Spotting Bad Apples Fast

Paul Martinsen and his colleagues at the Horticulture and Food Research Institute of New Zealand in Hamilton think that they have found a technique that can quickly spot bad apples. Using a time-delayed-integration spectrometer, users can measure the low-light near-IR spectra of moving apples on a conveyor belt, collecting light from each sample and tracking it electronically via time-delayed integration. According to Martinsen, spectra can be collected from a photon flux approaching 40,000 photons per second at the detector (or some 9 x 10^—15W at 800 nm). This measure is roughly six orders of magnitude lower than the output of a 1-kW quartz-halogen lamp.

Increased integration time provided by tracking an apple with a time-delayed-integration spectrometer allows the instrument to measure at light levels on the order of 40,000 photons per second at the detector.

Narrow field of view

One feature separating this tool from a conventional spectrometer is its long, narrow field of view, which covers the width of the sample and several times its length, instead of fitting within an area smaller than the size of the object (see figures). This enables the instrument to track the sample as it moves along the conveyor, and to collect light for a longer period.

The New Zealand researchers’ goal was to verify the use of time-delayed integration and spectroscopy to measure transmission spectra of apples at five to 10 apples per second. Their equipment featured a 300 x 25-mm field of view imaged onto a slit by a 25-mm f/1.4 CCD camera. To produce the required dispersion, they modified an imaging spectrometer from MK-Photonics Inc. of Albuquerque, N.M.

This device combined a 791-lines-per-millimeter transmission diffraction grating and aberration-corrected f/2.0 optics to produce a 6 (spatial) x 12-mm (spectral) image. A Hamamatsu back-thinned fast Fourier transform CCD detector located at the focal plane — with a CCD array that contained 532 pixels along the wavelength axis and 250 pixels along the spatial axis — collected the dispersed light.

The spectrometer’s spectral range was 650 to 950 nm, which covered the narrow band required for apple inspection. Bandwidth was 14 nm, and spatial resolution of the fruit was 1.2 mm in the direction of motion and 25 mm perpendicular to it.

Martinsen said that experiments indicate the viability of time-delayed-integration spectroscopy for high-speed near-IR measurements at low light levels at speeds required for production-level inspection of apples or other optically dense objects on a conveyor or traveling through a fluid stream.

Contact: Paul Martinsen, the Horticulture and Food Research Institute of New Zealand, Ruakura Research Centre, Hamilton; e-mail: pmartinsen@hortresearch.co.nz.