This article describes how Optical touch technology works

Optical touch (now obsolete) used a small camera in each corner of the monitor

Optical touch was only supported on Windows XP and Windows 7 and later.  It was fully functional right out of the box, requiring no touchscreen drivers and no alignment.

• Windows XP limitations: Single touch only; landscape orientation only; no right-click support
• Windows 7 and later limitations: None

 Touch alignment automatically follows any screen rotation done in the Windows Display Control Panel.
 
Multi-touch gesturing is supported by Windows 7 and later for applications that are multi-touch compliant. Many basic applications do not support multi-touch; consult your application’s documentation to determine applicability.  You can test Windows 7 (or later) multi-touch operation using a brush or drawing tool in the Paint accessory. If only one line can be drawn at a time, make sure multi-touch is enabled in the Pen and Touch item, accessed via the Control Panel.
 
Connections required: AC power cable, display cable (typically HDMI) and USB cable for touch (only the power cable is required if an Elo computer module is installed in the back of the monitor).

What happens if an Elo touch driver is installed?
Touch continues to operate normally if an Elo touch driver is installed, but the Elo Control Panel options will be non-functional and “no touchscreen found” may be displayed. In some cases, the Elo options will attempt to work but they will not function or display normally.  The Elo driver is not required and should not be installed.

Keep the cameras and light diffuser clean:
A translucent purple light diffuser separates the black plastic bezel from the front glass (see attachment).  Keep the light diffuser reasonably clean.  There is a tiny (1 mm) black dot in each corner; these are the cameras.  Make sure the cameras are not obstructed by dust or debris.
 
Optical touch technology was discontinued in 2014.