Terahertz Film Thickness Measurement via Interference
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Solution Overview
Problem
Existing methods for measuring the thickness of plastic films, particularly those less than 200 µm, face challenges due to overlapping pulse transit times in time-domain terahertz spectroscopy and limited resolution in frequency-domain measurements, making it difficult to achieve accurate and reliable results.
Innovation Solution
A method and device utilizing terahertz radiation with a substrate of higher refractive index than the plastic film, where the plastic film is clamped on a rotating cylinder to prevent air inclusion, allowing for interference-based measurement with enhanced resolution by canceling partial reflections, enabling the detection of significantly smaller film thicknesses without additional equipment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If time-domain terahertz spectroscopy is used to measure plastic film thickness, then measurement capability is provided, but for films less than 200 µm the pulses overlap and measurement becomes difficult
Solution Approach 1:
The patent transitions from time-domain measurement to frequency-domain measurement, changing the dimension of analysis. By measuring in the frequency domain and analyzing the spectrum of received terahertz signals, the method resolves the pulse overlap issue that plagues time-domain measurements of thin films, enabling reliable measurement of films thinner than 200 µm
Solution Approach 2:
The patent changes the measurement parameter from time-domain pulse transit time to frequency-domain spectral characteristics. By analyzing the frequency spectrum and using the relationship between frequency, refractive index, and film thickness, the method achieves improved measurement precision for thin plastic films
2Productivity
If frequency-domain measurement is used for plastic film thickness, then continuous measurement capability is provided, but resolution is limited
Solution Approach 1:
The patent optimizes the frequency range and spectral analysis parameters to simultaneously achieve fast data acquisition and high measurement resolution. By carefully selecting the terahertz frequency range and using appropriate spectral analysis methods, the system maintains both high productivity and improved measurement precision
3Device complexity
If plastic film is measured in air on both sides, then simple setup is provided, but measurement resolution is limited due to refractive index differences
Solution Approach 1:
The patent introduces a substrate with a higher refractive index than the plastic film as an intermediary medium. This substrate enhances the refractive index contrast at the interfaces, strengthening the partial reflections and improving the measurement resolution without significantly complicating the overall setup
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly improves the measurement resolution by a factor of 2, allowing for precise determination of smaller film thicknesses, specifically enabling the measurement of half as thick plastic films at the same frequency, without the need for extensive equipment modifications.
Implementation Method 1
an interference system is formed during a measurement in which terahertz radiation from a surrounding medium with a lower refractive index such as air, gas or vacuum first strikes the plastic film and then the background with a higher refractive index
Implementation Method 2
The plastic film is preferably measured in a number of frequencies, in particular in a frequency space or frequency range. The plastic film is thus guided on a substrate, specifically a rotating cylinder or roll, made of a material that has a higher second refractive index for the terahertz radiation than the first refractive index of the plastic film
Implementation Method 3
Partial reflection occurs on the upper side of the plastic film when the terahertz radiation enters from the medium (air) with the lower third refractive index into the medium with the higher first refractive index (plastic material)
Data Source
Figure 1~2
Figure 3~4
Figure 5a~5b
AI summary
The invention relates to a method and a measuring device (1) for determining a layer thickness (d) of a plastic film (2), comprising at least the following steps: laying or guiding the plastic film (2) with a first THz refractive index onto a base layer (4) having a second THz refractive index that is greater than the first THz refractive index; radiating THz radiation (3) through a surrounding medium (9) with a third THz refractive index, e.g. air, onto a surface (2a) of the plastic film (2) in the region in which it is in contact with the base layer (4), wherein the third THz refractive index is less than the first THz refractive index; measuring the intensity (I) of THz radiation (3b) reflected from the surface (2a) of the plastic film (2); and determining the layer thickness (d) of the plastic film (2) from the measured intensity of the reflected THz radiation (3b). The determining can be carried out in particular with multi-frequency radiation and a measurement in the frequency range, e.g. as the local minimum of the reflected radiation.