Terahertz Sensor Technology for Web Gauging

Terahertz radiation is part of the electromagnetic spectrum lying between microwaves and the far-IR. This region has frequencies ranging from 0.1 – 10 THz and wavelengths from 3 mm to 0.03 mm. This spectral region is often referred to as the “Terahertz gap” as these frequencies fall between electronic (measurement of field with antennas) and optical (measurement of power with optical detectors) means of generation. Historically, little study of the interactions between these wavelengths and matter has been undertaken. The reason for this was the difficulty in generating and detecting terahertz.

Recent advances in combining optical and electronic methods have allowed for generation and detection of very high signal-to-noise ratio and high data acquisition rates of 0.1 – 3 THz frequencies. Consequently, a surge of interest in and study of the terahertz region is underway.

The ability to measure individual layer thickness of co-extruded, calendered, or other multilayer continuous web products is critical to controlling quality, reducing material usage, and minimizing defects such as voids and delamination. Terahertz technology is used to simultaneously measure thickness, basis weight and density in a single sensor solution for transparent and opaque products, such as rubber tires, roofing, coextruded plastics, foam products, insulation, building products and many other high value applications.

The Thermo Scientific™ Terahertz Thickness, Basis Weight and Density sensor is a unique technology, developed by TeraMetrix®  (a division of LUNA) and incorporated into our web gauging measurement and control platforms. This sensor makes its measurement by detecting the time it takes for the terahertz energy to travel through the material being measured and reflect off the surfaces of each layer. This is accomplished by using a laser to generate pulses of terahertz waves at ultra high frequency and a collinear receiver to measure the waves as they come back from each surface. (See image below)

1. t1 to t4 are the time at which the terahertz wave is detected by the sensor as it reflects off each surface.
2. Despite the diagram's depiction, the transmitter and receiver are collinear.

Using a proprietary data analysis technique, the Terahertz sensor determines the thickness of each layer by calibrating it by the speed of light in the material and measuring the time difference between t1, t2, t3 and t4 and the index of refraction of the material. As material thicknesses increase, the time for the  terahertz energy to travel through it will also increase.

For simultaneous measurement of thickness, basis weight and density, an external reference structure must be used.

Web gauging systems help ensure accurate measurement and control of thickness and basis weight for plastic and polymer materials. Precise measurement is needed to help ensure customer requirements are met, as well as help reduce scrap, improve product quality, and increase productivity.

Terahertz technology is used in multilayer plastic sheet extrusion, multilayer foam extrusion, rubber tire calendering, multilayer conveyor belt, roofing shingles, single-ply roofing (e.g. TPO or EPDM) and many other web gauging applications.

Examples of web gauging terahertz technology use:

Rubber tires

Fabric calender: Simultaneous measurement of top gum thickness, fabric thickness, bottom gum thickness and total thickness using only one Terahertz sensor.

Wire cord calender: Simultaneous measurement of bottom gum thickness and total thickness using one Terahertz sensor. A second Terahertz sensor head can be added to measure top gum thickness.

Conveyor belt

Simultaneous thickness measurement of top cover rubber, nylon, polyester, bottom cover rubber and total thickness using only one Terahertz sensor.

Coextrusion, lamination, foam products, shingles and other single ply roofing

Simultaneous thickness measurement of top PP layer, EVOH barrier layer, bottom PP layer and total thickness using only one Terahertz sensor.