Calibration
In most cases, the touch surface must be initially calibrated with the display image. This process involves displaying a sequence of dots or crosses on the touch surface and having the user select these dots either with a stylus or their finger. This process is called alignment, calibration, or orientation. Fixed installations with projectors and boards bolted to roof and wall greatly reduce or eliminate the need to calibrate.
A few interactive whiteboards can automatically detect projected images during a different type of calibration. The technology was developed by Mitsubishi Electric Research Laboratories Inc and is disclosed in patent 7,001,023. The computer projects a Gray Code sequence of white and black bars on the touch surface and light sensitive sensors behind the touch surface detect the light passing through the touch surface. This sequence allows the computer to align the touch surface with the display; however, it has the disadvantage of having tiny fiber-sized "dead spots" in the resistive touch surface where the light sensors are present. The "dead spots" are so small that touches in that area are still presented to the computer properly.
Another system involves having a light sensor built into the projector and facing the screen. As the projector generates its calibration image (a process called "training"), it detects the change in light reflected from the black border and the white surface. In this manner it can uniquely compute all the linear matrix transform coefficients.
Read more about this topic: Interactive Whiteboard