90088262 MCS130B
CORNERSTONE 130B MONOCHROMATORS
30
9
8B
BLOCKING HIGHER ORDER RADIATION
Detailed information regarding grating physics can be found in the Appendix of this user’s manual. A
summary of grating physics is noted here.
•
Only wavelengths that satisfy the grating equation pass through the output port of the
instrument.
•
The remainder of the light is scattered and absorbed inside the instrument.
•
The grating is rotated to bring different wavelengths of light in line with the output.
•
A grating creates interference patterns when light is shown onto it.
•
Different wavelengths interfere at different angles off the grating.
•
Light occurs when there is constructive interference, called grating orders.
•
All wavelengths interfere at one specific angle of the grating. This is called t
he “zero order”.
•
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.
•
When a parallel beam on polychromatic light is incident on a grating, the light is dispers ed s o
that each wavelength satisfies the Grating Equation.
•
Usually only the first order is desired. The other wavelengths in higher orders may need t o be
blocked.
•
The input spectrum and detector sensitivity determine whether order sorting or blocking
filters are needed.
9.1
40B
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 more sensitive at t h e higher
order.
Erroneous measurements may be taken because what was thought to be a measurement wit h a
single wavelength was actually 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:
•
A monochromator is set to output 600 nm and the signal read by a UV -enhanced Si
detector with a spectral responsivity range of 200 nm to 1100 nm.
•
Additional output will be 300 nm (600/2) and 200 nm (600/3). These are the second
and third order wavelengths.
•
The second and third order wavelengths are within the responsivity range of this
detector.
•
In order to block these extra orders, a filter that blocks wavelengths below 300nm
and transmits light at 600nm needs to be inserted in the optical path.
Example 2:
•
A monochromator is set to output 1200 nm and the signal read by a Ge detector
with a spectral responsivity range of 700 nm to 1800 nm.
•
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.
Summary of Contents for Cornerstone 130B
Page 1: ...90088262MCS130B Rev B s 1 8m Monochromator Family User s Manual Cornerstone 130B...
Page 32: ...90088262 MCS130B CORNERSTONE 130B MONOCHROMATORS 32 Figure 23 Model 74010 Filter Wheel...
Page 44: ...90088262 MCS130B CORNERSTONE 130B MONOCHROMATORS 44 Figure 33 Model 74006 Mounting Plate...
