# Engineering Metrology Questions and Answers – Grazing Incident Technique to Evaluate Surface Flatness

This set of Engineering Metrology Questions and Answers for Aptitude test focuses on “Grazing Incident Technique to Evaluate Surface Flatness”.

1. How many data points can be analyzed in 20 seconds using grazing incidence interferometry?
a) 35,000
b) 45,000
c) 55,000
d) 65,000

Explanation: A laser-based system is developed to view part surface topography using grazing incidence interferometry. It is a microprocessor based system. It can analyze more than 65,000 data points and computer flatness in less than 20 seconds.

2. Which of the following option is true for the evaluation of the surface by using grazing incident technique with laser beam system?
a) It can two dimensional profiles with submicron accuracy
b) It measure half the part surface at a time
c) It can produce three dimensional profiles with submicron resolution
d) Video screen is used to view test results

Explanation: It measures the whole part surface at one time and produces 2 and 3-D profiles with sub-micron resolution and accuracy. Test results can be viewed with the help of video screen and hard copied on a printer.

3. How much a laser beam can be expanded inside interferometer using grazing incidence interferometry?
a) 200 mm
b) 200 nm
c) 400 mm
d) 400 nm

Explanation: In this instrument, inside the interferometer a laser beam can be expanded into 200 mm column of light. The interference pattern obtained by this process represents the shape of the part of the surface.

4. Which system is used to automatically interpret the interference pattern?
a) Phase measuring interferometric system
b) Multi-color microscopic interferometry system
c) Two point distance measuring interferometric system
d) Michelson interferometric system

Explanation: Phase measuring interferometric system or we can say PMI can automatically interpret the interference pattern. It can capture a large number of data samples from test surface.

5. In how many picture elements the test surface is divided if a high contrast video system captures the image of an interference pattern?
a) 455336
b) 55536
c) 65536
d) 75536

Explanation: If a high contrast video system is used to capture images of the interference pattern then it divides the test surface into 65,536 picture pixels or picture elements. The map or pixel map is superimposed over the interference pattern and its intensity profile is recorded.
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6. How many data samples can be captured by the PMI system?
a) 1 million
b) 2 million
c) 1.5 million
d) 4 million

Explanation: PMI system or phase measuring interference system can capture over 2 million data samples. After capturing, it interprets them and then compiles the data into a 3-D image of the test surface.

7. From which part the light column is divided into beams in grazing incident technique?
a) At the hypotenuse of the system’s precision prism
b) At base of the system’s precision prism
c) After reflecting from the test surface
d) At perpendicular of the system’s precision prism

Explanation: At the hypotenuse of the precision prism of the system, the light column is divided into two beams i) the reference and ii) test beam. When these beams recombine, they interfere with one another and create a series of light and dark bands or an interference pattern.

8. How the intensity variation appears at each pixel in the grazing incident technique with laser beam system?
a) As a cosine wave
b) As a sine wave
c) Straight line
d) Hyperbolic

Explanation: The intensity variation at each pixel appears as a sine wave and recorded by the microprocessor. It is used to determine its relative phase. The phase differences are used to define the surface topography.

9. Which of the following statement is true about the grazing incident technique used to evaluate surface flatness?
Statement 1: Grazing incidence design turns rough surface into polished mirror surface to facilitate measurement.
Statement 2: It is important to prevent test beam to strike at grazing angle for measurement performance.
a) F, F
b) T, T
c) F, T
d) T, F

Explanation: The performance of measurement can be improved by striking the test beam at a grazing angle. The grazing incidence design turns non-reflective, rough surfaces into a polished mirror surface to facilitate measurement.

10. How many diffraction gratings are used by interferometer in grazing incident technique with laser beam system?
a) 1
b) 2
c) 3
d) 4

Explanation: The interferometer uses 2 linear diffraction gratings. These gratings are used to split and recombine the light into reference and test beams. The diffracted light of first order from the first grating reflects off at a grazing angle and it is the test beam. The zero order diffracted light is reference beam and passes parallel to the surface.

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