Hyperspectral imaging

copyright © Onera-Institut d'Optique 2007 - All rights reserved
In this image with artificial colors, the red corresponds to the light emitted in the near infrared spectrum. Chlorophyll emits strongly in this part of the spectrum¹:  this clearly separates the vegetation from the rest of the image.

In reality, what is know as a hyperspectral image is in fact a whole series of images of the same scene but taken in several dozen wavelengths - which correspond to the same number of "colors".  Measurements like these provide much information about the physical properties of the objects observed.  Therefore, airborne spectral imaging is very useful for studying and monitoring the environment.  

To gain expertise in the field in preparation for an airborne infrared spectral imager project, Onera and the Optical Institute designed and made a demonstration hyperspectral camera for the visible domain.  This apparatus consists of a traditional black and white imaging system with an interferometer2 in front of it.  The interference created by the interferometer spatially modulate the image formed on the detector (see image below) and can thus be used to measure the spectral information wanted.

copyright © Onera-Institut d'Optique 2007 - All rights reserved
The interference created by the interferometer spatially modulate the image formed on the detector and can thus be used to measure the spectral information wanted.

The images presented here are the first operational results of this instrument.  They were acquired in the South of France over the Salins de Giraud, with the camera on board a Puma helicopter from the Istres Flight Test Center (French Armaments Procurement Agency).  The flight path on the ground of the extract presented is a rectangle of about 400 meters wide and 300 meters high (steps on the ground of 1 meter), and about 40 spectral bands were acquired simultaneously.  The image below is the result obtained when only three, corresponding more or less to the sensitivity of the eye, are combined:  the green vegetation, red tiled roofs, roads and grey marl can be recognized.  To obtain the image at the top of the page, we displayed the image corresponding to a wavelength in the near infrared spectrum in red.  Chlorophyll emits strongly in this part of the spectrum: this clearly separates the vegetation from the rest of the image.

copyright © Onera-Institut d'Optique 2007 - All rights reserved
This 3-color image is the result obtained when only three "elementary" images corresponding more or less to the sensitivity of the eye are combined: the green vegetation, red tiled roofs, roads and grey marl can be recognized.

Images: integrated Onera / Optical Institute team coordinated by Christophe Coudrain, engineer in the Theoretical and Applied Optics Department

Text: Yann Ferrec, engineer in Onera's Theoretical and Applied Optics Department and doctoral student at the Optical Institute

Glossary

• Spectrum
  The light spectrum is the breakdown of light into its different components in terms of frequency, energy of wavelength photons and the associated colors.
  
  The light spectrum, from ultraviolet to infrared, going through all the colors of the rainbow.

• Interferometer
  An interferometer is a measuring instrument that exploits the interference phenomena due to electromagnetic waves, to light in particular.  Among its many applications, it can be used to accurately measure and separate the wavelengths of the light it receives.