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M74050
1/4M IMAGING SPECTROGRAPHS
21
6.1 GRATING TYPES
Ruled grating masters are produced using a ruling engine with an extremely fine cutting tool.
Holographic gratings (also called interference gratings) are produced by recording interference
fringes in photoresist. The different techniques cause some differences in performance.
Holographic gratings are most frequently available at high groove densities due to manufacturing
limitations inherent in the technology. They are generally favored for work in the UV and through the
visible to about 600 nm. Holographic gratings produce less scattering, thereby reducing stray light
inside the spectrograph.
Ruled gratings typically have higher efficiencies. They may have periodic errors in the grating grooves
caused by minor defects in the ruling machine, resulting in anomalous readings or “ghosts”.
Holographic gratings do not suffer from ghosts, so interpretation of line spectra is simplified.
The signal-to-noise ratio (SNR) is the ratio of diffracted energy to unwanted light energy. Although it
may be assumed that increasing diffraction efficiency will increase SNR, stray light usually plays the
limiting role in the achievable SNR for a grating system. Note that the actual signal to noise ratio will
depend on the spectral content of the incident light and the detector.
*** Bandpass is measured across a 1 inch array, with grating centered at blaze wavelength.
Figure 15: Grating Properties Table
GRATING PROPERTIES
Configuration
Grating
Position Type
Groove
Density
(lines/mm)
Blaze
Wavelength
(nm)
Array
Bandpass
(nm) **
High Resolution
#1 Ruled 1200 360 80
#2 Ruled
1200
500 81
#3
Ruled 1200
750 80
Holographic
#1
Holographic 1200
250 80
#2
Holographic 1800
500 50
Extended Range
#1
Ruled 600
200 160
#2
Ruled 600
400 165
#3
Ruled 600
1000 160
Wide Bandwidth
#1
Ruled
300 500 325
#2
Ruled
300 1000 325
#3
Ruled
300 2000 325