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



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4. Creating 3D images

4.1 Using a laptop computer


Figure 3 illustrates how we can obtain 3D images from the liquid crystal display of a laptop computer3. The polarization directions are indicated by double-headed arrows.





(a)

(b)

By wearing polarizer glasses, the observer sees only the light paths represented by the solid lines in Fig. 1(c). The polarizer glasses reject the light paths represented by the dashed lines in Fig. 1(c).

The laptop computer wears the polarizer glasses instead of the observer.

Fig. 3 Converting a laptop computer screen into a 3D display.




The "polarizer" glasses shown in Fig. 3 are glasses in which the coverings for the eyes are constructed from polarizer sheets that have been cut down to a size suitable to fit into the cardboard frames of the glasses. With our laptop computer, the direction of polarization of the light from the screen is at 45 from the horizontal direction. (Note that the direction of polarization of light is not necessarily at 45; the angle depends on the type and make of the device. If it is not at 45 the difference should be accounted accordingly). An observer looking at this screen through polarizer glasses whose transmission axes are at 45 will be able to see the whole screen.

If, however, the right half of the laptop computer screen is covered by a cellophane sheet, the direction of polarization of the covered section is redirected to 135 (= 45 + 90 ). Let us first consider what the observer sees through his or her right eye while wearing the polarizer glasses. The transmission axis of the polarizer covering the observer’s right eye is at 45 , which means the right eye can no longer see the covered section of the screen. The right eye can see only the uncovered left half of the screen. In short, with this configuration, the right eye sees only the ball displayed in the left half of the laptop computer screen.

Next, we turn our attention to what the observer sees through his or her left eye. If the direction of the transmission axis of the polarizer covering the left eye is set at 135 , the left eye sees only the picture of the ball displayed in the right half of the laptop computer screen; it cannot see the ball in the left half of the screen.

In summary, with this configuration of the laptop computer screen (half covered by cellophane) and the orientation of the polarizers in the viewing glasses (left polarizer at 135 and right polarizer at 45 ), we can eliminate the light paths in the dotted lines in Fig. 1(c) and duplicate the desired crisscross light path shown in Fig. 1(b), thus generating the illusion to the observer that there exists an actual ball.


4.2 Using two camera phones


A similar experiment can be performed using two separate camera phones placed side by side as shown in Fig. 4. (The details are reported in Ref. 4.) The two camera phones are held with a spacing of about 6.5 cm, which is the average spacing between human eyes. A pair of pictures taken with this spacing is a pair of stereoscopic images. The object was a horizontally placed red pencil tip pointing toward the cameraman. This stereoscopic pair is sent by the transmitter phones to a pair of dialed distant receiver phones so that the two transmitted images are reproduced at the receiver site.

The receiver site has to do two things:



  1. Transpose the images. To transpose the images, the image taken by the left transmitting camera phone is sent to the right receiver phone, and likewise, the image taken by the right transmitting camera phone is sent to the left receiver phone. Otherwise, the tip of the reconstructed pencil image would point away from the observer and the depth information would be reversed (pseudoscopic image). The operation of the transpose is photographically indicated by the crossed arms in Fig. 4(b).





    (a)

    (b)

    Geometry.

    Corresponding photograph.

    Fig. 4 Viewing the transposed stereoscopic image in the receiver phones through polarizer glasses.




  2. Rotate the polarization direction of one of the images. The display of the left camera phone in Fig. 4 is covered with the cellophane half-waveplate with its fast axis (the direction perpendicular to that of the roll of the cellophane) in either the horizontal or vertical direction. Light from the Panasonic GD 88 display is polarized at 135 from the horizontal direction, while that of the cellophane-covered receiver phone is now rotated by -90 to 45 from the horizontal direction. Thus, the stereoscopic pair of images is displayed with orthogonal polarizations on the two phones side by side. Images in orthogonal polarization (135 on the right and 45 on the left) become separable by wearing the above mentioned pair of glasses of orthogonal polarization.
    (Note that the direction of polarization of light from the laptop was 45 whereas that of the camera phone is 135 and the cellophane sheet was placed on the left phone so that the same pair of glasses can be used for both the laptop and the camera phone). A sure check is to examine whether or not the right eye can see only the screen of the left phone, and similarly, the left eye can see only the screen of the right phone, thus generating the illusion to the observer that there exists an actual object off screen. (This is the second of the two most important tests in constructing the 3D display system. The first test was to verify the cellophane properties. The second test is to confirm these crisscrossed light paths by using your hand to cover one eye at a time.)
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