Interferometry explained

History of interferometry

Interferometry - the ultimate measurement standard.

As long ago as the 1880s, the use of light interference principles as a measurement tool was first demonstrated. Although the technology has been developed over the years since, the basic principle of using the very small, stable and accurately defined wavelength of light as a unit of measure has remained.

Environmental compensation

No matter how good your laser unit is (i.e. how accurate and "stable" it is) the measuring wavelength can be altered as the light passes through air because the air temperature, air pressure and relative humidity vary from the "standard" values.

Even the following small changes will increase laser wavelength (and measured lengths) by 0.25 ppm

˜0.26 ºC increase in air temperature

˜0.83 mbar decrease in air pressure

˜29% increase in %RH (at 40 ºC)

Without reliable and accurate wavelength compensation, errors of 20 ppm - 30 ppm would be common in linear measurement reading.

Environmental compensators ensure that XL-80 and ML10 measurements maintain accuracy over a wide range of conditions.

Note: Environmental compensation is NOT required for angular or straightness measurements when using a Renishaw laser system. This is because these are calculated from differences in two beam paths close together, where the environmental factors self cancel. As rotary axis, flatness and squareness measurements are also based on these measurements, they also do not require environmental compensation.

Remote interferometry

Some competitor systems locate the interferometer/beam splitter within the laser head. Thermal growth of the laser head changes the measurement path length, so additional warm-up time is needed before accurate measurements can be taken. By using a remote beam splitter, Renishaw avoids this problem. (Please refer to the technical paper below.)

Renishaw's interferometric pedigree

Renishaw has been designing, manufacturing and supplying laser interferometer systems for over 20 years. Its ML10 laser system has become a standard for accuracy and reliability in use, enabling linear measurements to be made to an accuracy of ±0.7 ppm. It has become the leading high accuracy, portable machine calibration and  measurement system worldwide.

In 2007 the ML10 was superseded by the XL-80 laser system which gives enhanced measurement performance (±0.5 ppm, 50 kHz, 4 m/s) in a highly compact, portable and easy-to-use package; bringing the benefits of laser interferometry to an even wider audience.

After specific requests for a closed loop laser feedback solution, our first laser encoder product, the HS10 long-range laser scale, was launched in 1994. To extend application suitability to smaller  machines, the RLE laser encoder was developed, based on the principle of combining interferometer performance levels with conventional encoder's simplicity. The first generation RLE10 was launched in 2001, with further developments since. Please refer to the Laser Encoders section of this website for further details of these products.

Specifications

Renishaw operates a policy of continual improvement. Specifications given in these pages are accurate to the best of our knowledge. However Renishaw reserves the right to change product appearance and specification without notice and customers should always seek confirmation at the time of enquiry.

Useful links

XL-80 measurement laser

QC20-W ballbar