M77781A
MS257
™ USB/RS232
MONOCHROMATOR AND SPECTROGRAPH
62
Grating Dispersion
A grating's angular dispersion is proportional to the number of lines that it has. The greater the dispersion,
the more the spectrum is spread out. A monochromator
’s dispersion is usually quoted as [reciprocal]
linear dispersion in nm/mm. A monochromator with a 1200 I/mm grating will have exactly half the linear
dispersion of a monochromator with a 600 I/mm grating. Note: this comparison only applies at the same
grating angle for both gratings since linear dispersion changes with angle. It does not apply at the same
wavelength.
It is often asked why gratings aren
’t made with many thousands of lines per millimeter. Usually gratings
with 2400 lines per millimeter are the upper limit. In the case of ruled gratings, it is expensive and difficult
to rule finer and more closely spaced lines on a grating. Even though it is possible to have thousands of
lines per millimeter on a holographic grating, there are fundamental reasons for not doing so. As the
number of lines increases, the wavelength range of the grating decreases until it is of little use, except for
very specialized applications. A 2400 I/mm grating has a maximum range of about 600 nm, but a 4800
I/mm grating would have a range of only about 300 nm - and then only in the UV to visible wavelengths.
Bandpass & Resolution
Bandpass is the range of wavelengths passed by the monochromator at any one wavelength setting. It
may be obtained by multiplying the slit width by the reciprocal linear dispersion and is usually quoted in
nanometers.
In a monochromator, the bandpass may be decreased by reducing the width of the slits until a limiting
bandpass is reached. The limiting bandpass is called the resolution of the instrument. For a properly
illuminated grating, the aberrations of the optical system determine the resolution. For any instrument, the
optical aberrations are minimized by illuminating only the central zone of the input slit.
Resolution is a measure of how finely a monochromator can differentiate between spectral lines. Since
resolution is closely related to dispersion, higher dispersion results in better resolution. If the number of
lines per millimeter is doubled the resolution is twice as good. Although there are several definitions for
calculating resolution, a common spectroscopy standard is the width of a single wavelength line at the
point of half maximum intensity - also known as FW HM, full width at half maximum (See Figure 24).
A traditional definition of resolution is the Rayleigh criterion. This definition ensures that two closely
spaced wavelengths will be differentiated if the point at which the peaks merge is between the 80%
intensity levels for both peaks. The resolution will vary with the size of both input and exit slits, but will be
limited by optical aberrations no matter how small the slits may be.
For example, a practical limit with 250 mm monochromators is a slit size of about 10 nm. Note that it can
be misleading to talk about
“raising” or “increasing” the resolution, when in fact the resolution number
decreases. Oriel Instruments uses the convention that raising or increasing the resolution is an
improvement, although the resolution number actually decreases, e.g. raising the resolution from 2 nm to
1 nm.
