Prism and Diffraction Gratings | Throughput and Bandpass Compared

Prism and Diffraction Grating
Properties Compared

Light Transmission Through Prisms: Of all spectrometer characteristics the efficiency with which light is transferred through the system is the most critical.

If signal is buried in noise then resolution or bandpass is meaningless.

The fact is, light throughput through a prism is unbeatable over the wavelength range from 365 to 950 nm. Internal transmittance is close to 100% and Fresnel reflection off the prism sides can be reduced to insignificance with anti-reflection coatings, see
Figure 1.

$Transmission curves for prism and diffraction gratings$

Figure 1. Light throughput and efficiency curves for the PARISS prism, ruled and holographic gratings. (These plots are for illustration only. Contact the manufacturers for actual curves)

The Efficiency of Diffraction Gratings: Diffraction gratings are not so fortunate. Diffraction gratings come in two flavors; ruled and holographic. In order to maximize the efficiency of any grating, the groove profile has to resemble a right triangle as shown in Figure 2.

This groove profile is that of a "blazed" grating, where the angles shown are selected to produce maximum efficiency at just one wavelength - the blaze wavelength. At all other wavelengths efficiency drops-off, sometimes precipitously also shown in Figure 1.

Low groove density holographic gratings (<600 g/mm) are very difficult to blaze or optimize; consequently, their maximum efficiency is significantly less than that of ruled gratings.

Most importantly all grating efficiency profiles favor shorter wavelengths in their range, making it difficult to get good results at longer wavelengths in the red.

Figure 2. Blazed grating groove profile

High Order Diffraction: To put it bluntly, diffraction gratings work like a shower-head, compared to a prism that directs light like a fire-hose. Watch the movie in Figure 3

$Diffraction grating movie$

Figure 3. The grating movie. Double click to see how convex ("concentric"), concave, and plane diffraction gratings, act like beam splitters when compared to a prism.

The Practical Effects of Second Order Spectral Pollution with Diffraction Gratings: Certain "spectral" features can appear "twice" once in first order, and then again in second order. The only way to prevent this is to add "filters" that cut-out the blue and UV. To learn more about second second order pollution click here.

Wavelength Dispersion: Both diffraction gratings and prisms deliver non-linear wavelength dispersion. Low groove density gratings (<600 g/mm) are less non-linear than high density gratings.

All prisms on the other hand present significant non-linearity just like Liquid Crystal Tunable Filters (LCTF), and Acousto Optic Tunable Filters (AOTF).

However, non-linear dispersion is a significant advantage because bandpass varies directly with wavelength dispersion.

Figure 4, illustrates how the QE curve of a CCD camera falls off at higher wavelengths. Note also that as the efficiency of the camera falls the bandpass of the prism falls with it.

Figure 4. showing a typical QE curve of a CCD camera and the change in bandpass of diffraction gratings and prisms.

The PARISS "Curved Prism" Hyperspectral
Imaging Spectrometer

All prisms present inherently low scattered light (noise) because their surface area is orders of magnitude less than the very best diffraction grating!

The spectrum below is a perfect illustration of the excellence of the PARISS design. This spectrum was acquired with a single 30 ms PARISS acquisition using a Q-Imaging Retiga 2000R as the wavelength detector. The above spectrum is a tribute to both the light-transfer efficiency of the spectrometer and the camera (it is not electron multiplied, such as an EMCCD). We could argue that we observed EMCCD performance at a fraction of the price.

Hg spectrum from 365 to 920 nm acquired on a PARISS Hyperspectral Imaging System

Figure 5: A Hg spectrum emitted by a wavelength calibration lamp and acquired by a PARISS Analytical Spectral Imaging System. The scan presents both outstanding spectral resolution (~1.5 nm FWHM at the 436 nm line) and spectral range. The PARISS system uses a prism as the wavelength dispersive element; consequently, there are no higher orders to pollute the spectrum. (Diffraction gratings commingle higher order diffraction). To see an example of second order pollution click here.

Many imaging spectrometer systems find it either difficult, or impossible to capture any Ar lines above 650 nm. This is because Ar lines fade rapidly to zero as a Hg/Ar lamp warms up (<10 sec.).

Although Ar fades rapidly with time to make it difficult to capture its red lines, it is just as tough in the UV because the Hg 365-436 nm lines only become bright after the Ar has faded. The net result is that only the most efficient spectrometers and detectors can hope to grab light at both 365 and 920 nm simultaneously, in a single acquisition.

In other words, a diffraction grating cannot hope to deliver
the spectrum shown in Figure 5 without spectral overlap

Prism and Diffraction Grating Properties Compared

This page compares the performance of prisms versus diffraction gratings

Prism and Diffraction Grating
Properties Compared