Inspection of fabrics with fluffy appearance by NIR image analysis

Abstract

Some fabrics, either woven or knitted, show a fibrous, fluffy appearance that hides their thread interlacing structure. In such a case tasks related with inspection of the fabric structure, identification, classification and fault detection are difficult to carry out. A remarkable improvement can be obtained when the fabric image to evaluate is captured under near infrared (NIR) illumination. NIR illumination penetrates in the material more than visible illumination and the reflected image contains more information about its structure. Although humans are not sensitive in this region, we can observe a NIR image of a fabric by exploiting the residual sensitivity of a conventional monochrome camera that reaches up to 1000 nm. The light source used is an array of NIR LEDs emitting in a band to which the camera is still sensitive. This inexpensive image acquisition system is completed with a monochrome TV monitor to display the NIR image and a computer for image analysis. Some results obtained by applying Fourier analysis to the fabric image obtained under either visible or NIR illumination are provided and discussed.