Many technologies exist that have the ability to digitize different types of input. There are two main touch sensing approaches: active and passive. Passive devices include those that are receptive in a predictable manner to resistive, capacitive, acoustic surface wave, or electro-optical variations due to contact or touch by, for instance, a human finger or a stylus. Active devices include inductive and RF devices.
The key downside of active approaches is that an explicit object must be used (e.g., a special pen), which is implemented with electronics (and potentially batteries). This is more complex to manufacture. For example, pens augmented with infrared light emitters on their tips can be used on the commercially available Microsoft Surface.
Therefore, there is a need to construct a passive input tool without the need for electronics (including e.g., indirectly or wireless powered or EM coupled components including coils) or other active components.