Interferometry explained

Laser interferometry is a well-established method for measuring distances with great accuracy.

Renishaw's machine performance measurement systems use remote interferometers for all measurement modes (not just linear) and a precision stabilised laser source to deliver exceptional precision and accuracy.

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.

Today "laser measurement" is recognised as a byword for accuracy. However systems vary widely from custom laboratory standards costing tens of thousands of pounds to hand tools for home use, costing a few pounds only. It is essential to pick the right system for the job.

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 40ºC)

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.  Fixing the interferometer to the laser means that thermal growth of the laser head requires that additional warm-up time needs to be allowed before high accuracy readings can be taken.  By using a remote beam splitter, Renishaw ensures high accuracy measurements every time. (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 ±0.7ppm. It has become the leading high accuracy, portable machine calibration and  measurement system worldwide.

In 2008 it was joined by the XL-80 laser system with enhanced measurement performance (±0.5ppm, 50kHz, 4m/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 geometry machines, the RLE laser encoder was developed, based on the principle of combining interferometer performance levels with encoder simplicity. The first generation RLE10 was launched in 2001, with further devlopments since.  Please refer to the Laser Encoders section of this website for further details of these products.

Today "laser measurement" is recognised as a byword for accuracy. However systems vary widely from custom laboratory standards costing tens of thousands of pounds to hand tools for home use costing a few pounds only. It is essential to pick the right system for the job and one that is accurate, reliable and durable in day-to-day use, where it really matters.

At Renishaw, our aim is to help you choose the right laser system for your application.

 

Specifications

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