Resumen de Use of acousto-Optic Filters in imaging applications. A contribution to the SmartSpectra project
This Thesis analyses the use of an acousto-optic tunable filter (AOTF) for the realisation of spectral imaging systems. An AOTF is an electronically tunable spectral bandpass optical filter. It consists on a crystal that, excited with acoustic waves, diffracts and then separates a single wavelength of light from a broadband source. The wavelength of selected light is a function of the frequency of the signal applied to the crystal. We have implemented a spectral imaging instrument called Autonomous Tunable Filtering System (ATFS) that includes an AOTF in order to acquire multispectral images with a monochrome digital camera. Careful design and calibration permits its use in spectroscopic applications. By using a convenient radio-frequency source, it is possible to configure the spectral filtering performance of the AOTF. This Thesis presents two different excitation techniques: the use of a Direct Digital Synthesiser, and the use of a high-speed Digital- to-Analog Converter. Both methods achieve a configurable broadening of the spectral bandwidth characteristic of the AOTF. In this Thesis we evaluate the use of AOTF’s in spectral imaging applications by means of the implemented instrument. We present a methodology to fully characterise the spectral performance of the AOTF’s respect to the parameters of the driving signal (frequency and power), and we apply this methodology for the characterisation of three different AOTF models from different manufacturers. Imaging performance of the AOTF’s presents some problems due to their low efficiency and chromatic aberration. However, we propose a methodology to overcome the drawbacks of this filtering technology. Finally, the Thesis demonstrates this spectral imaging technology in several applications, including the mapping of the spectral reflectance and transmittance indexes of plant leaves, and the multispectral imaging of different objects. This Thesis concludes that AOTF technology can be successfully applied to spectral imaging systems. Imaging performance of such systems is good, but a proper image processing is needed. Spectral performance of AOTF’s is accurate, allowing their use in quantitative measuring applications, although they require a careful calibration process.
