MCS260
CORNERSTONE 260 MONOCHROMATORS
23
6.2 GRATING EFFICIENCY AND BLAZING
Efficiency and its variation with wavelength and spectral order are important characteristics of a
diffraction grating. For a reflection grating, efficiency is defined as the energy flow (power) of
monochromatic light diffracted into the order being measured, relative either to the energy flow of
the incident light (absolute efficiency) or to the energy flow of specular reflection from a polished
mirror substrate coated with the same material (relative efficiency). Efficiency is defined similarly for
transmission gratings, except that an uncoated substrate is used in the measurement of relative
efficiency.
High-efficiency gratings are desirable for several reasons. A grating with high efficiency is more
useful than one with lower efficiency in measuring weak transition lines in optical spectra. A grating
with high efficiency may allow the reflectivity and transmissivity specifications for the other
components in the spectrometer to be relaxed. Moreover, higher diffracted energy may imply lower
instrumental stray light due to other diffracted orders, as the total energy flow for a given
wavelength leaving the grating is conserved (being equal to the energy flow incident on it minus any
scattering and absorption).
Control over the magnitude and variation of diffracted energy with wavelength is called blazing, and
it involves the manipulation of the micro-geometry of the grating grooves. The energy flow
distribution (by wavelength) of a diffraction grating can be altered by modifying the shape of the
grating grooves. The blaze wavelength is the wavelength where the grating efficiency is enhanced
by shaping the grating grooves. Although holographic gratings are not shaped like ruled gratings,
the peak grating efficiency wavelength is also referred to as the blaze wavelength.
The choice of an optimal efficiency curve for a grating depends on the specific application. Often
the desired instrumental efficiency is linear; that is, the intensity of light transformed into signal at
the image plane must be constant across the spectrum. To approach this as closely as possible,
the spectral emissivity of the light source and the spectral response of the detector should be
considered, from which the desired grating efficiency curve can be derived. Usually this requires
peak grating efficiency in the region of the spectrum where the detectors are least sensitive.
In many instances, the diffracted power depends on the polarization of the incident light. For
completely unpolarized incident light, the efficiency curve will be exactly halfway between the P and
S efficiency curves. Anomalies are locations on an efficiency curve (efficiency plotted vs.
wavelength) at which the efficiency changes abruptly. These sharp peaks and troughs in an
efficiency curve are sometimes referred to as Wood's anomalies.
The efficiency curves shown are relative (not absolute) and were measured using an in-plane near
Littrow configuration. Please use the curves as a guide and not as absolute data. Grating
diffraction is dependent on the polarization of the radiation incident on the grating.
Software such as the Mono Utility and Oriel’s TracQ Basic may be configured to switch between
gratings at a specific wavelength. Typically, the most efficient grating is selected, so this switchover
wavelength would be where the two efficiency curves meet. To determine empirically the ideal
switchover wavelength, the output should be measured by an optical detector. Run a scan in the
crossover region using only Grating 1. Repeat the scan using only Grating 2. Where the detector
readings are closest is the optimal switchover wavelength. For a triple grating instrument, this
process can be repeated for Grating 2 and Grating 3.
If the selected grating’s efficiency has a sudden increase or decrease at a particularly critical
wavelength and the application demands extreme accuracy, it may be more desirable to select the
grating with the more gradual change in efficiency.
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