Prism produces a spectrum

Prism versus Diffraction Grating

Either a prism or a diffraction grating is used as the wavelength dispersive element in most optical spectrometers.

This page contrasts and compare the performance characteristics of each.

Performance comparison between prisms and diffraction gratings

All wavelength dispersion spectrometers use either a diffraction grating or prism as the wavelength dispersive device. This page describes the Pros and Cons of each.

Parameter

Prism

Diffraction Gratings*

Spectral resolution

Wavelength range

Light throughput

2nd order pollution

NIR efficiency

1 – 10-nm variable

365 – 920-nm+

>60% 365 – 420-nm
>90% 420 – 1000-nm

None

Excellent

1 – 10-nm constant

One octave: For example, 400 – 800-nm

Max 70% at blaze, dropping at shorter and longer wavelengths

Yes. 2nd order overlaps1st order

Poor, due to QE of the spectrum camera and low diffraction efficiency *Typical of those used in hyperspectral imaging systems

Detailed diffraction grating characteristics

Grating “Pros”

  • Off the shelf variety of groove densities and wavelength dispersion options
  • Wide selection of blaze wavelengths

Grating “Cons”

  • Produced either with 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.
  • Classically “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 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.

 

Prism “Pros”

  • 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 prism based imaging spectrograph click here

Prism “Cons”

  • Compared gratings, prisms are very expensive.  Only high end instruments addressing challenging applications use them
  • Wavelength dispersion is non-linear meaning that bandpass an resolution change from high in the blue to lower in the red.  Linearizing dispersion is trivial in the software, but does not compensate for changing bandpass. Prisms share non linear dispersion with both AOTF and LCTF devices (Acousto optic tunable filters and Liquid Crystal Tunable filters)

Prism versus diffraction grating curves

Figure 1: Prism vs diffraction grating efficiency curves.  Diffraction grating efficiency profiles vary, but never equal the efficiency of a prism.

Diffraction gratings present overlapping "orders" that pollute 1st order

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.

Changing spectral resolution compensates for falling camera QE in the red

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.  A the spectral resolution of a prism goes a long way to compensate offer high S/N at long wavelengths that are a problem for diffraction gratings.

Prism versus diffraction grating performance

Nanoparticle characterization in darkfield scatter

How PARISS hyperspectral imaging works

PARISS modes of operation: SnapShot and Field-Scan

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.