MCS260B
CORNERSTONE 260B MONOCHROMATORS
31
9 BLOCKING HIGHER ORDER RADIATION
Detailed information regarding gra
ting physics can be found in the Appendix of this user’s manual. A
summary of grating physics is noted here.
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Only wavelengths that satisfy the grating equation pass through the output port of the instrument.
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The remainder of the light is scattered and absorbed inside the instrument.
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The grating is rotated to bring different wavelengths of light in line with the output.
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A grating creates interference patterns when light is shown onto it.
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Different wavelengths interfere at different angles off the grating.
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Light occurs when there is constructive interference, called grating orders.
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All wavelengths interfere at one specific angle of the grating. This is called the “zero order”.
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When a parallel beam of monochromatic light is incident on a grating, the light is diffracted from
the grating in directions corresponding to m = -2, -1, 0, 1, 2, 3, etc.
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When a parallel beam of polychromatic light is incident on a grating, the light is dispersed so that
each wavelength satisfies the Grating Equation.
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Usually only the first order is desired. The other wavelengths in higher orders may need to be
blocked.
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The input spectrum and detector sensitivity determine whether order sorting or blocking filters
are needed.
9.1 ORDER SORTING FILTERS
For meaningful spectral measurements, care should be taken to remove unwanted orders of
radiation, particularly if the input radiation is intense or the detector is more sensitive at the higher
order.
Erroneous measurements may be taken because what was thought to be a measurement with a
single wavelength was a measurement using radiation at that wavelength
– but contaminated with
higher order radiation. Consider using Newport’s Colored-Glass Alternative Filters for blocking
higher order diffraction.
Example 1:
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A monochromator is set to output 600 nm and the signal is read by a UV-enhanced Si
detector with a spectral responsivity range of 200 nm to 1100 nm.
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Additional output will be 300 nm (600/2) and 200 nm (600/3). These are the second and
third order wavelengths.
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The second and third order wavelengths are within the responsivity range of this detector.
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In order to block these extra orders, a filter that blocks wavelengths below 300 nm and
transmits light at 600 nm needs to be inserted into the optical path.
Example 2:
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A monochromator is set to output 1200 nm and the signal is read by a Ge detector with a
spectral responsivity range of 700 nm to 1800 nm.
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Additional output will be 600 nm (1200/2) and 400 nm (1200/3). These are the second
and third order wavelengths.
•
No order sorting filter is needed as the second and third order wavelengths are outside the
responsivity range of this detector.
