile, all-digital method using spatial light modulators to emulate complex random scattering media with precise tunability and reproducibility, overcoming the limitations of physical samples for applications in structured light, biological imaging, and optical communications.

Original authors: Kelsey Everts, Cade Peters, Andrew Forbes

Published 2026-05-13

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Kelsey Everts, Cade Peters, Andrew Forbes

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to shine a flashlight through a foggy window or a piece of frosted glass. The light hits the tiny bumps and particles, gets scattered, and comes out the other side looking messy, blurry, and distorted. Scientists call this "scattering," and it's a huge problem for things like seeing inside the human body, sending data through fiber optic cables, or even just taking clear photos through the atmosphere.

Usually, to study how light behaves in these messy situations, researchers have to use physical samples. They might use a jar of white paint, a sheet of paraffin wax, or a special gel that mimics human tissue. Think of this like trying to learn how to drive in a snowstorm by actually driving out into a blizzard. It works, but it's messy, hard to control, and every time you try it, the snow falls differently. You can't easily say, "Let's make the snow exactly 10% heavier than last time," and you can't perfectly recreate the exact same storm for a second test.

The "Digital Phantom" Solution

This paper introduces a clever new tool called a "digital phantom." Instead of using a jar of paint or a piece of wax, the researchers use a computer screen (specifically a device called a Spatial Light Modulator, or SLM) to create the messiness digitally.

Here is how it works, using a simple analogy:

1. The Digital "Frosted Glass"

Imagine the SLM is a giant, high-tech pixelated screen. The researchers program this screen to act like a "digital frosted glass." They divide the screen into tiny square blocks. For each block, they randomly decide whether to delay the light passing through it or not.

The Analogy: Think of a marching band walking across a field. If the ground is flat, they all step in time. If the ground has random bumps (the "scattering"), some marchers step on a bump and get delayed, while others step in a hole and speed up. By the time they reach the other side, their formation is scrambled.
The Digital Version: The researchers program the screen to create these "bumps" (phase shifts) instantly. They can make the bumps tiny and frequent (heavy scattering) or large and sparse (light scattering) just by changing a number on their computer.

2. Two Knobs to Control the Chaos

The paper highlights two main ways to tune exactly how "messy" the light gets, giving them total control that physical samples can't offer:

Knob 1: The "Block Size" (Distortion Strength)
Imagine the marching band again. If the bumps on the ground are huge (large blocks), the band stays mostly in order. If the bumps are tiny and frequent (small blocks), the band gets completely scrambled.
The researchers can shrink or grow these digital "blocks" instantly. If they make the blocks smaller, the light gets more distorted. They proved that their digital simulation matches perfectly with what happens in real-world physics simulations.
Knob 2: The "Blur Filter" (Spatial Filter)
Imagine looking at the marching band through a telescope. If you close the telescope's lens down a bit, you block out the fine details of the individual steps, leaving only the big, blurry movements.
The researchers use a physical lens setup to "block out" some of the fine details of the light pattern. This makes the distortion look even more severe, allowing them to fine-tune the messiness without changing the digital code.

3. Testing the Tool

To prove this digital phantom works, they tested it on different types of "light shapes":

Simple Beams: They took a standard, clean beam of light and ran it through their digital fog. It came out looking just as scrambled as light passing through real white paint or biological tissue.
Structured Light (The "Twisted" Light): They used special beams of light that carry "twist" or "spin" (like a corkscrew). When these beams hit the digital scatterer, they got scrambled just like they would in real life. The researchers measured how much the "twist" was lost and found their digital results matched real-world experiments perfectly.
Vectorial Light (The "Polarized" Light): They even tested light where the direction of the wave changes across the beam (like a rainbow of polarization). The digital phantom scrambled the patterns exactly as expected, without breaking the fundamental rules of how the light behaves.

Why This Matters (According to the Paper)

The authors argue that this method is a game-changer because:

It's Instant: You don't need to mix chemicals or wait for a gel to set. You just type a number, and the "scattering" is ready in a split second.
It's Repeatable: You can recreate the exact same scattering pattern a thousand times, which is impossible with real paint or tissue.
It's a Perfect Match: The digital results line up perfectly with computer simulations, meaning scientists can trust the digital model to predict what will happen in the real world.

In short, the paper presents a way to turn a computer screen into a perfectly controllable, reusable, and predictable "messy window" for light, replacing the need for messy, unpredictable physical samples. This allows researchers to study how light behaves in difficult environments (like inside the body or through the atmosphere) with a level of precision and control that was previously impossible.

Technical Summary: Towards Digital Phantoms: Emulating Scattering with a Spatial Light Modulator

Problem Statement
The propagation of light through complex random media—such as biological tissue, underwater environments, atmospheric turbulence, and optical fibers—inevitably leads to the distortion of light's degrees of freedom. These distortions manifest as aberrated intensity profiles, scrambled phases, speckle patterns, depolarization, and pulse broadening, which degrade spatial resolution, contrast, and communication bandwidth. Current methodologies for studying these effects rely heavily on physical scattering samples (e.g., ground glass, white paint, tissue-mimicking phantoms). While effective, these physical methods suffer from significant limitations: they lack precise control over distortion profiles and severity, require laborious fabrication and external characterization, are difficult to reproduce between studies, and degrade over time. Furthermore, physical samples cannot be perfectly matched by numerical simulations, creating a disconnect between theoretical models and experimental validation.

Methodology
The authors propose a "digital phantom" approach that leverages the versatility of spatial light modulators (SLMs) and computer-generated holograms to emulate scattering media without physical samples. The core methodology involves encoding binary random phase masks (alternating between 0 and $\pi$ ) onto an SLM. These masks are composed of super-pixel square blocks with spatially varying random phases.

The strength of the digital distortion is quantified by a unitless parameter, $\Omega = d/l_0$ , where $d$ is the input beam diameter and $l_0$ is the transverse correlation length (the size of the phase blocks). By varying $l_0$ , the authors control the disorder of the medium. The experimental setup utilizes a 4f imaging system and a Sagnac interferometer to generate and analyze both scalar (Laguerre-Gaussian and Hermite-Gaussian) and vectorial structured light. The system allows for full-field measurements, including intensity, phase retrieval, and Stokes polarimetry.

The authors demonstrate two primary methods to tune the distortion strength:

Varying $\Omega$ : Changing the encoded block size ( $l_0$ ) alters the number of intensity nulls and the severity of amplitude fluctuations in the near-field.
Spatial Filtering: Adjusting the aperture size in the Fourier plane of the 4f system controls the thickness of the intensity null lines by filtering out high spatial frequency components, thereby further modulating the distortion severity.

Key Contributions

All-Digital Implementation: The paper presents a simple, reproducible, and fully digital method to simulate random scattering using binary phase masks on an SLM, circumventing the fabrication and characterization challenges of physical phantoms.
Precise Tunability: The authors provide two distinct mechanisms to precisely tune distortion strength ( $\Omega$ and spatial filter size), allowing for a continuous range of scattering severities.
Validation against Physical Samples: The study validates the digital approach by comparing results against real-world physical scatterers (e.g., parafilm, biological hydra, bubble wrap). The digital phantoms successfully replicate the scintillation index ( $\sigma^2_I$ ) and distortion characteristics of these physical samples.
Structured Light Analysis: The toolkit is applied to scalar and vectorial structured light, demonstrating its ability to emulate scattering effects on orbital angular momentum (OAM) modes and polarization states.

Results

Agreement with Simulation: There is excellent agreement between simulated and experimental results for both intensity and phase profiles across various distortion strengths. The scintillation index ( $\sigma^2_I$ ) increases monotonically with $\Omega$ , matching theoretical predictions.
Scalar Light Distortion: When applied to Laguerre-Gaussian (LG) and Hermite-Gaussian (HG) modes, the digital phantom causes an exponential decay in modal fidelity as $\Omega$ increases. The study observes modal crosstalk that manifests not only along the diagonal but also around the anti-diagonal, consistent with the behavior of twisted light in complex media.
Vectorial Light Stability: For vectorial beams (superpositions of orthogonally polarized LG modes), the digital phantom distorts the spatially varying polarization structure. However, the degree of non-separability (concurrence) remains extremely stable across all distortion strengths, confirming that the phase-only masks act as a unitary channel that preserves the beam's vectorial nature, with minor deviations attributed to spatial frequency loss in the imaging system.
Scalability: The digital phantom can reproduce a wide range of scattering strengths, from weak to strong ( $\sigma^2_I \sim 1$ ), covering the regimes of various physical samples.

Significance and Claims
The authors claim that this digital phantom approach offers a versatile, accessible, and highly controllable alternative to physical scattering samples. By enabling the rapid generation of simulated channels that are directly comparable to numerical simulations, the method facilitates more systematic and controlled studies of light propagation in random media. The paper positions this toolkit as a significant advancement for applications at the intersection of structured light, biological imaging, and optical communications (both classical and quantum). The authors note that the approach is easily extensible to include amplitude modulation, absorption, and birefringence, and can be synchronized with high-speed display technologies to study time-varying media. Ultimately, the work aims to increase the ability to investigate scattering channels and improve the effectiveness of imaging and communication techniques in complex environments.

1. The Digital "Frosted Glass"

2. Two Knobs to Control the Chaos

3. Testing the Tool

Why This Matters (According to the Paper)

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