Lec. 26, Thursday, April 15 Chapter 14: Holography. Hologram

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1 Lec. 26, Thursday, April 15 Chapter 14: Holography We are here How to make a hologram Clever observations about holograms Integral hologram White light hologram Supplemental material: CCD imaging Digital light projector 1 Hologram A hologram looks like a 3-D image. What is it really? A hologram is a photograph of an interference pattern (which is also called a diffraction pattern.) Example: two laser beams hitting a screen create an interference pattern of light and dark areas: 2

2 Clever observation One laser beam (beam 1) falling on the recorded pattern will be scattered from the pattern in a way that reconstructs the missing beam 2. Do this first to record the pattern: 3 Then this happens with one beam falling on the pattern: The hologram recreates the missing light when one beam is shined on it. 4

3 Second clever observation This also works if beam 1 is a laser beam and beam 2 is laser light scattered off some object (coffee cup). Then the beam 2 that is recreated looks like light from that object (a coffee cup). 5 The holographic image appears if the film is viewed from front or back (but some holograms are one-sided): 6

4 Third clever observation The holographic image is 3-dimensional. Why? When viewing the hologram, the left eye sees light scattered in the direction of the left eye and the right eye sees light scattered in the direction of the right eye, so the images seen by the two eyes are different. 7 Fourth clever observation Cut the hologram in half and you see the whole image, with some fine detail lost. Sounds like a great computer memory. Data is not lost if part of memory goes bad. 8

5 Fifth clever observation Laser light is not always necessary for viewing. 9 Integral hologram These go 360 degrees around the object. 1. The object is rotated on a turntable. 2. The film is passed through the camera simultaneously so that different views are on different sections of the film. 10

6 What do I see in the integral hologram? As you move around the curved film you see views from different directions. The image appears to rotate, or a dancer appears to dance. 11 Technical details 1. Film has fine grain ( mm). The image has details on the scale of the wavelength of light. Ordinary film records details of 0.01 mm. 2. The silver bromide film coating is thicker so that the pattern is many wavelengths deep. 3. The objects can t move during the picture-taking (except for integral holograms) 4. The laser beam must be expanded with lenses so that the light covers the object. 12

7 Lec. 26, Thursday, April 15 Chapter 14: Holography How to make a hologram Clever observations about holograms Integral hologram White light hologram We are here Supplemental material: CCD imaging Digital light projector 13 Digital Cameras Film is replaced by a CCD chip (and a memory chip.) Supplemental material 14

8 CCD is a charge coupled device A CCD is rows and columns of sensors which convert light to electric signals. It produces a bitmap image directly. One sensor element 15 Color CCD chips have filter arrays 16

CCD chip seen with a microscope. 17 How is the image stored? The easiest file type to understand in the bitmap (.bmp) which is a matrix of numbers. Bitmaps files are often too large (2 MB or more).

9 CCD chip seen with a microscope. 17 How is the image stored? The easiest file type to understand in the bitmap (.bmp) which is a matrix of numbers. Bitmaps files are often too large (2 MB or more). You could not store many images on a small memory stick. JPEG (joint photographic experts group) files are smaller (compressed about 10:1) and some detail is lost. They are converted back to bitmaps to be displayed. 18

10 Examples of JPEG compression 23 KB 8.4 KB 4 KB 19 What is a digital light projector (DLP)? It is an array of tiny mirrors which can tilt. Each mirror is 16 micrometers square. Maximum resolution now is 1280 x 720. Mirror array with insect leg. 20

Light shines on the mirrors and is projected (or not) onto the screen For color images, color filters are needed. Often a rotating wheel with color filters is used.

Interior view of a single-chip DLP projector, showing the light path.

11 Light shines on the mirrors and is projected (or not) onto the screen For color images, color filters are needed. Often a rotating wheel with color filters is used. 21 Internal components of an old 1998 InFocus LP425z single-chip DLP projector, with a 4-segment color wheel. Interior view of a single-chip DLP projector, showing the light path. Light from the lamp enters a reverse-fisheye, passes through the spinning color wheel, crosses underneath the main lens, reflects off a frontsurfaced mirror, and is spread onto the DMD (red arrows). From there, light either enters the lens (yellow) or is reflected off the top cover down into a light-sink (blue arrows) to absorb unneeded light. Wikipedia.com 22

What is a digital image?

What is a digital image? Lec. 26, Thursday, Nov. 18 Digital imaging (not in the book) We are here Matrices and bit maps How many pixels How many shades? CCD Digital light projector Image compression: JPEG and MPEG Chapter 8: Binocular