3D LCD panels
Any type of matrix display can be used as the basis of the 3D display. However, the recent fall in cost of LCDs has made some stereoscopic dis- play methods viable for mass production of 3D displays. The display itself is a standard mass produced LCD panel, with some additional optical ele- ments. There are several types of 3D LCDs:
● Parallax barrier: This type uses a series of black vertical lines in front of the display. These lines, known as a parallax barriers, are positioned so that each of the eyes only sees half the pixels, every other pixel. By care- ful selection of the barrier geometry, it is possible to adjust the viewing window position and angles for a 3D viewing. Apart from halving the resolution of the display, the main disadvantage to parallax barriers is that the barrier reduces the brightness of the display. The parallax bar- riers can be switched off to view a perfect 2D image.
● Lenticular or integral display uses tiny lenslets (very small lenses) attached to the display with high accuracy to split the image into left and right. Lenticulars are often slanted to improve the transition between viewing zones for a multiview display. The main advantage of lenticu- lars is that they transmit full brightness. The disadvantages include the fact that it is harder to switch the function off to achieve 2D.
● Time-sequential displays: This type employs a directional light source placed behind the display whose direction can be altered. In one frame, the light is directed to the left eye while the left image is displayed. In following frame, the light is directed to the right eye while the right image is displayed. In this way, full resolution is maintained for both views. However, it has the disadvantage that the refresh rate of the dis- play must be doubled.
Power supply requirements
As far as the electronic sections of a projection system are concerned, the power requirements are the normal nominal 3, 5 and 12 V d.c. These volt- age rails also serve the motor drivers for the focal unit. The light source, however, needs its own separate power supply incorporating a ‘ballast’ as well as power factor correction (PFC) circuits. Essentially, a ballast provides a suitable starting voltage for the lamp and then limit its current flow dur- ing steady-state operation. It is normal for the ballast itself to provide the power factor correction. It will also regulate the lamp output against line voltage changes, minimise power losses and obtains a high power factor.
Traditionally, electromagnetic (EM) ballasts were used which are designed to operate at line frequency (50 or 60 Hz) with very low light output per consumed power watt. Current ballasts are electronic switch- ing circuits with a high frequency (20–60 kHz) and increased efficiency.
Figure 19.19 shows a typical ballast circuit with half-bridge inverter circuit configuration. The inverter block consists of two controlled switching transistors T1/D3 and T2/D4, a coupling capacitor Ce, a resonant induc- tor L2, a resonant capacitor Cr and a lamp. The operation of the circuit is similar to that of a resonant converter and PFC described in Chapter 11. The rectifier D1/D2 and filter circuits convert the a.c. power into d.c. The half-bridge inverter converts the d.c. voltage into a high frequency a.c. voltage to start the lamp and stabilise the lamp current. When it is in a steady-state operation, the gating signals from the dimming and control IC determine the conduction of the switching transistors T1 and T2 and diodes D3 and D4. By controlling the duration when T1 is on relative to T2, the power transfer to the lamp can be adjusted. The energy going into the lamp is in the form of chopped d.c. which triggers tuned circuit L2/Cr into oscillation at a frequency of about 60 kHz.