Interferometric Method for Transparent Thin Films (transparent, 2µm to 250µm)

If you have ever seen light reflected through a thin layer of petrol on a puddle of water or bubbles in the bath then you have almost certainly been amazed by the spectrum of colours exhibited. The phenomenon is caused by interference of the white light reflecting from the upper and lower surfaces of the thin layer of petrol or the inner and outer surfaces of the bubbles.

Bubbles  Interference explained

In the interferometric optical probes from Precitec Optronik white light from a variety of sources (halogen, LED, xenon etc) is coupled into an optical fibre in the controller unit and transmitted to the optical probe. The emitted light undergoes reflection from the layers and is collected back into the optical probe, passes back up the fibre where it is collected into an analysis unit. The modulated signal is subjected to a fast Fourier transform to deliver a thickness measurement.

Spectrometer interferometryFFT graph

While thickness measurement of materials (glass, plastic etc) can be performed using the chromatic confocal method the interferometric method provides better accuracy and is better at measuring thickness with a varying measuring distance, for example on a production line where the working distance of the sample might change by several mm during measurement. The chromatic confocal method of thickness measurement can operate over a range of 30µm to 35mm but really requires a fixed working distance to ensure maximum accuracy.

Due to the physics behind the mode of operation, the interferometric thickness method is only applicable for layers of thickness between approximately 2µm and 250µm; below this lower limit there are not enough maxima and minima to perform an accurate FFT, above the upper limit there are too many.

Not  enough fringes  Too many fringes

The interferometric method of thickness measurement is especially applicable to such applications as the measurement of thin plastic films, layers of varnish on metals or soft gel coatings etc.

  • Halogen, xenon or LED sources available
  • Transparent or translucent materials
  • Thickness range between 2µm and 250µm
  • Independent of color and temperature
  • Speeds up to 66kHz
  • Easy to integrate inline or with other sensors
  • Multi-channel systems available
  • CHRocodile_S
  • CHRocodile 2 S
  • CHRocodile_SE
  • CHRocodile 2 SE
  • CHRocodile_E
  • CHRocodile_LR
  • CHRocodile_M4

If you have an application that you feel might benefit from using non-contact interferometric thickness measurement and would like to discuss it in more detail please either contact us at +44 (0) 1604 654 220 or click through to our contact form and we'll be in touch as soon as we can.

Price: £0.00