Using cellophane to convert a liquid crystal display screen into a three dimensional display



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Using cellophane to convert a liquid crystal display screen into a three dimensional display (3D laptop computer and 3D camera phone)


Keigo lizuka

Department of Electrical & Computer Engineering


35 St. George Street
University of Toronto
Toronto, Ontario, Canada M5S 1A4

Contents

      Abstract


1.   Properties of Cellophane
2.   Stereoscopic Principles
3.   Manipulating Polarized Light
4.   Creating 3D Images
5.   Eye Fatigue
6.   Eliminating the Need to Wear Glasses
7.   Using a single camera phone
8.   Method Summary
9.   New Applications
10. Conclusion
       References

Abstract


We present a novel, inexpensive, stereoscopic technique for generating 3D images from cellophane on a liquid crystal displays which are most typically used for a laptop screen or a camera phone screen. Stereoscopy requires independent manipulation of the left and right eye views. 1 Our technique takes advantage of two facts; the first is that the light from the liquid crystal display of either a laptop computer or a camera phone is polarized light 2, and therefore we can easily manipulate its transmission with a polarizer sheet. The second fact is that a cellophane half-waveplate can change the direction of polarization of light. The direction of polarization of one half of the liquid crystal screen was rotated by the cellophane half-waveplate. Two images displayed with orthogonal polarization on two halves of the screen become separable by wearing a pair of glasses of orthogonal polarization.

A distinct advantage of our technique is its simplicity; either a laptop screen or a camera phone screen can be converted into a 3D display with minimal knowledge of optics. An additional advantage of our technique is that we can eliminate the need for the observer to wear special glasses by making the screen wear the glasses instead. This is possible because there is normally only one viewer at a time, and the relative orientation of the viewer's head and the screen is sufficiently stationary. A further significant discovery is that we verified that cellophane (costing mere pennies) proved to be a better half-waveplate than a commercial half-waveplate (costing hundreds of dollars for the required size) for rotating the polarization of white light.


1. Properties of cellophane


Let us begin by examining the properties of cellophane. Cellophane is fabricated by protruding an alkaline viscose solution through a narrow die into an acid bath. Because of the unidirectional strain during the protruding process, cellophane is an anisotropic material and it behaves like a calcite crystal. The refractive index ny of cellophane measured by a light wave component polarized in the direction of the longer dimension of the rolled cellophane (in the y direction) is larger than nx, measured by a light wave component polarized in the direction of the shorter dimension (in the x direction).

As a result, the component polarized in the x direction propagates through the medium faster than the component polarized in the y direction. After transmission through such a medium, a phase difference arises between these two light wave components. The difference ny-nx in the refractive index and the thickness of the cellophane determine the amount of the phase difference between the components polarized in the x and y directions. A medium that creates a 180o phase delay is a half-waveplate. The phase difference incurred in our plain ordinary colorless cellophane sample was measured to be 170.2o , which is about 95% of the phase delay of an ideal half-waveplate. Our cellophane sample was purchased from Lewiscraft. It was sold under the brand name "cello GIFT WRAP clear," and its SKU number was listed as #17606. We measured the cellophane thickness to be 25 microns. The 170.2o phase delay of is within acceptable limits for a number of practical applications that do not require a precise 180o phase delay. (For an unknown kind of cellophane, a simple test can be performed by inserting the cellophane in question between two polarizer sheets polarized in the same direction. As you rotate the cellophane sheet you may see a variation in the transmittance of the polarizer sheets. If you can find an angle for which the polarized sheets become almost completely opaque, the cellophane passes the test. If there is no angle for which the sheets become opaque, the cellophane sheet is of no use. There are two very important tests that need to be done in order to construct your 3D system. This is the first important test, which is confirming that the cellophane has the necessary properties. The second test, which confirms the the crisscrossed paths, is explained later).



One of the most important functions of a half-waveplate is its ability to rotate the direction of polarization of the transmitted light. We found that cellophane’s performance in rotating the direction of polarization of white light was superior to that of a commercially available half-waveplate designed for a specific wavelength. An added bonus is that cellophane is very inexpensive. Before describing the role of a half-waveplate in generating 3D images, we need to introduce some basic stereoscopic principles.

2. Stereoscopic principles


Figure 1 explains the basic principle of a 3D display based upon the parallax effect1.








(a)

(b)




The observer sees the ball in front of his or her eyes.

A picture of the ball is drawn on the screen by extending the lines from the eyes to the ball.











(c)

(d)




The observer sees two balls on the screen, and there is no stereoscopic effect as yet. To produce a stereoscopic effect, we must find a way to eliminate the views represented by the dashed lines. The simplest although not the most practical way is to block the dashed line paths by extended hands.

For a more practical method, polarized light is used. Polarizer glasses are worn to block the dashed line paths yet pass the solid line paths.




Fig. 1 Principle of stereoscopy.




Figure 1(a) shows what an observer’s left and right eyes see when a Fusen (a traditional Japanese ball) is flying toward the observer. The left eye sees the ball to its right, while the right eye sees the same object to its left. Our brain judges the distance to the ball using this information as well as other factors. Figure 1(b) is an attempt to fool the brain. Two pictures of the ball are drawn on the screen located behind the ball by extending the lines of sight onto the screen. As far as the light paths between the center-crossing point and both eyes are concerned, they are the same as the light path that would have been created by the actual ball.

Do these two ball pictures on the back screen alone give the observer the illusion that the ball exists off the screen? The answer is emphatically "No!" The reason is that each eye sees both pictures of the ball on the screen as indicated by the solid-line traces and the dashed-line traces in Fig. 1(c). In order to create the illusion that the ball exists off the screen, we must find a way to ensure that each eye sees only one picture of the ball as indicated by the solid-line traces blocking the dashed-line traces. Try to block only the dashed-line traces and pass the solid-line traces by extending your hands. You will find the picture becomes instantly three dimensional. If we eliminate the pictures that correspond to the dashed-line traces in Fig. 1(c), the light paths in Fig. 1(c) become identical to the light paths shown in Fig. 1(b), and the illusion that the ball exists outside the screen is created. One way to accomplish this without using hands is by using polarized light and wearing polarizer glasses as shown in Fig. 1(d). Manipulation of polarized light makes it possible for each eye to see only the picture corresponding to the solid-line traces.


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