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Visible and invisible semiconductor light for tomorrow’s TV

Visible and invisible semiconductor light for tomorrow’s TV

Technology News |
By eeNews Europe



In order to perceive a two-dimensional TV picture spatially, people need both eyes. They perceive objects from different angles and send two different images of an object to the brain, which then puts the two together in one image with perspective.

This is why two cameras are always needed for 3D technology to record the image from different angles. Both fields are then transmitted to the TV set and shown in chronological sequence. “Shutter-glasses” then ensure you can see the first camera’s images with one eye and the second camera’s images with the other.

The digital eyepatch

Looking at shutterglass technology in more detail, the fields are no longer dyed as they used to be or polarised as they are in cinemas but simply placed in sequence or shown alternately. The images and perspectives therefore change in fractions of a second. Shutterglasses include LCD lenses, which cycle for each eye rapidly between transparent and opaque. So while one eye sees the TV picture for a brief moment, the other is momentarily blind. This happens so rapidly that what the viewer perceives is a flowing, uninterrupted image that is three-dimensional. To black out at the right moment, the TV communicates with the glasses via infrared light. Infrared light-emitting diodes, or IR LED for short, which are installed in the TV set, synchronise the glasses with the TV picture.

These diodes are characterised by their short response times and small scale. They have to produce maximum output to provide the signal to black out the glasses in the 200Hz–600Hz range at the right moment. One or two silicon photo-diodes are incorporated in the glasses on the other hand, which convert the IR LED visual signal into electrical signals and therefore control switching of the LCD lenses.

The rapid change of angles is also possible due to the high repeat rates of modern LCD TV sets. At 100–600 frames per second (100Hz–600Hz) shots from different angles can be easily placed after each other without disrupting the image flow. Human beings can no longer distinguish individual shots from around 25Hz. The brain therefore converts them into a flowing sequence of moving images.

LED light for a perfect view

However, 3D TV sets have to do more than produce rapid frame rates. The interaction of glasses and TV requires particularly powerful backlighting of LCD displays.

With shutterglasses only some of the display’s brightness output reaches the viewer; almost 50 percent is swallowed by the glasses’ polarisation. The glasses’ liquid crystal lenses also absorb another 5–10 percent of the light in the translucent state. The special glasses also use different systems: some that are open during the entire frame and others that black out again after half the time. This often means less than 20 percent of the original light density remains. Apart from less brightness the picture contrast also deteriorates.

To improve picture quality, a “blinking backlight” or “scanning backlight” can be used. The blinking backlight makes the entire LED backlighting flash white in synch with the image and therefore makes up for the reduced brightness caused by the glasses. With the second approach, the scanning backlight, a black strip is drawn line by line through the backlighting. Both versions increase the channel separation or selective perception for the right and left eye. Ghosting, the overlapping of individual images for left and right, is avoided and sharpness improved, especially with moving pictures and contrast overall.

Backlighting solutions

When using light diodes for backlighting liquid crystal displays, there are essentially two different approaches – edge and direct backlights.

With an edge-light LEDs emit light laterally into a light PCB behind the LCD. Depending on the brightness required, layout and component specification, the LEDs can be attached on one to four sides and couple their light into the edges of the light conductor. The special structuring on the upper surface of the light conductor ensures the light is coupled out to the LCD evenly.

Image 1: OSLON edge backlighting

The second approach involves direct backlighting, whereby the LEDs cover a PCB behind the display in a matrix arrangement. Without deflection the light is shone into the film stack and the LCD. In order to guarantee homogeneous backlighting where height is limited, apart from special diffuser foils, a large number of LEDs are also used. With 40 inch TVs they total more than a thousand.

Direct backlighting enables technologies that provide even more brilliant TV pictures. Contrast depth is therefore increased via local or area dimming. Behind dark areas in the TV picture, individual LEDs or LED arrays are dimmed or switched off.

Besides this, dimming techniques can achieve energy savings of up to 50 percent and meet the environmental guidelines for LCD TV sets.

As fewer LEDs are required and with the opportunity to design thinner TV sets, more edge-light solutions are used. The light PCB, usually made of acrylic glass (PMMA), guarantees ideal, homogeneous and seamless backlighting of the display and constitutes the core of the edgelight system. With particularly flat light PCB designs of less than 2mm thickness it is possible to produce very slim LCD TVs, far flatter than displays with direct backlighting. Adapted, slim LEDs, whose output window is smaller than 2mm, are used here.

Light conductor solutions have become the standard with small display applications such as mobile phones, PCs or satnavs and are catching on in large displays too. Even though dimming of individual areas is not directly possible, work is underway on various image-improving techniques.

Thin-film technology

Diode light in backlighting LCD displays has been given a boost by thin-film technology. With thin-film LEDs, 97 percent of the light is focused and directed forwards without lateral scattering. This is then available directly for coupling into the light conductor. Unlike conventional, volume LEDs, where some of the light escapes laterally from the chip, almost no luminance is lost here. This makes these diodes especially efficient. Plus, due to its superior efficiency, it produces less heat, which would affect the light conductor’s linear expansion. This is particularly important when used in display backlighting. The remaining heat generated has to be discharged backwards with good system design via the PCB and enclosure. Another advantage of thin-film technology is the diodes’ luminance increases proportionate to the chip size. This means large-scale LEDs can be produced if required. LEDs with conventional substrates lose their efficiency on the other hand as the area increases.

Products: brilliant colours with Osram Opto Semiconductors’ LEDs

In order to perceive sufficient brightness despite shutterglasses and overlapping pictures on TV sets, 3D displays have to be very bright. To achieve this, Osram Opto Semiconductors provides a wide range of products. At Electronica 2010 the TOPLED Compact 4520 was presented for the first time. These LEDs score well with their very high efficiency of 85lm/W (at 150mA) and they are perfectly suited to pulse operation, as they can take very high current and produce high-resolution pictures.

Products: communication with IR LED

For communication between TV sets and shutterglasses the IR LEDs type SFH 4250S are especially suitable. They are the first IR-powered TOPLED with a stack chip. These Osram Opto Semiconductors’ IR LEDs are very compact and provide a lot of output in a small area. Osram’s stack chips have already proved themselves in numerous applications beyond TV sets, such as mobile devices or for security systems such as night view cameras in vehicles. The light is emitted at a wavelength invisible to humans of 850nm and is therefore ideally coordinated to silicon photo-diodes, which convert the visual signals back into electrical ones. Diodes that are particularly suitable for this are the models SFH 235 FA or SMT version BPW 34 FAS, which both come with a daylight blocking filter.

Outlook

Virtually all the major TV manufacturers sell 3D TV models and are launching their first systems, initially with displays of over one meter diagonally. With only slight price increases for 3D capability these new systems are being marketed aggressively. This year, the market expects several hundred thousand systems to be sold but as soon as 2013 there will be far more than 30 million of them. As even the latest TV sets cannot be upgraded, 3D TV has the potential to be the driver of market growth in coming years, especially for the LED market. Because only with light diodes can TV take over tomorrow’s living rooms.

About the author: Janick Ihringer is employee of Osram Opto Semiconductors. He is working on applications for Osram’s General Lighting segment.

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