Question E6F05:
Which of the following describes an optical shaft encoder?  

Choices:  
A: A device that detects rotation by interrupting a light source with a patterned wheel  
B: A device that measures the strength of a beam of light using analog-to-digital conversion  
C: An optical computing device in which light is coupled between devices by fiber optics  
D: A device for generating RTTY signals by means of a rotating light source    

Correct Answer is  A: A device that detects rotation by interrupting a light source with a patterned wheel  

Correct Answer: A — A device that detects rotation by interrupting a light source with a patterned wheel

1) Why A is correct
- An optical shaft encoder uses a light source (typically an LED) and a photosensor with a rotating disk that has slots or patterns.
- As the shaft turns, the patterned wheel interrupts the light, creating pulses. These pulses correspond to angular position and/or speed.
- This is the standard definition of an optical shaft encoder.

2) Why the other choices are incorrect
- B: “Measures the strength of a beam of light using analog-to-digital conversion” describes a photometric sensor system, not a shaft encoder. Encoders produce digital pulses related to rotation, not general light intensity measurements.
- C: “Light coupled between devices by fiber optics” describes optical interconnects or optical computing, not the function of a shaft encoder.
- D: “Generating RTTY signals by means of a rotating light source” is unrelated. RTTY generation is not what shaft encoders do.

3) Useful formulas and rules of thumb
- Pulses per revolution (PPR) sets angular resolution:
  $$\text{Angular resolution (degrees/step)} = \frac{360^\circ}{\text{PPR}}$$
- Rotational speed from pulse frequency $f$ (pulses/second):
  $$\text{rev/s} = \frac{f}{\text{PPR}}, \quad \text{RPM} = \frac{60 f}{\text{PPR}}$$
  $$\omega \text{ (rad/s)} = \frac{2\pi f}{\text{PPR}}$$
- Quadrature encoders use two sensors 90° out of phase; with x4 decoding you effectively get $4 \times \text{PPR}$ counts per revolution and direction information.
- Absolute encoders use coded tracks (often Gray code) to give unique position for each angle; incremental encoders provide relative movement via pulses.