Difference between revisions of "Holograms"
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See Setups for more information on making these types of holograms.
A Hologram Defined
hol·o·gram (hŏl'ə-grăm', hō'lə-) n. A Diffraction Pattern which, when properly lit, produces a three-dimensional image. Typically, holograms are made using a laser as a light source and and a very high resolution film or glass plate to record the diffraction pattern resulting from interference between light coming directly from the laser and light reflected from the object.
This was the one of the first holograms made. A transmission hologram is made when the reference beam and light from the object enter the recording material from the same side. The recorded interference fringes form a transmission grating which diffracts light passing through the hologram.
- looks like a blurry rainbow image when viewed with white light
- viewable as a sharp image only by shining laser light through the hologram
- recording material requirements are more relaxed (less resolving power is needed)
- simple set-up
- greater depth of the scene is possible
- the scene can be projected by shining a collimated laser beam through the hologram
A reflection hologram is made when the reference beam and light from the object enter the recording material from opposite sides.
- viewable in regular light
- very simple Denisyuk style setup can be used
- finished hologram is monochromatic (a single color) for each laser color used
- color can be shifted by pre or post shrink/expanding recording material
H1 to H2 copies
H1 refers to a first generation (master) hologram. H2 refers to a copy made from the H1. H1s are usually transmission holograms and H2s are usually reflection. They may use different recording materials.
- Somewhat complex setup requirements
- Objects can be made to appear to be coming out of the plate towards the observer
- Once a usable master H1 is made and the setup constructed, many copies can be produced easily
Rainbow holograms are transmission holograms which are produced in such a way as to be viewable in regular white light. Depending on the viewing angle, the color changes (hence the term rainbow)
- perspective information is lost in one axis (for example, you may not be able see a change in perspective when looking from above or below)
Open-Aperture Transmission Hologram
An open-aperture transmission hologram is simply a transmission hologram the has the image very close to the film plane and is designed to be viewed in white light.
- White light viewable.
- Image blurs colors as the image move in front of, or behind the film plain.
- 2 cm usable depth of field.
- The image is achromatic.
Multiplexed holograms store many different holograms on one piece of film usually as multiple exposures.
- simple animations are possible
- diminishing quality as more holograms are stored
Edge Lit Holograms
Edge Lit Holograms have the reference beam entering the plate from one edge instead of one face. This allows the illumination to remain hidden from the observer and makes for a fairly compact display.
- They are difficult to make.
Edgelit holography:Extending Size and Color by Ryder Sean Nesbitt
Embossed holograms are made by forming a rainbow transmission hologram in thermoplastic and bonding it to a mylar mirror. It is the kind of hologram seen on credit cards.
- very low per-unit cost when mass-produced
- shallow hologram depth (usually just a few millimeters)
- durable and flexible
- mass production can use existing equipment and technology (e.g. CD production)
Pulsed Holograms can be either transmission or reflection. The key difference is the pulsed laser emits a short, powerful pulse of light rather than a continuous beam. This pulse (about 20ns) is short enough to stop moving objects and make an image. Even bullets can be stopped with the correct setup. Most often used for portraits.
- The flash photography of holography
- Stability requirements are greatly diminished, allowing for holograms of people, melting ice, flowers, animals, etc. to be made
- Pulse lasers are very expensive
- Setup and testing can be tricky and dangerous
True Color Holograms
True color holograms are a variety of reflection hologram made with more than one laser color. There have been good true-color holograms made with two, three and four colors of lasers. The resulting hologram displays the same colors as the original object.
- Since true color holograms are multiplexed holograms, recording material need to be capable of holding a lot of information
- Lasers and equipment can be expensive and tricky to set up