Phase Light Modulation: The Complete Guide

What is Phase Light Modulation?

Phase Light Modulation is (PLM) is a technique for Digital Light Processing (DLP®) that shapes light by controlling the phase of its wavefront pixel-by-pixel . Where a conventional Digital Micromirror Device (DMD) controls brightness by manipulating if light gets through for each pixel, PLM uses software-based control to delay or advance the wavelength of light at each pixel.

These fractional, incremental adjustments create a programmable phase pattern. As the phase-shaped light propagates, the waves interfere, creating the desired intensity pattern at the image plane.

How does PLM actually work?

Texas Instruments’ PLM is a MEMS micromirror array device derived from DMD technology but with the micromirrors operating in piston mode. Unlike a conventional DMD, which tilts each mirror between two angles to send light either into the optics (“on”) or away (“off”), resulting in amplitude modulation, PLM moves each micromirror up and down across multiple discrete height levels to create a controllable phase shift on the reflected beam.

Each mirror sits above an electrode on a standard CMOS array. Applying a voltage pulls the mirror down by a precise, discrete amount so the mirror pattern becomes a phase map, and the chip behaves as a programmable diffraction grating.

The phase map is a computer-generated hologram computed by iterative algorithms, which adjusts the switching rate of the mirrors fast enough to allow for dynamic wavelength modulation. Because PLM systems operate in the Fourier domain rather than the image plane, the phase map they produce creates a far-field diffraction pattern delivering the desired light distribution, rather than projecting an image onto a sensor.

Why does controlling light phase matter?

Most projection and imaging devices are amplitude modulators that control how bright each pixel is. A DMD is the classic example, switching each pixel on or off on a single, 2D image plane.

PLM instead controls phase across the entire light field—what the light does at every point in space, not just its brightness on one plane—offering far greater flexibility in how light is distributed.

By shaping the wavefront directly, a single software-controlled PLM chip can:

• Project onto curved surfaces at arbitrary depths with no optics adjustment required
• Project an image at variable depth between the chip and the target, or at multiple depths at once.
• Steer a beam to any point in space without a scan mirror or scan lens
• Generate single spots, multi-spot arrays, lines, top-hat profiles, or grayscale dose distributions

Compared to an amplitude modulator like DMD, this delivers two clear benefits. First, optics become programmable. Variability that once required moving parts or optical elements, such as lenses, is solved by software. Second, there’s less wasted energy. Where amplitude modulation discards the light from every “off” pixel, a phase modulator redistributes it for greater efficiency.

How does PLM compare to other light modulation technologies?

For phase modulation, two other technologies offer wavefront control, but each with tradeoffs.

Deformable mirrors use a flexible membrane manipulated by hundreds of actuators. These are fast and are the established standard in adaptive optics for astronomy, where they’re used to unbend starlight distorted by the atmosphere for better visualization and analysis.

But the continuous surface and low actuator count can’t produce the discrete, high-resolution phase steps that holography demands. PLM offers three to four orders of magnitude more control points with over 2 million independently addressable mirrors on a 0.98” array.

Liquid Crystal on Silicon (LCoS) SLMs shift phase by applying variable voltage to change the liquid crystal’s refractive index. They modulate only one polarization state and are temperature- and viscosity-dependent, which limits switching speed and requires active thermal management. They’re also prone to inter-pixel crosstalk, which can affect accuracy.

PLM’s discrete piston mirrors are polarization-agnostic and rated for a wider temperature range (-40^° to 95^°C) without active compensation, and the reflective MEMS architecture tolerates industrial-grade laser power.

Between LCoS and PLM, there is a trade-off in bit depth: 8-bit phase on LCoS versus 4-bit on visible PLM, or 5-bit on near-infrared PLM. For applications where phase resolution is the limiting factor, LCoS holds an advantage. Where speed, robustness or optical power dominate, PLM delivers performance that was previously unattainable.

What are the applications for PLM?

PLM is ideal for applications requiring laser-based, programmable light distribution in multiple image planes, including:

• Holographic projection and displays, including Augmented Reality (AR) and Heads Up Display (HUD) systems, providing realistic depth-of-field and the ability to shift the focal plane of an image in real time. Imagine 3D wayfinding indicators appearing in your field of view as you drive at the actual point at which you should make a turn, rather than floating on a fixed plane of glass.
• Beam steering and LiDAR for scanning without moving parts and in arbitrary patterns, rather than fixed raster lines.
• Lithography and laser sintering for writing arbitrary patterns onto flat or curved surfaces, potentially eliminating scan lenses and moving mirrors from the system entirely.
• Volumetric additive manufacturing with the ability to project images and cure material across multiple depth planes.
• Adaptive optics for flattening distorted wavefronts in astronomy and microscopy.
• Optical computing and communications, including high-speed optical switching, precise free-space beam alignment, and neuromorphic architectures.

Wherever a system currently requires a scan mirror, a scan lens or a focusing mechanism to move light, PLM can potentially replace it with a phase map.

Where did PLM come from?

The theoretical groundwork for computer-generated holography was laid in the 1960s, but it wasn’t until 1972 that Gerchberg and Saxton published their iterative algorithm for turning a target image into a phase map.

But the display remained a challenge. Precision holography requires a device that can physically hold a discrete phase pattern across many pixels and change it quickly. For decades, LCoS was effectively the only technology that could, but it came with speed, polarization and power constraints.

Deformable mirrors can shape and correct a continuous wavefront, but can’t form the discrete steps a hologram requires. They can fix distorted light, but not display it.

Texas Instruments spent nearly 30 years perfecting a different approach to micromirror arrays: the DMD, the chip at the heart of DLP, featuring millions of tilting mirrors on standard CMOS. In 2019, TI’s DLP team demonstrated the first prototype that operated those mirrors in piston mode rather than tilting, allowing each mirror to impose a phase delay rather than brightness modulation.

Built on the existing DMD manufacturing process on top of standard DLP CMOS, PLM went from prototype to production-grade in just a few years. Since then, TI has refined the technology for more phase levels and better efficiency, and extended the mirror travel to reach near-infrared wavelengths. Lifetime testing beyond 1,000 hours and new high-volume production test methods have brought it to production readiness.

The result is a phase modulator that switches in tens of microseconds and handles industrial-grade laser power, with commercial development kits now available on the market.

What does In-Vision’s PLM Development Kit offer?

The potential for PLM is enormous, but realizing it takes more than just a chip. An experienced partner can help you turn programmable phase control into a novel application—and make all the difference in whether a project succeeds or stalls.

In-Vision’s PLM development kits are derived from proven, industrially deployed electronic systems, with support available directly from the developers who built them.

The DLPM980E is engineered for industrial use and high-end optical R&D. It ships as a complete kit, running on In-Vision's own software modules out of the box, with a Linux-based operating system, a control API for parametrizing complex exposure processes, and a graphical tool for initial commissioning.

That means algorithmic work can start immediately, rather than requiring driver development first. The open FPGA platform allows you to run your own code, and the compact controller and PLM boards drop easily into any experimental setup or OEM assembly for smooth integration with in-field applications.

With our legacy in optics, In-Vision's expertise covers the full opto-electronic chain. Alongside the electronics, we design and manufacture precision optics and DLP-based light engines for lithography, additive manufacturing, and metrology and can help you achieve optical control without limits.