# Engineering Metrology Questions and Answers – Laser Inspection

This set of Engineering Metrology Multiple Choice Questions & Answers (MCQs) focuses on “Laser Inspection”.

1. Which of the following is not true about metrology lasers used in laser inspection?
a) These are high-power instruments
b) Used for rapid non-contact gauging of delicate parts
c) Have low optical cross-talk
d) Wide dynamic range

Explanation: These are low-power instruments. Laser systems have wide dynamic range, high contrast and low optical cross-talk. Laser systems can be used in inspection of surfaces and in dimensional measurements and are very useful in precision and accuracy measurements.

2. What is the wavelength of light produced by He-Ne laser in laser inspection?
a) 6988 Å
b) 5328 Å
c) 5928 Å
d) 6328 Å

Explanation: He-Ne lasers are used in laser inspection. Light produced by He-Ne lasers is at a wavelength of 6328 Å or 0.6 μm and that is in phase and coherent. This source is a 1000 times more intense than other monochromatic sources.

3. Which technique is not suitable to measure large diameter parts or large gaps?
a) Diffraction pattern technique
b) Scanning laser technique
c) Photodiode array imaging
d) Laser triangulation sensor

Explanation: Diffraction pattern technique is not suitable for diameters larger than few mm and is used to measure small gaps and small diameter parts. In this method, a parallel laser beam which is coherent is diffracted by a small part, and the resultant pattern is focussed on a linear diode array by a lens.

4. What is the accuracy of scanning laser gauge for objects of 10-50 mm diameter?
a) ±0.15 μm
b) ±0.25 μm
c) ±0.35 μm
d) ±0.45 μm

Explanation: Scanning laser gauge can provide an accuracy of ±0.25 μm for objects of 10 to 50 millimeter diameter. It can also be used for objects of diameter from 0.05 to 450 millimeter and offers repeatability of 0.1 μm.

5. Which of the following is true about resolution in two frequency laser interferometer?
a) Straightness resolution – 90 nm
b) Angular resolution – 3 arc seconds
c) Flatness resolution – 2 nm
d) Linear resolution – 1 nm

Explanation: The advantage of two frequency laser interferometer technique is that system does not depend upon the beam intensity and provides high sensitivity, it is also independent of noise due to air turbulence. The linear resolution by two frequency laser interferometer technique is 1 nm, angular resolution is 0.03 arc seconds and straightness resolution is 40 nm.
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6. How much accuracy can be achieved by photodiode array imaging?
a) ±0.05 μm
b) ±0.15 μm
c) ±0.5 μm
d) ±0.25 μm

Explanation: In photodiode array imaging method which is a laser inspection technique in which the stationary part’s shadow is projected on a solid-state diode array image sensor. For large parts, 2 arrays are used i.e. one array for each edge. Accuracies as high as ±0.05 μm can be achieved by this method.

7. Which application is ideally suited for two frequency laser interferometer?
a) Pitch and yaw measurement
b) Hole diameters
c) Thickness measurement
d) Measurement of edge locations

Explanation: 2 frequency laser interferometer techniques are ideally suitable for measuring linear positioning, pitch, yaw and straightness in two planes. The two-frequency laser head provides, one frequency with a P polarisation i.e. M (measuring) beam and another frequency with an S polarisation i.e. R (reference) beam.

8. The location of the image spot directly depends on which factor in laser triangulation sensor technique?
a) Wavelength of laser
b) Measuring range
c) Standoff distance
d) Focal length of lense

Explanation: The location of the image spot in laser triangulation sensor technique depends directly upon the standoff distance between the sensor and the surface of object. If standoff distance changes, it will result in a lateral shift of the spot along the sensor array.

9. Which technique from given laser inspection techniques is useful for measuring the diameter of hot steel bars?
a) Laser scanning gauge
b) Frequency laser interferometer
c) Laser triangulation sensors
d) Photodiode array imaging

Explanation: The Laser scanning gauge technique is very useful to measure the diameter and roundness of hot steel bars under the vibration conditions to an accuracy of 0.025 millimeter over diameters of 5 to 25 millimeter.

10. Where does wire is placed in a measurement of diameter using the technique of gauging wide diameter from the diffraction pattern formed in a laser beam?
a) Between collimator and lens
b) Between collimator and source
c) Between collimator and plane of measurement
d) Between collimator and detectors

Explanation: Gauging wide diameter is a method of the measurement of the thin wire diameter with the use of the interference fringes resulting from diffraction by the wire in the laser beam. Output variation from the photodetector is caused by changes in fringes.

11. How many sensors are needed to measure part thickness by using laser triangulation sensors?
a) 1
b) 2
c) 3
d) 4

Explanation: It is possible to measure part thickness or the inside bore diameter with the help of 2 sensors in laser triangulation sensor technique. The measurement accuracy and measurement range are directly related.

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