The imaging scanning principle is a more conventional method using either reflected or transmitted light depending on the measuring substrate. The scanning assembly consists of the light source, a focusing lens, a scanning reticle with equally spaced lines, and photocells. The scale or glass disk carries the lines matching the scanning reticle. Relative motion of the components creates pulses of light. When the lines of both the reticle and the glass disk exactly align, no light passes through. Once they begin to move, light gets through until it reaches maximum intensity and the reverse starts to happen, diminishing intensity. This alternating sequence of light passes through to the photocells creating sinusoidal electrical signals.

The last angle-encoder category is modular encoders or encoders without an integral bearing. These encoders are for integration into a system. Their scanning head must be adjusted to the measuring standard. They're also well suited for applications requiring no additional starting torque, like an air bearing table, or for systems with large internal diameters to 10 m. Because these encoders have no integral bearing, certain models can reach mechanical speeds of 40,000 rpm. A tape scale lets them measure a segment of 360° with as little as 50° of traverse.

Consider several factors when evaluating modular encoders. For starters, look at the system accuracy which includes the positional integrity within one revolution and the position deviation within one signal period. Also, consider the error caused by eccentricity of the graduation to the bearing as well as error from radial deviation of the bearing. If the encoder is to deliver optimal performance it must adhere to the mounting tolerances for that particular model. Ignoring installation tolerances can lead to additional error. The amount of error will differ depending on the scanning technique or type of encoder.

When making the final choice of an angle encoder, there are several considerations other than the mechanical package. For example, the proper interface and output of the encoder are important. There are also mechanical and environmental issues such as contamination and washdown. Protective covers are a good idea for modular encoders. Devices with an integral bearing can accommodate an air purge system for extra protection. Mechanical issues to consider include temperature, acceleration, shock, and vibration.