How Hyperspectral Imaging Works

"Hyperspectral Imaging Instruments use either:
Filter-Flippers (LCTF, AOTF, interferometer)
or a Wavelength Dispersive Imaging Spectrometer"

Wavelength dispersive systems
Wavelength dispersive systems (WDS) use either a prism or a diffraction grating as the wavelength dispersive element. WDS systems sweep across an unlimited field of view (FOV) in what is referred to as a "push-broom" system later on this page you can see how this works in an animation

All wavelengths are acquired simultaneously over the entire spectral range. In the case of the PARISS system this is from 365 to 920 nm. The exact wavelength range is determined by the camera that acts as the wavelength detector.

The PARISS Wavelength Dispersive Hyperspectral Imaging System (WDSI)
The PARISS imaging spectrometer uses a unique patented curved prism as the wavelength dispersive element as shown in the illustration above. This approach guarantees the integrity of a spectrum because there is no delay in acquiring all the wavelengths of a spectrum; and there is absolutely NO second order contamination. (Higher orders do not exist with a prism; which partially explains the very high light throughput). See the "Imaging Spectrometer - tutorial"

The PARISS imaging spectrometer uses a prism that is more like a "lens with a wedge". The added optical power, and the ability to change the curves of both the prism and back reflector, combine to produce outstanding image quality in a compact and light weight system

The "Filter Flipper" (LCTF/AOTF/Interferometer)
Filter flippers include Liquid Crystal Tunable filters, (LCTF), Acousto Optic Tunable filters (AOTF) and interferometers.

Filter flippers accumulate the spectra, in each pixel of a CCD camera, of objects in a rigidly fixed FOV.

Each of these devices acquires wavelength data sequentially over a period of time. Because the FOV is fixed the method fails when either an object moves or a chemical or biological interaction changes the spectral content. In order to "speed things up" it is not uncommon to take as few wavelength data points as possible.

Interferometers function in an analogous way to filter flippers because they too take wavelength information sequentially. An interferogram is constructed either by scanning a mirror or translating an optic. The net result is that it emulates a filter system that acquires wavelengths sequentially to form a "spectrum" in each pixel of a CCD.

The PARISS and "Filter Flipper" Animations
Click the link below to play animations that compare filter flippers with wavelength dispersive hyperspectral imaging systems such as PARISS. You may have to say "Yes" to Internet Explorer security restrictions before you can play the movies. The animation will open in a separate window.

There is a set of three animations - with each animation shown in the left hand column. You can click on any segment to select or replay any animation

1. The first animation illustrates how LCTF, AOTF and interferometers acquire a spectral image
2. The second how wavelength dispersive systems work based on the PARISS system
3. Finally a side by side comparison of the two techniques.

Security settings and your ability to play the animations
Your computer may have security settings that will not allow the flash application to run unless you have deliberately enabled it. You may have to keep your finger on the "CTRL" when you click the link or answer "yes" to a message at the top of the content window. If you have a problem getting the animation to run, or you would like to discuss it, call me!

Most if not all versions of Windows XP already has a Flash player installed. If you have Win 2000 or an older version of XP you can download a free version of Macromedia Flash here. Note you may have to reboot your computer before it takes effect.

New! After eleven incredible years read the PARISS update!

Prism assembly is manufactured under a license to US patent 5,127,728 owned by The Aerospace Corporation.
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