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Discussion on the Technique of Relative Optical Interference Intensity for the Measurement of Lubricant Film Thickness


The technique of relative optical interference intensity (ROII) has been developed for measuring lubricant film thickness. Here, we describe in detail the principle and the characteristics of the film thickness measurement system based on the existing ROII method. Some researchers have reported a large calculation error in film thickness with the use of ROII method. The description and analysis of the resolution, accuracy and important factors influencing the measurement results of this measurement technique presented here illustrate that the ROII method should be used in a specific optical system with negligible multiple beam reflections, but not in an ordinary one. The reported calculation error in film thickness using the ROII method comes from using an inappropriate optical system and inappropriate coating parameters. In fact, the ROII method, which is a very accurate method for measuring the thickness of a thin lubricant film, with a high resolution in film thickness of 0.5 nm and a horizontal resolution of 1.4 μm, can only be used in a two-beam interference system.

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Fig. 6
Fig. 7
Fig. 8
Fig. 9


e :

The extinction coefficient

h :

Lubricant film thickness

Δh :

Variation of the lubricant film thickness

I :

Light intensity

I 1 :

Light intensity of beam 1

I 2 :

Light intensity of beam 2

I 3 :

Light intensity of the secondary reflection component

I max :

Maximum interference light intensity

I min :

Minimum interference light intensity

I max(n) :

Maximum interference light intensity of the interference order n

I min(n) :

Minimum interference light intensity of the interference order n

ΔI :

The grey value gradation

\( \overline{I} \) :

Relative interference light intensity

λ :

Wave length of the incident light

k :

Reflective index of lubricant


Phase difference of two beam interference


Phase change caused by the surfaces of the chromium layer and the steel ball

\( \overline{I}_{0} \) :

Relative light intensity when the lubricant film thickness is zero

n :

Interference order


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This work was supported by the National Natural Science Fund China, the 973 project, and the International Science & Technology Cooperation Project (No. 2008DFA51190). The authors would also like to express their thanks to Amatsuji Steel Ball Mfg. Co. Ltd, Japan for providing the highly polished steel ball.

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Correspondence to Chenhui Zhang.

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Ma, L., Zhang, C. Discussion on the Technique of Relative Optical Interference Intensity for the Measurement of Lubricant Film Thickness. Tribol Lett 36, 239 (2009).

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  • Film thickness
  • ROII method
  • Thin film lubrication