Detailed Diffraction Grating Characteristics
- Off the shelf optional groove densities and wavelength dispersion options
- Wide selection of blaze wavelengths
- Produced either mechanically with burnished grooves (incorporates “ruling errors”) or holographically with limited groove density potential
- All have a peak efficiency at just one wavelength (the “blaze” wavelength). Diffraction efficiency drops off rapidly to shorter and gradually to longer wavelengths. See Figure 1
- Grating efficiency curves present “anomalies” or discontinuities in the efficiency profile.
- Classical “mechanically ruled” gratings are more efficient that those produced holographically
- The efficiency curves of all diffraction gratings are affected by light diffraction into higher “orders”
- Each order is one octave, for example, 400 to 800-nm. Second order will be 200 to 400-nm and overlay first order. See Figure 2
- To remove second order overlap order sorting filters are required
- In actual practice peak efficiency of a diffraction grating is almost always less than 70% and can drop to near zero at the extremities of its spectral range
- Wavelength dispersion (nm/mm) is non-linear varying as the diffraction angle and the distance to focus at each wavelength. In practice, wavelength dispersion is linear enough to present near constant resolution especially for low resolution instruments
Bottom line: Ruled gratings are more efficient than many holographic gratings when considered over a wide wavelength range. Nevertheless diffraction gratings cannot be used over a wavelength range greater than an octave without order sorting filters.
Detailed Prism Spectrometer Characteristics Pros and Cons
The major attraction of a prism is the near <90% average transmission efficiency at all wavelengths above ~365-nm. The efficiency profile is flat with no drop-off after ~400-nm. In terms of efficiency a prism will outperform all diffraction gratings.
- Ironically non-linear wavelength dispersion! As the QE of a camera decrease at longer wavelengths bandpass falls to compensate. Consequently, a prism delivers significantly higher signal to noise ratio over an extended wavelength range than a diffraction grating. See Figure 3.
- Transmission efficiency is a flat > 90% over the bulk of a wavelength range above ~400-nm outperforming all diffraction gratings
- Refraction does not result in “overlapping orders,” consequently a prism operates over greater than one octave without requiring filtering. Prisms work from 365 to 920-nm or above
- To see the PARISS imaging prism spectrograph click here
- Compared gratings, prisms are very expensive. Only high end instruments addressing challenging applications use them
- Wavelength dispersion is non-linear, consequently bandpass and resolution change from high in the blue to lower in the red. Linearizing dispersion is trivial in the software, but does not compensate for impact of changing bandpass.
- Prism spectrometers share non-linear dispersion with both AOTF and LCTF devices (Acousto optic tunable filters and Liquid Crystal Tunable filters)
Figure 1: Prism vs diffraction grating efficiency curves. Diffraction grating efficiency profiles vary, but never equal the efficiency of a prism.
Figure 2: With some exceptions most diffraction gratings are used in first order. However, gratings also diffracts light into plus and minus orders that overlap first order.
Figure 3: All spectrometer components can present wavelength efficiency issues. Signal to noise ratio S/N is a product of bandpass and efficiency. The efficiency curve of most cameras falls with increasing wavelength. The spectral resolution of a prism goes a long way to compensate and offer high S/N at long wavelengths that are a problem for diffraction gratings.
How PARISS Analytical Hyperspectral Imaging Works
How PARISS Hyperspectral Wavelength Dispersive Imaging Works
All PARISS Hyperspectral systems are custom configured to meet the needs of an application.
The above configuration is for guidance only. Specifications can and do change without notice.