What is Volume Reflection Hologram Technology?
Volume reflection hologram technology (also called Denisyuk holography or thick hologram technology) is a method of recording holograms in a thick, light‑sensitive medium (such as dichromated gelatin or photopolymer) where the interference pattern is recorded as a periodic variation of refractive index throughout the volume of the material – not just as a surface relief. When the finished hologram is illuminated with ordinary white light (e.g., sunlight or a halogen lamp), it selectively reflects a narrow band of wavelengths, producing a bright, true‑colour, three‑dimensional image. Unlike embossed holograms (which are rainbow‑coloured and viewed in reflection from a metalised surface), volume reflection holograms can display realistic colours (e.g., skin tones, green leaves) and are extremely durable because the holographic structure is embedded inside the material. They are widely used for high‑security applications such as passport data pages, ID cards, credit cards, and tamper‑evident labels. Holoseal does not manufacture volume holograms; we focus on embossed holograms for mass production. This entry is for educational completeness.
🔬 Key Takeaway: Volume reflection holograms are recorded in thick photosensitive media. They reflect white light like a coloured mirror, producing bright, true‑colour images with excellent durability – ideal for passports and high‑security ID documents.
🔬 How Volume Reflection Holography Works (Denisyuk Method)
The Denisyuk method, invented by Russian physicist Yuri Denisyuk in 1962, is the simplest way to make a reflection hologram. Here is the step‑by‑step process:
- Recording setup – A single expanded laser beam (usually from a continuous‑wave laser, e.g., He‑Ne 633 nm or DPSS 532 nm) illuminates the object. The light scattered from the object travels back toward the recording plate. A reference beam (the same expanded laser) also illuminates the plate from the opposite side.
- Interference in the volume – The forward‑going reference beam and the backward‑going object beam interfere inside a thick (10–100 µm) photosensitive layer (e.g., silver‑halide emulsion, dichromated gelatin, or photopolymer). The interference pattern creates planes of varying refractive index – essentially a Bragg grating – throughout the thickness of the layer.
- Processing – After exposure, the plate is chemically processed. The refractive index modulation is fixed, creating a volume phase hologram. Unlike surface‑relief holograms (which have a physical pattern on the surface), volume holograms have no surface structure; the hologram is embedded inside the material.
- Reconstruction (viewing) – When illuminated with white light (e.g., sunlight or a spotlight), the hologram acts as a narrow‑band Bragg filter. Only a specific wavelength (colour) that satisfies the Bragg condition is reflected back to the viewer, forming a virtual image. Different colours are reflected from different sets of planes, producing a full‑colour image if recorded with multiple wavelengths or if the material swells appropriately.
📦 Key Properties of Volume Reflection Holograms
- True colour – Because the hologram reflects only narrow bands of wavelengths, it can reproduce natural colours (reds, greens, blues) without the rainbow effect of embossed holograms. This is achieved either by recording with multiple lasers (RGB) or by post‑processing the emulsion to swell or shrink, shifting the reflected colour.
- Viewing in white light – No laser or special lighting required; a standard incandescent bulb or sunlight works perfectly.
- High brightness – Volume holograms can have diffraction efficiency up to 90% or more, producing very bright images.
- Extreme durability – Because the hologram is embedded in the volume, it cannot be scratched off or easily damaged. The surface is smooth and can be laminated or coated.
- Angle and wavelength selectivity – The hologram reflects only a specific colour at a specific viewing angle. This intrinsic property makes counterfeiting very difficult; a counterfeit would need to replicate the exact Bragg planes.
🛡️ Volume Reflection vs. Embossed (Surface‑Relief) Holograms
| Aspect | Volume Reflection Hologram | Embossed Hologram |
|---|---|---|
| Recording medium | Thick photopolymer, gelatin, or silver halide (10–100 µm) | Thin photoresist or lacquer on PET (0.5–2 µm) |
| Structure | Refractive index variations (volume) | Surface relief (embossed grooves) |
| Colour | True colour (can be full‑colour) | Rainbow (diffractive) |
| Viewing light | White light (sun, halogen) | White light (rainbow effect) |
| Durability | Very high – embedded | Moderate – metal layer can oxidise |
| Mass production | Expensive, limited volume | Very high (embossing from nickel shim) |
| Cost per unit (volume) | High (₹50–500+ per piece) | Low (₹0.20–10 per label) |
| Typical applications | Passport data pages, ID cards, banknotes | Security labels, stickers, packaging
🔐 Manufacturing Process for Volume Reflection Holograms
Volume reflection holograms are not mass‑produced by embossing. Instead, each hologram is either:
- Individually recorded – The master is recorded on a plate; then contact copies are made by exposing a fresh photopolymer plate through the master in a closely spaced optical setup. This limits volume but is suitable for high‑security documents (e.g., passports, ID cards) where quantities are in the millions but not billions.
- Mass‑replicated by casting – A master volume hologram is used as a mould to cast replicas in UV‑curable photopolymer. This is slower than embossing but can produce thousands of copies per master.
- Laminated as a patch – The finished volume hologram film is cut into patches and hot‑stamped or adhesively applied to documents. This is common for passport data page holograms.
🌍 Applications of Volume Reflection Hologram Technology
- Passport data pages (e‑passports) – Many countries use volume reflection holograms as the primary overt security feature on the polycarbonate data page. The hologram typically includes the passport holder’s portrait, document number, and a colour‑shifting background.
- High‑security ID cards (national IDs, driver’s licences) – Volume holograms embedded in polycarbonate or laminated onto the surface, offering tamper‑evident protection.
- Banknotes (colour‑shift patches) – Some currency notes use volume hologram patches that change colour when tilted – a result of the Bragg selectivity.
- Credit cards (premium lines) – Some high‑end credit cards feature volume holograms (e.g., metallic‑looking reflective logos) that are more durable than traditional embossed holograms.
- Brand protection (high‑value products) – For luxury watches, jewellery, and pharmaceuticals where the cost of the hologram is less important than its durability and counterfeit resistance.
⚙️ Advantages and Limitations for Security
- Advantages
- True colour – more memorable and harder to copy with simple diffraction foils.
- Very difficult to counterfeit – requires original laser recording setup or advanced optical knowledge.
- Durable – cannot be scratched off (surface is smooth).
- Integrates well with polycarbonate card bodies (laser‑engravable).
- Limitations
- Higher cost per unit compared to embossed holograms.
- Not suitable for very high volumes (billions of units) due to slower replication.
- Requires precise angle and lighting for optimal viewing; colour may change with viewing angle.
❓ Frequently Asked Questions About Volume Reflection Hologram Technology
- What is the difference between a Denisyuk hologram and a rainbow hologram? – A Denisyuk hologram is a volume reflection hologram viewed in white light; it reproduces true colours and is recorded in a thick medium. A rainbow hologram is an embossed surface hologram that shows rainbow colours and is viewed in white light but does not have true colour reproduction.
- Can volume reflection holograms be mass‑produced? – Yes, but not by embossing. They are produced by contact copying (photopolymer to photopolymer) or casting. This allows millions of copies, but the unit cost remains higher than embossed holograms.
- Why are volume holograms used on passports? – They are extremely durable (cannot be scratched), offer true‑colour images (e.g., portrait and national emblem), and are very difficult to counterfeit because the Bragg grating structure is hard to replicate without the original setup.
- Do volume holograms require a laser for viewing? – No – they are designed to be viewed in ordinary white light (sunlight, halogen, LED). The hologram itself acts as a colour filter, reflecting only the recorded wavelengths.
- What is the typical thickness of a volume hologram recording layer? – For photopolymer, thickness ranges from 10 µm to 100 µm. Thicker layers provide higher wavelength selectivity but narrower viewing angle.
- Can I order volume reflection hologram labels from Holoseal? – Holoseal specialises in embossed hologram labels (mass production, low cost). For volume hologram needs (e.g., passport patches, ID card overlays), we can recommend specialised suppliers. Contact us for guidance.
