Blocking Higher Order Radiation

Monochromators vs. Spectrographs

In a monochromator, the design concentrates on the path of light from the input slit, off the grating and to the output slit. Light of other wavelengths is absorbed. At any grating setting, only a very small range of angles around the diffraction angle D, figuratively “one” wavelength, passes through the monochromator.

A spectrograph looks simultaneously at a wide range of angles D, and therefore at the range of wavelengths that satisfy the grating equation for this range of angles D. The optics of Oriel’s spectrographs are designed for this purpose. A slit is replaced at the output of a spectrograph by long open area over which the various wavelengths are spread in a known fashion.

Blocking Higher Order Radiation

Figure 1. Polychromatic light diffracted from a grating.

The dual output port models of the MS260i and MS257 spectrographs allow users to employ the instruments as monochromators by having a slit installed at the lateral output port. The topic of higher order radiation is applicable to both spectrographs and monochromators. Therefore it is beneficial to understand the effects of higher order radiation as it applies to both types of instruments.

In general, when a monochromator is set for wavelength λ, radiation at λ/1, λ/2, λ/3 can also be present at the output. The denominator 1, 2, 3, etc. is called the order of diffraction. Radiation at λ is called first order radiation, which is the desired wavelength when the monochromator is set to output λ. Radiation at λ/2 is called second order radiation, λ/3 is called third order, etc. All radiation at λ/2, λ/3, etc. can be collectively referred to as higher order radiation. As an example, when a monochromator is set to output 600 nm, any radiation present at 200 nm or 300 nm will also pass through the monochromator. Similarly, if the grating is set for 1000 nm, then 500 nm, 333 nm, 250 nm and 200 nm radiation also passes through the monochromator.

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 the higher order. Erroneous measurements may be taken because what was thought to be a measurement with 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.

As an example, if a scan from 450 to 1100 nm is desired, choose a 10CGA-420 filter, which transmits from 420 nm. This will eliminate second order radiation to ~780 nm. To measure longer wavelengths, stop the scan at 700 nm and insert a second filter, 10CGA-695, for second order blocking to ~1300 nm. If scanning over a large wavelength range, as with Oriel multiple grating Cornerstone monochromators, consider using a filter wheel to switch easily between filters during a scan.