Holograms are made by recording patterns of light interference onto a light-sensitive material, most commonly photopolymers, silver halide emulsions, or dichromated gelatin. Unlike a photograph, which captures only brightness and color, a hologram captures the full wave pattern of light bouncing off an object, including depth information. The physical hologram itself is a thin film or plate containing microscopic fringe patterns, sometimes spaced only half a wavelength of light apart.
The Recording Materials
Three families of light-sensitive materials account for most holograms ever made. Each one responds differently to laser light and suits different applications.
Silver halide emulsions work much like traditional photographic film. A thin layer of silver halide crystals (compounds of silver with chlorine, bromine, or iodine) is coated onto glass or plastic. When laser light hits the emulsion, it exposes certain crystals, which are then chemically developed into metallic silver. The resulting pattern of silver deposits forms the hologram’s interference fringes. Industrial silver halide plates like PFG-03M have been workhorses of holography for decades because they offer fine resolution and well-understood processing chemistry.
Photopolymers are the dominant material in modern commercial holography. A typical formulation combines two acrylic monomers in a roughly 3:7 ratio with a light-sensitive initiator that responds to blue or green laser wavelengths. When exposed, the monomers polymerize (harden) in the bright regions of the interference pattern while remaining liquid in the dark regions. This creates a permanent variation in the material’s density and refractive index. Some advanced photopolymers incorporate silicon oxide nanoparticles, around 14 nanometers in size, to boost the sharpness and brightness of the final image. Dyes like Rhodamine B can be added to shift color properties.
Dichromated gelatin is considered the highest-quality holographic recording medium available. It’s essentially animal-derived gelatin (the same protein used in food) mixed with a chromium compound that makes it light-sensitive. After exposure and processing, it produces holograms with extremely low scattering and high brightness. Its main drawback is sensitivity to humidity: the gelatin can swell or degrade in moist environments, so dichromated gelatin holograms need protective sealing.
Why Laser Light Is Essential
A hologram can only be recorded using coherent light, meaning light waves that stay perfectly in step with each other over a usable distance. Lasers produce this kind of light because they emit an extremely narrow band of wavelengths. The narrower the emission band, the longer the coherence length, which is the distance over which the light waves remain synchronized enough to create clean interference patterns.
Ordinary white light has a broad spectrum and almost no coherence length, so it can’t produce the precise interference fringes a hologram requires. During recording, a single laser beam is split in two: one half illuminates the object, and the other serves as a reference beam. Where these two beams meet on the recording material, they create the interference pattern that encodes the 3D image. Visible-light lasers used in holography typically operate between 400 and 700 nanometers, with red (633 nm) and green (532 nm) being the most common choices.
Transmission vs. Reflection Holograms
The internal structure of a hologram depends on how it was recorded, and this determines how you view it.
In a transmission hologram, the interference fringes run perpendicular to the surface of the plate, spaced more than half a wavelength apart. You view it by shining laser light through it from behind. The light diffracts off the fringe pattern and reconstructs the original 3D image on the other side. These holograms tend to produce vivid, deep images but require a laser or very specific lighting to see.
In a reflection hologram, the fringes run parallel to the plate surface and are packed much more tightly, roughly half a wavelength apart. You view it with ordinary white light, like a desk lamp or sunlight, shining from the front. The tightly spaced layers act like a natural color filter, reflecting only the correct wavelength back to your eyes. This is the type you’re most likely to encounter on credit cards, museum displays, and art prints.
Security Holograms on Cards and Banknotes
The shiny holographic patches on credit cards, ID badges, and currency are mass-produced differently from laboratory holograms. A master hologram is first created using laser recording, then stamped into a thin metallic foil through a process called embossing. The foil is typically a layer of aluminum vacuum-deposited onto a plastic carrier film. On polymer banknotes, this carrier is often biaxially oriented polypropylene (BOPP), a plastic stretched in two directions for strength and clarity.
The embossed surface contains microscopic ridges that diffract light into shifting rainbow patterns. Because replicating these ridges at the nanometer scale requires expensive, specialized equipment, security holograms serve as an effective anti-counterfeiting measure. Additional features like intaglio printing (raised ink applied on top of reflective metallic patches) add further layers of security that are difficult to reproduce.
Digital Holograms Without Film
Not all holograms require a physical recording material. Computer-generated holography (CGH) uses a device called a spatial light modulator to shape a laser beam in real time. The most common type is the liquid crystal SLM: a flat panel containing a grid of individually controlled pixels, each filled with liquid crystal. By applying different voltages to each pixel, the device changes the crystal’s optical properties, which shifts the phase of the laser light passing through it. A computer calculates the exact phase pattern needed to reconstruct a desired 3D image, and the SLM applies that pattern thousands of times per second.
This approach skips the chemical processing entirely. The “hologram” exists as a calculated phase map rather than a physical etching. It’s the technology behind holographic microscopy, optical trapping systems used in neuroscience research, and experimental holographic displays.
Volumetric Displays Often Called Holograms
Many things marketed as holograms aren’t holograms at all. The floating projections you see at concerts and trade shows typically use Pepper’s Ghost, an old stage illusion where an image is reflected off an angled transparent screen. It looks three-dimensional from the audience’s perspective, but it’s a flat 2D reflection with no actual depth information encoded in a wave pattern.
True volumetric displays do create points of light in open air, but through entirely different physics. Laser-induced plasma displays focus a powerful femtosecond laser to a point in space where the energy ionizes the air, creating a tiny glowing dot of plasma. By scanning rapidly, the system draws shapes in three dimensions. Acoustic trap displays use arrays of ultrasonic speakers to levitate small particles (often tiny beads of expanded polystyrene) and illuminate them with colored light. Current acoustic trap technology can create images up to about 117 millimeters across at a 10 Hz refresh rate.
Both of these produce genuinely three-dimensional images you can walk around, unlike a true hologram, which is confined to a plate or screen and appears flat or invisible when viewed from the edge. But neither one records or reconstructs a light wavefront, which is what makes a hologram a hologram.
How Long Holograms Last
Durability depends almost entirely on the recording material. Silver halide holograms, especially those that have been bleached to convert the dark silver deposits into transparent compounds for brighter playback, are stable under normal indoor conditions but degrade in very high humidity. Dichromated gelatin is even more moisture-sensitive and typically needs to be sealed between glass plates or laminated. Photopolymers are generally the most robust of the three, since the polymerized regions are chemically stable once fully cured. Embossed security holograms on plastic films can last for years under normal handling, protected by the polymer substrate itself.
UV light, extreme heat, and moisture are the main enemies across all holographic materials. A well-protected hologram stored in stable conditions can remain readable for decades.