Machine Vision Sees Contaminants We Can’t

Scientists at the ARS Instrumentation and Sensing Laboratory in Beltsville, Md., are developing “machine-vision” systems that can detect contamination the human eye often can’t see called machine-vision systems. These are quicker and more accurate than the human eye and don’t require anyone to handle the fruit or cut it up.

Yud-Ren Chen, an agricultural engineer, and his colleagues Kuanglin Chao, an agricultural engineer; Moon Kim, a biophysicist; and Alan Lefcourt, a biomedical engineer, have built a prototype “multispectral imaging” apple-inspection system.

Chen leads a team that specializes in developing machine-vision systems using visible and near-infrared light. A mechanical engineer, an electrical engineer, a computer scientist and a USDA Food Safety Inspection Service (FSIS) industrial engineer are also in the group.

The team has tested machine vision on a commercial apple-sorting line, using a digital spectral camera. It can take pictures at different wavelengths simultaneously creating multiple images. This once required two or more cameras, each with its own light filter. Using a hyperspectral imager, the team can find the wavelengths best suited to spotting fecal contamination or cuts and bruises that can harbor bacteria. Some wavelengths are chosen because of their identifiable relationships to photosynthetic pigments in apples.

Hyperspectral Imaging

Biophysicist Kim came to ARS in 1999 from NASA, where he used reflectance and fluorescence for sensing vegetation remotely from airplanes to check on the planet’s environmental health.

Now, to detect fecal contamination, he senses photosynthetic pigments from plants, but on a much smaller scale, working barely 2 feet from his targets rather than several thousand.

Kim and Lefcourt upgraded and modernized the lab’s existing hyperspectral imaging equipment. The lab uses the hyperspectral system to design commercial inspection systems for poultry and produce.

The instrument, designed and hand-built by the Beltsville team using commercially available components, is called hyperspectral rather than multispectral because it can capture images at up to 256 different wavelengths; a multispectral system generally uses only two to four wavelengths.

“In the research stage, we use over 100 images at many different wavelengths,” Kim says. “But it takes several minutes to scan objects at that many wavelengths. So hyperspectral imaging wouldn’t be practical for commercial operations. But it is valuable because it lets us visualize images across a range of the spectrum. We can then choose a few optimal spectral bands that will get the job done with enough speed and accuracy when used in multispectral imaging systems.”

A multispectral imaging system can scan a whole object in a fraction of a second and is more suitable for real-time use in processing plants, Kim says. The hyperspectral imaging system has “scientific-grade” imaging spectrograph and halogen and fluorescent lamps, all packaged in one unit that sits above a motorized positioning table where the apple is placed. The imaging spectrograph is connected to a computer. For reflectance sensing, visible to near-infrared light comes from quartz halogen bulbs connected to the unit through fiber-optic lines, while fluorescence imaging uses fluorescent lamps. ARS-developed computer software analyzes the hyperspectral images.

Each Apple Scanned a Hundred Times

The imaging spectrograph scans a moving apple hundreds of times, each time sensing a line across the apple’s surface. The light on each point on the line is spread out like a rainbow by the spectrograph, creating a three-dimensional image.

The positioning table lets the researchers run hundreds of scans of the apple surface, placing the apple in many different positions, while recording the exact position of the apple so a scan can be repeated later. Mathematical algorithms interpret the multiple images.

“The hyperspectral imaging system is versatile and has many research applications besides food safety,” Kim says. Chen agrees that the lab’s hyperspectral imaging equipment can be used in many disciplines and with a variety of agricultural products.