What is Transmission Hologram Technology?
Transmission hologram technology is a method of recording and reconstructing holograms where the hologram is illuminated from the opposite side of the viewer, and the reconstructed image is seen through the hologram plate. The hologram is recorded by allowing a reference beam and an object beam to interfere on a photosensitive plate from the same side. During reconstruction, a laser beam (or, less commonly, a white light source with filtering) shines through the developed hologram, and the diffracted light forms a virtual image behind the plate. Transmission holograms are known for their high brightness, deep 3D effect, and wide viewing angle. They were the first type of hologram to be demonstrated (by Leith and Upatnieks in 1962). Today, transmission holography is used in holographic microscopy, optical data storage, holographic optical elements, and certain high‑security verification applications where a laser reader is acceptable. Holoseal focuses on embossed reflection holograms for product labels; this entry is for educational completeness.
🔦 Key Takeaway: Transmission holograms are viewed with laser light shining through the plate. They produce very bright, deep 3D images but require a laser for viewing – not suitable for everyday consumer authentication.
🔬 How Transmission Holography Works (Leith‑Upatnieks Method)
The classic transmission hologram, invented by Emmett Leith and Juris Upatnieks in 1962, is recorded with a laser and viewed with a laser. Here is the step‑by‑step process:
Recording
- Laser beam splitting – A continuous‑wave laser (e.g., He‑Ne 633 nm) is split into an object beam and a reference beam.
- Object beam – The object beam is expanded, filtered, and directed onto the object. Light scatters from the object toward a high‑resolution photosensitive plate (silver‑halide or photoresist).
- Reference beam – The reference beam is expanded and directed to the same plate from the same side as the object beam, but at an angle.
- Interference – The two beams interfere on the plate, creating a microscopic pattern of fringes. This pattern encodes both the amplitude and phase of the light from the object.
- Processing – The plate is chemically developed, fixing the interference pattern as variations in transparency (absorption) or refractive index (phase).
Reconstruction (Viewing)
- Illumination – The developed hologram is placed in a laser beam identical to the original reference beam (same wavelength, angle, and wavefront curvature).
- Diffraction – The laser light diffracts off the recorded fringes. Two diffracted orders are produced: the +1 order (reconstructs the original object wavefront) and the −1 order (conjugate image).
- Image formation – The +1 order diverges from the hologram exactly as the original object wavefront did. The viewer, looking through the hologram, sees a bright, three‑dimensional virtual image positioned behind the plate.
- Parallax and depth – Because the full wavefront is reconstructed, the image exhibits realistic parallax – moving your head reveals different sides of the object.
📦 Key Properties of Transmission Holograms
- Laser viewing required – The hologram must be illuminated by a laser (or a very narrow‑band filtered light) to reconstruct the image. This makes them unsuitable for consumer product authentication but ideal for laboratory and security verification where a reader is available.
- High brightness and resolution – Transmission holograms can have very high diffraction efficiency (>50%) and produce extremely sharp, bright images.
- Deep 3D effect – Because the full wavefront is reconstructed, the image depth can be many centimetres or even metres.
- Large viewing angle – The image can be seen over a wide angle, up to 180° in some cases.
- Image behind the plate – The virtual image appears behind the hologram plate, as if looking through a window into the scene.
🛡️ Transmission vs. Reflection Holograms
| Aspect | Transmission Hologram | Reflection Hologram |
|---|---|---|
| Recording geometry | Object and reference beams on same side of plate | Object and reference beams on opposite sides |
| Viewing illumination | Laser (or narrow‑band filtered light) transmitted through the plate | White light reflected from the plate (e.g., sunlight, lamp) |
| Image location | Virtual image behind the plate | Virtual image in front of (or behind) the plate, depending on type |
| Colour | Monochromatic (colour of the viewing laser) | True colour (if recorded with RGB lasers) or rainbow (if embossed) |
| Viewing angle | Wide (up to 180°) | Narrower (typically 30–60°) |
| Typical applications | Holographic microscopy, optical data storage, lab research | Security labels, passports, ID cards, art |
| Consumer verification | Difficult (requires laser) | Easy (tilt under white light) |
🔐 Applications of Transmission Hologram Technology
- Holographic microscopy (digital holographic microscopy – DHM) – Transmission holography is used to capture quantitative phase images of transparent specimens (e.g., living cells, tissue cultures). The hologram records both intensity and phase, allowing 3D reconstruction of the specimen’s thickness and refractive index.
- Optical data storage (holographic data storage) – Data is recorded as holograms in a photosensitive medium (e.g., photorefractive crystals or photopolymers) and read out with a laser. Transmission geometry allows page‑wise storage and high data density (terabytes per disc).
- Holographic optical elements (HOEs) – Transmission gratings, lenses, and beam splitters can be recorded holographically. These are used in head‑up displays (HUDs), laser scanners, and telecom components.
- Holographic interferometry (double‑exposure transmission holography) – Two exposures of the same object (one before and one after deformation) are recorded on the same plate. When reconstructed, interference fringes reveal displacement with sub‑wavelength accuracy. Used for stress analysis, vibration measurement, and nondestructive testing.
- High‑security verification (specialised) – Some banknote security features incorporate transmission hologram elements that are verified by laser scanners (e.g., machine‑readable DOVIDs). The laser reader detects the reconstructed image without human intervention.
⚙️ Materials and Recording for Transmission Holograms
- Silver‑halide emulsions – High resolution (up to 5000 lines/mm), high sensitivity, but require wet chemical processing. Used for scientific and art holograms.
- Photoresist – Used for master origination of embossed holograms (surface relief). Not typically for final transmission holograms.
- Photopolymers (e.g., DuPont Omnidex, Bayer Bayfol HX) – Self‑developing, dry process, good for mass production of transmission holograms (e.g., HOEs).
- Photorefractive crystals (e.g., lithium niobate, BSO) – Reusable, used for dynamic holography (real‑time interferometry) and data storage.
🔍 Transmission Hologram Verification – How It Works in Security
Some high‑security documents use transmission hologram patches that are read by a small laser scanner (e.g., at border control). The process:
- A laser diode (e.g., 655 nm) shines through the hologram patch.
- A photodetector array captures the reconstructed image (often a 2D barcode or a specific pattern).
- If the image matches the expected pattern, the document is authenticated.
This method is more secure than visual inspection because the hologram’s exact optical response is machine‑verified. It is used in some e‑passports and visa stickers.
❓ Frequently Asked Questions About Transmission Hologram Technology
- Why do transmission holograms require a laser for viewing? – Because the fringe spacing is extremely fine (hundreds of nanometres). Only coherent light (laser) can diffract from such fine gratings to reconstruct the image. White light would produce severe blurring.
- Can I view a transmission hologram with an LED? – Only if the LED is highly monochromatic (e.g., a laser diode or a filtered LED). Standard white LEDs will not work because their spectral width (~50 nm) blurs the image.
- What is the difference between a transmission hologram and a rainbow hologram? – A rainbow hologram is a type of transmission hologram modified with a slit to allow viewing in white light (at the cost of vertical parallax). Standard transmission holograms have full parallax but require a laser.
- Are transmission holograms used on credit cards? – No – credit cards use embossed reflection holograms that are visible in white light. Transmission holograms are impractical for consumer products.
- Can transmission holograms be mass‑produced? – Yes, by contact copying or by embossing surface‑relief transmission holograms (these are less common than reflection embossed). Most commercial transmission holograms are produced as HOEs in photopolymer by casting or copying.
- Does Holoseal offer transmission hologram labels? – No. Holoseal specialises in embossed reflection hologram labels for brand protection. For transmission hologram needs (e.g., machine‑readable optical security), we can recommend specialised suppliers.
