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Rapid Spectral Imaging Determines Grapes’ Ripeness

Photonics Spectra
Mar 2000
Michael D. Wheeler

In the long history of winemaking, determining when to harvest the grapes has been a task marked as much by guesswork as by skill. The conventional means to gauge ripeness involves selecting sample grapes and testing them with a sugar refractometer, which designates the grapes' sugar concentration. Because pigments strongly influence wine quality, visual evaluation of the grapes' skin is also important. But this approach is subject to human error.

A team of researchers at the Carnegie Mellon Research Institute hopes to change that with a customized acousto-optic tunable filter that performs rapid spectral imaging, enabling a more quantitative method to determine ripeness.

The acousto-optic tunable filter detects the interaction of light and acoustic waves in certain crystals. The diffraction by the phase grating formed by acoustic waves enables the device to monitor the chemical composition of the grapes. The optical absorption or reflection spectrum in the presence of ambient light reveals the composition. An analyzer can rapidly acquire these data and process them into instantaneous images for evaluation. And because the filter images grapes from different clusters as well as from different places on the same cluster, it can measure the mean ripeness of the crop.

A research group at Carnegie Mellon Research Institute has developed an acousto-optic tunable filter that can help vineyard operators determine the best time to harvest their crops.

"The advantages of the new instrument are that it does not require subjective judgment of pigmentation and is capable of taking averaged readings over a large number of grape bunches," said Milton Gottlieb, one of the principal researchers involved in the project. "This is more meaningful than measurements on individual grapes, which will vary widely at any particular time. The instrument can also be used to sort the harvest from any particular day into batches of greater uniformity."

Since the instrument was first tested at a vineyard near Pittsburgh about a year and a half ago, the team has made several improvements, including better image analysis software. The system's new optical design also provides better spatial resolution than the prototype.

The team is working to extend the sensitivity of the instrument to the far-IR by growing IR crystals that are highly efficient, high quality and suitable for commercial production. Gottlieb said the group is seeking partners to bring the cost of the instrument into the range of a few thousand dollars -- a price that should be affordable to wine producers. Besides looking for other agricultural applications, the group aims to address battle area surveillance and target acquisition, mineral exploration and mining.

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