• An organic filter that allows only near-infrared (NIR) light to pass through has been developed by scientists at the CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST) based in Thiruvananthapuram.
• The new NIR filter can be used for night vision glasses, night photography, and will have applications in security and forensics such as identifying blood stains on a dark fabric.
• Currently available inorganic filters are expensive and brittle whereas organic filters are easy to process and flexible too.
• The filter was prepared by mixing a black dye (diketopyrrolopyrrole or DPP) having an amide group that helps the molecules to be in close contact with each other and interact, leading to changes in their optical properties.
• The organogel-based filter has the ability to absorb both ultraviolet and visible light while allowing the near-infrared light alone to pass through. 
• The nanofibres formed through the self-assembly of the DPP molecules are responsible for the broad light absorption of the material, making it appear dark.

How Scientists Developed It

• The researchers developed the filter by mixing the organogel with a transparent polymer (polydimethylsiloxane).
• The addition of the dye turns the transparent polymer into a semi-transparent one and the filter appears black as it absorbs most of the ultraviolet-visible light.
• Only very little of the organogel has to be added to the polymer to make the filter.
• The material is present throughout the polymer matrix even though very little is added.
• The filter was found to absorb light from 300-850 nm (both ultraviolet, visible and a part of NIR light) and transmit NIR light from 850-1500 nm.
• The researchers tested it for night photography and found the filter responsive only to NIR light.
• A potential application of the new material is in the design of hidden security codes on documents which can be viewed only through a NIR-readable camera.

Source:TH