Super low-profile optical touch screens thanks to powerful miniature LEDs

Although optical touch screens are available in a wide variety of designs they do share a common principle in that the touch of a finger will either cast a shadow or reflect light. In basic versions an array of infrared emitters (IREDs) and detectors create a grid of vertical and horizontal beams. The components are housed in a low-profile frame, known as the bezel, measuring a half to one millimeter in depth around the screen. “To make this frame even shallower particularly small IREDs and detectors are needed, “ said Bianka Schnabel, Product Marketing Manager at OSRAM Opto Semiconductors. “This is especially important for mobile devices such as tablet PCs and laptops because of the positive effect on the design aspect.” The new CHIPLED SFH 4053 measures just 0.5 x 1 mm so it takes up very little space. With a height of 0.45 mm in industry standard 0402 (length/width) it is one of the smallest on the market.

In addition to size, the high light output of these components plays an important role, particularly for camera-based touch screens. Such a display setup requires considerably fewer IR emitters than the basic version but the emitters must have a very high output because they flood the display with infrared light from two corners. Next to the IR emitters are camera chips that detect a change in the signal when a pen or finger touches the display.
Thanks to OSRAM’s thin-film chip technology the SFH 4053 is another high-efficiency IRED that offers high output from a small package for this application, rated at 35 mW from an operating current of 70 mA. In pulse mode it can achieve as much as 260 mW from 700 mA. “This CHIPLED therefore provides enough light to make it easy and cost-effective to expand camera-based touch screen systems to larger screen diagonals”, added Bianka Schnabel.

The new CHIPLED is part of a broad range of components that are being used in more and more applications – from narrow-angle emitters for light grids to low-profile components for optimum injection into light guides. LEDs are available in 0603 sizes with lens types from +- 40° to +-10° and different chip technologies (thin film, nanostack).

The wavelength of the CHIPLED is ideal for the requirements of these applications. At 850 nm the light from the IRED is barely discernible to the human eye but receiver components exhibit very high sensitivity to this wavelength.