# Holography Simulator - Record & Reconstruct Holograms, Twin Image, Fourier

> Simulate holography: place points, record fringes, refocus via FFT angular spectrum, compare in-line vs off-axis, cut the plate, draw a Fourier hologram.

- URL: https://qilingtools.com/en/physics/holography
- Category: Physics Tools (https://qilingtools.com/en/physics)
- Tags: [Physics](https://qilingtools.com/en/tags/physics), [Education](https://qilingtools.com/en/tags/education)
- Last updated: 2026-10-29
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

A hologram does not store a picture of the object but the interference fringes between object light and a reference beam, which keep both amplitude and phase. This tool computes those fringes on a 256×256 grid and propagates the light back with an FFT angular-spectrum method: drag the focus slider and points at different depths sharpen one at a time, showing the virtual image, the real image and the twin image. You can also cut the plate in half, or draw a shape and watch its Fourier hologram rebuild the original and an inverted conjugate copy.

## How to use

1. On the Record tab, tap to add object points, drag them into place, set each point's depth and tilt the reference beam.
2. Zoom into the hologram to see how fringe spacing follows the reference angle, and how the spectrum splits into three orders.
3. Switch to Reconstruct and drag the focus distance, or press Sweep depth, to bring each point into focus at its own depth.
4. Try virtual only, real only or plain illumination, and options such as cutting off the right half, and compare the results.
5. On the Fourier tab, draw a shape and find the image, twin image and zero order in the reconstruction.

## Features

- Real interference of point objects with a tilted plane reference; zoom the hologram down to single fringes
- Hand-written 2D FFT and angular-spectrum propagation, fast enough to refocus and sweep on a phone
- Digital spatial filtering separates zero order, virtual and real images, with the filter window drawn on the spectrum
- Gabor in-line and Leith–Upatnieks off-axis holograms side by side, so the twin-image problem is plain to see
- Cut-the-plate demo: any piece still rebuilds every point, only at lower resolution
- Draw a shape to make a Fourier hologram, with optional random-phase diffuser and binary CGH

## FAQ

### How is a hologram different from a photograph?

A photo records only intensity and loses the phase, so it shows a single viewpoint. A hologram records the interference fringes between the object wave and a coherent reference: fringe position encodes phase and fringe contrast encodes amplitude. Lit by the reference beam, the fringes diffract a copy of the original wavefront, so the object appears to be there, with real depth and a changing view as you move.

### What is the twin image and why does off-axis holography remove it?

The recorded intensity contains both O·R* and O*·R. On replay one recreates the object wave (the virtual image) and the other is its conjugate, which focuses into a real image on the other side of the plate: the twin image. In Gabor's in-line hologram both overlap the straight-through beam, so a defocused twin always lies over the virtual image. Tilting the reference by θ puts the three terms at spatial frequencies 0 and ±sinθ/λ, so they leave in different directions and can be viewed or filtered separately.

### Why does half a hologram still show the whole image?

Light from every object point reaches the whole plate, so every piece records every point. What remains after cutting still rebuilds the full scene, but the aperture D is smaller, each image spot grows to about λz/D and the viewing angle narrows, like looking through a smaller window.

### How fine are hologram fringes, and what film do they need?

With a full angle θ between beams the spacing is Λ = λ/(2 sin(θ/2)). For a He-Ne laser at 30° that is about 1.2 µm, over 800 lines per millimetre; reflection holograms with nearly opposed beams need about λ/2n, more than 3000 lines/mm. Holography therefore uses ultra-fine-grain silver halide, dichromated gelatin or photopolymers, and the whole setup must stay still to a fraction of a wavelength during exposure.

### How are security holograms on banknotes and cards made?

Most are embossed rainbow holograms. A master is shot with a laser, a slit removes vertical parallax so it can be seen in white light, and the surface-relief fringes are electroformed into a nickel shim that is hot-stamped onto metallised foil like a seal. The relief is only microns deep and very intricate, so it is hard to copy, and it is often combined with kinetic diffractive images and microtext.

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