To characterise the performance of filters, detectors and components for optical radiation measurement applications, it is essential to be able to produce radiation of known wavelength and power level, and to make high accuracy measurements the radiant power should be stable over long periods.

The radiant power of lasers can be stabilised to a higher degree than is possible with incandescent lamps. Tunable lasers such as dye or solid-state crystal lasers are particularly useful, having a narrow spectral band width and tunable over a wide spectral range. The radiant output of lasers is also highly directional and easily manipulated, which is useful if working with detectors needing a fine spatial resolution, such as CCD arrays.

The normal output beam of a laser, however, is not spatially uniform, but exhibits a Gaussian distribution across its width, peaking in the middle. This cannot easily be smoothed out by a diffuser, or other optical device. Another potential problem is that laser radiation tends to be highly polarised which can also cause difficulties when characterising optical components and devices. This has often limited the application of laser radiation for characterising filter radiometers, for example.

NPL has developed a novel technique in which light from a laser is rendered uniform by passing it through an optical fibre immersed in a bath of water vibrated at ultrasonic frequency, before it enters an integrating sphere. A third port is added to the integrating sphere, at right angles to the entry and exit ports. The output of a detector mounted in this port is used to stabilise the radiance of the exit port of the sphere using a variable attenuator in the laser beam before it enters the optical fibre.

This source can then be used to characterise the spectral characteristics of any optical component or instrument ranging in scale from a simple filter radiometer such as a pyrometer through to a space bound satellite instrument. An example of the systems use is shown in the filter radiometer calibration facility.