Measurement of liquid films in micrometer range - LPSM (Laser Pattern Shift Method)
Using the new Laser Pattern Shift Method (LPSM) films can be analyzed through a transparent window. The design of the system is comparatively inexpensive and, in combination with a measuring rate greater than 500 kHz, makes simplified in-line process or continuous quality monitoring during operation possible. The system offers many advantages over conventional CCI measurement and also allows the determination of very small film thicknesses (< 2 µm).
The measurement of thin, transparent layers, such as lubricating films, is often a challenging and at the same time safety-relevant measurement task. In this context, it might in some cases not be possible to perform the measurement from above.
It might be easier to carry out the measurement from below, through an inspection window, for example. Especially if lubricating films inside of components need to be monitored, like in bearings, the new LPS method shows its benefits.
Depending on the field of application, different methods are used to measure film thickness. Confocal chromatic interferometry (CCI) is a common method for measuring film thickness in the micrometer range. CCI is used to carry out measurements from above as shown in figure a). However, the sensor head can interfere with the process, e.g. if liquids are to be sprayed. In addition, extremely favorable measuring conditions are required in order to be able to measure film thicknesses of below 100 µm. Therefore, it is hardly possible or not very economical to use CCI for the measurement of lubricating films.
Based on the novel ‘Laser Pattern Shift Method’ (LPSM) it is possible to measure coating thickness by means of an optical access to the measuring section, e.g. a glass window. The measurement can thus be performed from the rear side. The measuring principle is shown in the attached figure b). Via a prism, the light beam is coupled into the liquid layer. A total internal reflection occurs on the film surface. A change in layer thickness can be calculated very accurately from the shift of the reflected light beam on the detector. This innovative method thus allows for the measurement of layers of various thicknesses, even layer thicknesses of 2 µm can be reliably measured. Thinner layers can also be detected provided that the measuring system is modified slightly.
- Coating thicknesses from 2 µm to several millimeters can be measured
- Thinner layers can also be measured with slightly modified setup
- Measuring rates of 500 kHz or higher depending on the system design
- Inexpensive components
- Rear access to the measurement through transparent window
Specific applications for this innovative method include the measurement of thin films such as lubricating films, of paint layers after application, the inspection of film or glass thicknesses, quality control during the pre-treatment of surfaces (application of primers). LPSM could also be used to prevent "dry-out" in heat exchangers or to develop more precise wiper sensors.