The best choice for measuring individual layers
Principle of operation
Infrared (IR) sensors are designed to measure the weight or thickness (when density is constant) of constant composition materials or specific layers of multiple layer materials. It's also well suited for measuring the moisture content of relatively light materials with densities up to 200g/m2.
A light source located in the head emits a light spectrum having frequencies extending into the infrared region. Four detectors located in the NIR4 receiving head, and fitted with different input filters, selectively measure different infrared wavelengths. The signal from each detector is compared with and without material between the heads. Although more complex than measurement by x-ray or beta absorption, IR is also a transmission method.
The absorption is due to the different vibration modes of the molecules constituting the material. When the wavelength matches a molecular vibration mode, the energy is strongly absorbed. When the mass of material increases, so is the absorption. Conversely, wavelengths that do not correspond to a molecular vibration mode are poorly absorbed. Therefore each material is characterized by a specific infrared spectrum.
For example, below is the spectrum of a 190μm polyester film and its chemical formula.
The measurement of a given material is first done by selecting a suitable absorption peak. The peak’s height is a measure of the absorption and is therefore an indirect measurement of the material amount. It is clear that this method is much more complex than other absorption techniques where the detector signal, representing the energy absorbed, is itself enough to make the measurement.
First the proper wavelength must be physically selected by filtering the light by means of a specific filter. Then the background under the peak must be removed to improve the signal to noise ratio. Figure 2 shows in red the wavelength of the peak, in the green part of the spectrum that must be removed, and in blue the effective absorption.
To determine the effective absorption other wavelengths are necessary. Therefore a single measurement requires multiple detectors each with a specific filter selecting the required wavelength.
The effective absorption is then related to the basis weight or thickness by the Beer Lambert law (exponential decay) for which one has to determine the attenuation coefficient (figure 3).
The principle of infrared transmission dictates that the detector must be configured for each given material. This is of course a strong limitation, but it is also a very interesting feature since it can be used to selectively measure layers provided that the IR spectra differs sufficiently from layer to layer. The measurement of humidity is another example where the sensor is configured to measure the vibration mode of water molecules.
To simplify, there are two possible implementations:
- 1. Wavelengths can be selected in the emitter. In this case the filters must be placed one after the other in the light beam. This necessitates a rotating device inside the transmitter. Hence a single detector is sufficient in the receiving head. The great disadvantage of this design is that the measurement is not punctual. Thickness variations, if present during the rotation cycle, will introduce errors in the measurement.
- 2. Wavelengths can be selected in the receiver. The transmitter is much simpler, the measurement is punctual, but it is necessary to have as many detectors as wavelengths.
Scantech selected this second solution in order to have a single spot measurement independent of line speed.
- Measuring ranges: Scantech sensors operate in an extended wavelength range from 1 to 4μm. This provides more choices when selecting absorption peaks and widens the overall measurement range. Indeed, the attenuation coefficients are different from one peak to another. Therefore some peaks can be assigned to certain thickness ranges. For example, for polypropylene a first set of filters is well suited for low thickness (below 30 / 35μm). A second set allows measurements up to several hundred microns. In general the chosen peak, hence the set of filter, will depend on the desired measuring range and obviously on the nature of the material.
- Good accuracy: The NIR4 sensor offer good accuracy. Their intrinsic noise is especially low and the resolution is about 0.1 g/m2 for small weights (below 50g/m2).
- Very short response time: The intrinsic response time and the data processing time are very short. Individual measurements are averaged over steps corresponding to 10msec. sampling time. This avoids any artificial smoothing effect of the profile.
- Good streak resolution: The streak resolution is related to the spot size and is about 10 to 12 mm.
- Emitter: Rather than using a lamp with a short lifespan and whose efficiency very poor, Scantech uses diodes.
- No paperwork or legal authorizations: Rather than using a lamp with a short lifespan and whose efficiency very poor, Scantech uses diodes.
Our design has the following major benefits:
- Single spot measurements.
- No drift over time regardless of variations in temperature, pressure and humidity. No electronic drift (for a film of 20μm, the standard deviation for a trend of 10 hours is less than 0.02 μm).
- No influence due to variations in ambient temperature.
- An air gap of at least 4cm.
- Negligible influence due to misalignment of heads.
- Negligible influence of variations in the distance between heads.
- Not sensitive to variations in the intensity of ambient light.
- Not sensitive to light interferences.