Ultra Thin Film Measurement (EHD2)

The EHD is a fully automated computer controlled instrument for measuring the film thickness and traction coefficient (friction coefficient) of lubricants in the elastohydrodynamic lubricating regime. The instrument can measure lubricant film thickness down to 1 nm with a precision of +/- 1 nm.


The instrument measures the lubricant film thickness properties in the contact formed between a ¾“ diameter steel ball and a rotating glass disc by optical interferometry. The lubricant film thickness at any point in the image can be accurately calculated by measuring the wavelength of light at that point. The system measures the wavelength of the light returned from the central plateau of the contact and then calculates the central film thickness.

How its done?

  1. The contact is illuminated by a white light source directed down a microscope through a glass disc on to the contact.

  2. Part of the light is reflected from the Cr layer and part travels through the SiO2 layer and fluid film and is reflected back from the steel ball.

  3. Recombining the two light paths forms an interference image which is passed into a spectrometer and high resolution black and white CCD (charge-coupled device) camera.

  4. The camera image is captured by a video frame grabber and analysed by the control software to determine the film thickness


  • Compact bench top and fully automated

  • Independently driven specimen enable a wide range of contact conditions to be replicated across all three lubrication regimes

  • Unique configuration eliminates induced spin in the contact improving accuracy of test results

  • A wide range of specimen geometries and materials available


The Spacer Layer Imaging Method (SLIM) uses a high resolution RGB CCD colour camera to grab an image of the whole contact. The SLIM software uses a previously determined colourspace calibration to match the colours in the image to oil film thickness. The system can then produce a film thickness map of the whole EHD contact in a few seconds. This makes is a unique tool for examining conditions such as parched or starved lubrication, grease lubrication, rough surface EHL and additive boundary film formation.


  • Evaluation of film forming oils and greases.

  • Starvation and reflow characteristics of grease lubricated components.

  • Fuel economy prediction of candidate crankcase oils.

  • Performance prediction of oil in water emulsion rolling mill lubricants.

  • Fundamental investigations of the high pressure/high shear behaviour of fluids such as liquid crystals.

  • Investigation of boundary additive performance