Quadrature Encoder
This
encoder is designed to be read by two sensors. The disk pattern is designed to
create two square waves that are 90° out of phase. Given the two signals and
the two phases there are four signals from this encoder (hence the Quad
designation). You can also use a rate encoder
as a Quadrature encoder if you put your sensors out of phase by 90°.
The image above is being shrunk by your browser to be 100 pixels across, its native resolution is 600 pixels so if you print it on a 600 DPI laser printer it will be a 1" encoder and at 300 DPI a 2" encoder.