How black is ‘Super Black’?

Scientists at the National Physical Laboratory (NPL), Teddington, Middlesex, UK have good news for manufacturers and users across the optical instrumentation industry. Based on existing processes developed in the US and Japan, a team of researchers at NPL has developed a new technique for commercial manufacturing of ultra-black coatings, which represent one of the blackest, lowest reflectance surfaces developed so far.

Performance of optical instrumentation depends on the quality of materials used in their manufacture. For the accuracy of measurement in the ultra-violet, infra-red and visible regions, optimal radiation detection and minimisation of stray light is crucial.

By studying the effect of different methods of chemical etching on various compositions of nickel-phosphorous alloys, researchers have come up with the most effective commercially available black coating to date. With reflectance as low as 0.35% in the visible region, the coating, known as NPL Super Black or Ni-P black – as it is based on a nickel -phosphorous compound – is set to have a major impact in fields such as radiometry, spectroscopy, optical metrology, and within the aerospace and defence industries.

NPL Super Black has been produced on a small scale at the National Physical Laboratory in the UK for a number of years. Its efficiency in detecting radiation, and reducing stray light in instruments is well known. Until recently however, the process has not been fully understood, and a growing demand for more efficient low reflectance surfaces prompted NPL to conduct the first in-depth research of its kind.

Dr Richard Brown, Senior Research Scientist at NPL, says, “The results are very exciting. The improved understanding of the process means that NPL Super Black will be available to a wider range of users across many areas of science and technology, and its benefits are enormous. One of the advantages of this new black is that it can withstand cryogenic temperatures without cracking.”

NPL’s increased understanding of the process by which the black nickel-phosphorous coating is produced will enable larger scale production of an even higher quality, more effective optical black. As well as increased quality of optical measurement, one of the greatest benefits to manufacturers and users of optical equipment will be the reduction in instrument size and weight, made possible as a result of the increased efficiency of the coating.

Further good news is that the latest manufacturing process allows NPL Super Black to be produced in larger sample sizes. The largest available coated plates were formerly no bigger than 1 to 3cm2, making them too small for many applications. Now, plates can be manufactured in sizes of up to 12cm x 12cm, making them a viable alternative for a wide range of uses.

The advantages of Ni-P black over other coated surfaces are already well established. In addition to its higher absorbance, nickel-phosphorous black coatings do not age significantly compared with painted surfaces, and whereas the painted surfaces would crack at cryogenic temperatures NPL Super Black’s performance is unaffected.

This, combined with the fact that NPL Super Black has the potential to be plated onto a range of materials of different shapes and sizes, including glass and ceramics, will allow greater flexibility of instruments across a range of environmental conditions. Because of its excellent ageing properties, instrument lifespan will also be extended, making NPL ‘Super Black’ an extremely attractive and cost-effective alternative.