Holography & Musical Instruments

1. Holography & Musical Instruments Pat Tiedemann Physics of Music – Professor Grossman 3-21-05

2. The term holography comes from the Greek holos (complete) because a holograph is a complete image of an original object. Holography captures both light intensity and phase, distinguishing it from photography, which can only record intensity. A photograph lacks the parallax that a holograph can display. Basics of Holography

3. Basics of Holography • Holography is able to record the phase of light waves from an object by interfering two light waves out of phase. The human eye cannot determine phase, but it can detect the phase difference between these two beams of light. The original object can be reproduced with a reconstruction beam used on the hologram, creating an apparently 3-D image. This image is really just focused light.

4. Simple Hologram Setup • A simple hologram can be made with the following setup:

5. Simple Hologram Setup • single laser beam is split by beam splitter • one beam continues uninterrupted to film • other beam is aimed at object, then at film • when the two beams meet on film, they interact and cause an interference pattern • the film records this pattern through a chemical reaction that changes the index of refraction of the gratings inside the film

6. Reproducing the Image • When light is shone through the holographic film after it is processed, the complex variations in index of refraction within the grating refract the light and produce a 3-D image which appears exactly like the original object. The way that the light is affected by the different indices of refraction as it passes through the film is called the ‘Bragg scatter.’

7. Holographic Interferometry • Interferometry uses two consecutive holograms of the same object that interfere with each other, usually by a double exposure. If the film grating is tiny enough (individual particle sizes in the order of 30 nanometers), then the film will be able to detect tiny differences in the phase of the light waves coming from the object. The goal is not to create a 3-D image anymore, but instead to show phase difference (on a 2-D plane).

8. Interferometry and Musical Instruments • When an instrument is played, its body vibrates in a complex pattern called a vibrational mode (like the Chladni plates). Although undetectable to the human eye, small vibrations on an instrument can be recorded with holographic interferometry.

9. Interferometry and Musical Instruments • The stripes that appear on the image indicate areas that have moved by the same amount. • Dark stripes indicate destructive interference, meaning that the two light waves that were recorded from the dark stripes are out of phase by n/2 λ. • Light stripes indicate constructive interference, meaning that the two light waves that were recorded from the light stripes are in phase.

10. Interferometry and Musical Instruments • Holographic interferometry can be used to visually display the vibrational modes of musical instruments. • This is especially useful for non-uniformly shaped instruments or other instruments where the Chladni sand experiment would be too messy or inconvenient (consider the curves and f holes on a violin). • Generally, holographs are a more effective and powerful way of displaying the vibrational modes of an instrument.

11. Interferometry and Musical Instruments • Using the data from a 2-D holographic image of an instrument, it is possible to build a 3-D animation of its vibrational mode. interference holograph and corresponding animation

12. Brief History of Holography • Holography was first used by Denis Gabor in the 1940s with a mercury arc lamp. His discovery did not immediately attract much attention due to the ‘incoherent’ nature of the mercury light, and the resulting low quality results.

13. Brief History of Holography • With the 1960s invention of the laser (light amplification by stimulated emission of radiation), holography now had the ‘coherent’ light source needed to produce high-quality images. Coherent light is monochromatic, of a single wavelength, and from a single point source.

14. Brief History of Holography • 1962 Emmett Leith and Juris Upatnieks create first laser transmission holograph of 3-D objects.

15. Brief History of Holography • 1962 Dr. Yuri N. Denisyuk makes it possible to recreate holograms with incandescent light.

16. Brief History of Holography • In 1968 Stephen Benton discovered a way of eliminating the so-called ‘rainbow smear’ effect. Using his method, only one color would be seen. This gave rise to the "rainbow" effect; as the viewpoint is moved vertically, the color of the hologram changes. Benton’s discoveries made ‘embossing’ and mass-production of holographs possible