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A natural dye extract may protect our eyes from harmful laser

Read time: 3 mins
A natural dye extract may protect our eyes from harmful laser

The Indigofera tinctoria plant.
[Image Credits: Wikimedia Commons / CC BY-SA 3.0]

The blue dye extracted from the famed Indigo plants has been used over the years to colour clothes and clothing materials. Although synthetic indigo dyes are now available, the natural variety also is in common use. It is extracted from the leaves of the Indigofera tinctoria plant, following standard protocols in scientific laboratories. Scientists can extract the natural dye from the commercially available Indigo leaf powder by dispersing it in a solvent. The procedure involves applying ultrasonic vibrations to the resulting dispersion and then revolving it at more than 100 rotations per second for a few minutes.

Researchers from the Raman Research Institute (RRI), Bengaluru, and Kensri School and College, Bengaluru, have studied the optical properties of the natural Indigo dye. They have shown that the dye can act as a device to protect human eyes from harmful laser radiation. The study, funded by the Department of Science and Technology, Government of India, was published in the journal Optical Materials.

The natural dye has a violet hue. The researchers extracted the dye and stored it in a refrigerator below 4º Celsius to preserve its natural properties. By shining light on the dye, they studied how much it absorbed light at different wavelengths of the electromagnetic spectrum. They found that the absorption is maximum in the ultraviolet region of the spectrum, at a wavelength close to 288 nanometre, and in the visible region, close to 660 nanometre. The absorption is comparatively high for green light as well.

“Indigo absorbs light because of molecular absorption bands. The maximum absorption wavelength can vary over several nanometers depending on the dye’s solvent and concentration,” explains Reji Philip, professor at RRI and a co-author of the study.

The absorption’s variation with wavelength indicated that chlorophyll, an organic compound that takes part in photosynthesis, is present in the dye.

Organic dyes often show an additional absorption when the input light intensity is high, which the researchers were after. They measured how much light the dye absorbed at optical wavelengths when an intense laser pulse passed through it. They shone laser pulses of green colour, with a wavelength of 532 nanometre, and measured the fraction of light the dye transmitted when light pulses of different intensities passed through it.

The researchers could dial up the energy of the laser up to 100 times. Instead, they adopted a technique in which the laser’s energy is held constant while it is focused on the sample with the help of a lens. “We increased the optical intensity by using a lens to focus the laser beam. The intensity is maximum at the lens’ focal point, but it significantly reduces towards either side,” says Reji. As the sample moves away from the focus by a distance of 10 cm, the laser intensity the dye experiences varies over 100,000 times.

The team found that when they increase the intensity of the laser pulse, the dye absorbs more light. That is, it is more opaque to higher intensity light. Scientists refer to such materials as an ‘optical limiter’.

Optical limiters are useful in weakening the potentially harmful radiation emitted by powerful lasers. Thus, they can protect human eyes or other sensitive optical devices from accidental damage in an environment where such lasers are in use.

“Making a prototype optical limiter using natural Indigo is the next logical step, followed by a commercially viable product,” Reji signs off.

This article has been run past the researchers, whose work is covered, to ensure accuracy.