Camera-Based Optical Touch Technology

ambiguous term because there are several different methods of using light to detect touch. This article describes “camera-based optical touch,” in which two or more CMOS infrared (IR) cameras are placed on top of a display, looking across the surface of the display in order to detect the presence of a touching object. Several other types of optical touch technology are described in the following paragraphs but not covered in detail here. Traditional Infrared: Traditional infrared touch technology uses an array of infrared light-emitting diodes (LEDs) on two adjacent bezel edges of a display, with IR photosensors placed on the two opposite bezel edges. When a touching object interrupts the grid of IR light beams, a controller calculates the X-Y touch coordinates. Waveguide Infrared: RPO’s “Digital Waveguide Touch” uses one or two IR LEDs to provide a planar sheet of IR light projected from two adjacent bezel edges, along with polymer optical waveguides at the opposite bezel edges to direct the light into 10-μm channels leading to a small photosensor array. As in traditional infrared, a touching object interrupts the light projected across the display and a controller calculates the X-Y touch coordinates. Vision-Based: Vision-based touch systems employ one or more IR imaging cameras positioned so that an image of the entire screen can be captured. Because this usually means that the camera must be located a significant distance away from the screen, most vision-based touch systems are therefore implemented with the detecting cameras located behind a projection-screen surface. After capture, screen images are deciphered by image-analysis software to determine the coordinates (and often the geometry) of touching objects. LCD In-Cell Optical: LCD in-cell optical touch, also called “in-cell light-sensing,” functions by adding a photo-sensing transistor into some or all of an LCD’s pixels (i.e., in the TFT backplane). In its original concept, this technology used visible light, sensing either the shadow of the touching object from ambient light or the reflection from the backlight. Currently, the trend is toward the use of infrared light sourced by IR LEDs added to the LCD’s backlight. In this configuration, IR photosensors receive light reflected by touching objects; a controller samples each photosensor and calculates the X-Y coordinates of touching objects from the light intensities. In the remainder of this article, “optical touch” refers only to camera-based optical touch.