Technical Data Sheets
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Tel #: 215-412-8400
Fax #: 215-412-8450
Fax #: 215-412-8451
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Example
Certificate Of Calibration
EMS Catalog #79505 Sira Calibration Specimen
Certificate of Calibration
This specimen has been calibrated by diffraction measurements taken from an area approximately 1 mm diameter at the center of the grating area.
A laser beam of wavelength 632.8nm illuminated the specimen, and the angular separation of the diffracted beams measured on a certified calibrated rotary table.
The lined scribed on the face of the mount was first set horizontal so that the rulings perpendicular to this diffracted the incident beam in a horizontal plane (orientation A).
Owing to the small ruling spacing, the first diffracted beam is at an angle of almost 45º to the incident beam. It is therefore only possible to measure the first diffracted beam on either side of the central beam. In view of the large angle however, the possible errors in angle measurement are very small.
The specimen was then rotated to bring the scribed line to a vertical position so that the rulings at right angles could be measured (orientation B)
The complete series of readings were performed in triplicate.
The relationship between the diffraction angle and the grating spacing is given by
2 d sin 
=
n 
Where n = diffraction order
= Illumination
wavelength 632.8 nm
d = Grating spacings in nm
2 =
angle between corresponding diffraction orders on either side of the
central beam
In each case the mean angular separation of the diffracted beams was calculated.
- Orientation A = 43º 7.0'
- Orientation B = 43º 6.9'
In each case the uncertainty of measurement was within ±1' of arc.
Calculation of grating spacing.
Orientation A
- = 43º 7.0'
- sin =
0.68349
- d = 0.463 micrometer
Orientation B
- = 43º 6.9'
- sin = 0.68347
- d = 0.463 micrometers
A spacing of 0.463 micrometers corresponds to a grating ruled with 2160 lines/mm.
Measurements obtained with the diffraction apparatus in our laboratory on a similar grating were compared with independent measurements on the same grating certified by the National Physical Laboratory (NPL)
The reference of the NPL certified grating is 08A031/90005/ SEM 4/3
The instrument used by the NPL for the calibration of this grating was a scanning electron microscope fitted with a Helium-Neon laser interferometer. The measurements gave a pitch of 0.46 micrometers and the uncertainty in the measurements of 10 pitches was ±0.05 micrometers. The uncertainty in the calibration of the average pitch grating is therefore ±0.005 micrometers. The measurements obtained with the diffraction technique of our laboratory were in agreement with the certified calibration of the NPL to within the tolerances quoted.
The calibration of a grating using the diffraction technique effectively integrates the pitch spacing over a number of lines and the resultant measurement uncertainty is estimated to be ±0.001 micrometers.Notes for User of Calibrated Specimens
It should be pointed out that the diffraction measurements integrate over a considerable number of rulings, and therefore give a mean spacing with a high degree of accuracy. The results of a study at the National Physical Laboratory (NPL) suggest that the specimens are in fact very consistent in spacing in different areas of the surface. However, the measurement uncertainty in attempting to measure single line spacing does not permit us to assume that any individual line spacing is necessarily equal to the mean value. It is therefore prudent always to measure over at least 10 line spacings where possible, to reduce the potential for error.
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