|
|
Prism and Diffraction
Grating Spectrometer
Properties Compared
Light Transmission Through Prisms: Of all spectrometer characteristics the efficiency with which light is transferred through the system is absolutely 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.
|
|
 |
| Figure 1. Light throughput efficiency curves for prisms, ruled and holographic gratings. * |
| |
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
|
|
 |
| |
Figure 3. The grating movie. Double click to see how convex, concave, and plane diffraction gratings, act like beam splitters when compared to a prism. |
| |
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.
From Figure 4, note 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.
The result is that a prism delivers significantly greater net light throughput in the red when compared to a diffraction grating |
|
 |
Figure 4. showing a typical QE curve of a CCD camera and the change in bandpass of diffraction gratings and prisms. |
* The plots shown in the above graphs are for illustrative purposes only. Contact the device manufacturers for specific, up to date, information.
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.
To learn more about the physics
of imaging spectrometer systems download
Imaging Spectrometer Fundamentals for Researchers in the Biosciences - a Tutorial |
The PARISS "Curved Prism" Hyperspectral
Imaging System
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.
 |
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) |
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, diffraction grating based instruments cannot
hope to
reproduce the spectrum shown in Figure 5 without spectral overlap
| |
|
|
©
1990-2010 LightForm, Inc all rights reserved |
|
|
|
|
|
| |
|
