Reactable

building the table
table & surface

A camera and a projector with wide-angle lenses need to be placed underneath the table, so they can both cover the entire surface. Alternatively a mirror can be used in order to achieve a larger projection distance. For the interactive surface itself a normal perspex board can be used in conjunction with some ordinary tracing paper on the top side for the projection. This material is completely transparent for objects and hands in direct contact with the surface.
illumination

For the tracking, the objects need to be properly illuminated, so the camera and thus the computer vision application can see them correctly. For the projection onto a table, the surface needs to be dark though, so the user can see the projected image well enough. Since these two necessary steps logically exclude each other, the solution is to operate in two different spectra:
The projection has to be visible to the user, so the computer vision component needs to operate in a different, invisible spectrum such as infrared. Most CCD cameras are perfectly sensitive within the near IR spectrum, therefore infrared LED lamps can be used to illuminate the table. All light from the visible spectrum needs to be filtered in the camera, so the computer vision algorithm is not disturbed by the projection. Eventually an existing infrared blocker needs to be removed from the camera sensor.
camera

In general cameras with CCD sensors should perform better than those with a CMOS chip. Additionally you should make sure that the camera has an acceptable lens. These two criterias basically exclude cameras below the 50$ range. DV cameras supporting full-frame mode are suitable, while those with interlaced mode only, will not work at all. For lowest latency and best performance we recommend firewire cameras from the top range, such as industrial cameras with a high framerate and resolution. These cameras usually also come with high quality lenses. Cheaper firewire cameras, such as the unibrain fire-i also allow optional wide-angle lenses.