ROBOT.LK

YOU are looking at a colour no one can see. If you visited these grassy fields in Nepal's Annapurna Himalayan range, as New York-based photographer Sean Lynch did, they would look bright green. Crucially, though, they also reflect near-infrared light, which lies just beyond the range our eyes can sense. The fuchsia colour in Lynch's photo is created when this invisible light hits a special dye in the photographic film. "None of us will ever actually experience this 'colour'," says Paul Lucey, a geophysicist at the University of Hawaii in Honolulu.

Fields like these are a great way to showcase this imaging technique because plants reflect infrared light especially well. They do this for the same reason they shun green light: to make photosynthesis, which prefers blue and red light, as efficient as possible.

Such invisible reflections aren't just cool – they reveal important information. Although two plants may look similar to the naked eye, healthy leaves store more chlorophyll and so reflect more near-infrared light than unhealthy ones. This allows farmers to monitor plant health, aided by sensors mounted on aeroplanes and satellites. For entirely different reasons, oil slicks – and their clean-up – can be monitored using the same infrared reflections.

This sort of application requires greater precision than film provides, however, so digital cameras that can pick up invisible light have emerged. That may help explain why Kodak chose to discontinue the film on which Lynch shot these hills. Although it is still possible to buy the film, the supply is now limited. Dedicated photographers hoard the last of the dwindling stock in their refrigerators. "Every time we make an improvement, we lose a little and gain a lot – but that little bit is really important to somebody," says Lucey.

Read more: "The invisible issue: The world as you don't see it"

This article appeared in print under the headline "Imaging the invisible"