Canada's Ultimate Light Ruler

For NRC scientists turning on the lights is never a simple task. They inevitably ask what kind of light and how much? Which is why they're so excited about a new tool that will soon be Canada's ultimate light ruler. lundi

Called an ultra high-temperature blackbody, this rare physics tool now being readied at the NRC Institute for National Measurement Standards (NRC-INMS) in Ottawa will soon be one of the world's most accurate ways to measure ultraviolet, or UV, light. These UV measurements are critical for a wide range of environmental and health issues, emerging industrial technologies, and regulatory requirements pertaining to global trade.

While it's called a high-temperature blackbody, it's all about measuring light. All objects emit some form of electromagnetic radiation. The electromagnetic radiation of a high-temperature blackbody is predominantly in the optical radiation region. This region ranges from the infrared through the visible spectrum to the UV. While you're reading this at room temperature your body is emitting invisible infrared radiation that would be visible with infrared, or "night", goggles. What's special about a blackbody is that it's a perfect emitter. When it's heated, at any particular temperature it emits a distinct amount of energy at each wavelength of light. Thus if you know the blackbody's temperature you can use a physics calculation to determine the amount of light being emitted at any wavelength.

"We now have a light source whose absolute output I can calculate. In the measurement business you need a ruler against which you can measure things. The high-temperature blackbody will become our ruler in light measurement," says Arnold Gaertner, the NRC-INMS research officer leading the blackbody project.

The state-of-the-art equipment, built in Russia and installed in part by two technicians from Russia's National Metrology Institute in Moscow, addresses a growing Canadian demand for more accurate UV measurements. In order to deliver these measurements, UV lamps must be calibrated against a known source. This is where the high-temperature blackbody comes in. Its known UV radiation will be used to calibrate NRC UV standards, which will then be used by NRC-INMS staff to calibrate commercial UV equipment.

Until now, NRC's measurement scientists have used incandescent lamps as Canada's UV, visible and infrared light standards. However, these lamps are not perfect emitters. Nor do they provide measurable light below 300 nanometers in wavelength. This is like having a metre stick that doesn't have the centimetres and millimetres marked beyond the 90-centimetre mark. Along with improving the ability to measure in the UV range, the high-temperature blackbody will improve the accuracy of NRC-INMS light measurements over the entire optical wavelength range.

"With the blackbody we'll be able to improve our calibration uncertainties up to ten fold. So we're excited that it will open the door to new collaborative R&D opportunities with Canadian industries that are developing UV-dependent technologies and provide much needed traceable calibration services," says Joanne Zwinkels, Group Leader of the NRC-INMS photometry and radiometry group.

The high-temperature blackbody requires all the meticulous control you might imagine for a tool operating at the extremes of precision and accuracy, and Gaertner says the project is one of the highlights of his 30-year NRC career.

Precise UV Measurements benefit the Health and Safety of Canadians

The depletion of the ozone layer in the upper atmosphere has resulted in dangerous levels of solar UV radiation reaching the earth. This increased UV exposure has resulted in an alarming incidence of skin cancers and cataracts. However, UV radiation also has significant beneficial health and safety applications. Health applications of UV radiation include purification of water, dental light curing, artificial tanning, photodynamic therapy (treatment of psoriasis and jaundice and new photoactivated anti-lung cancer treatments), medical diagnostics, and sterilization of fruit and vegetables to increase shelf life. Apart from health and safety issues, there is an increasing use of UV in various industrial applications, such as non-destructive testing and inspection, curing of epoxies, and photofabrication. For these reasons, the use of UV radiation is one of the largest growth areas for optical radiation in industry. It is therefore crucial to have precise and accurate means of measuring UV radiation levels in the range 200 nm to 400 nm to support industries that use UV technology and to ensure public health and safety.

The core of the high-temperature blackbody is a hollow tube of a special form of graphite that can withstand intense heating. In order to produce the required UV radiation, the graphite core is heated to approximately 3230 ºC, a temperature at which almost all metals melt – hence the name an ultra high-temperature blackbody. The graphite is gradually heated over the course of several hours in the same way a stove element is heated – by running an electrical current through it. The graphite core is insulated with many concentric layers of carbon cloth which are water-cooled.

At 3230 ºC any oxygen would react instantly with the graphite, causing a fire. So, during operation the entire core is flushed with argon, a non-reactive gas. At operating temperature, the high-temperature blackbody produces an intense beam of light that's emitted from a tiny eight millimetre hole – a beam of light that will soon be Canada's ultimate light ruler.

In addition to its role in UV light measurements, the blackbody may be used, in collaboration with the NRC-INMS thermometry group, as part of an international effort to extend fixed points on the temperature scale. Fixed temperature points are ones that act as international temperature beacons – they're the same anywhere on Earth. Most fixed points are the melting or freezing points of ultra pure metals. At present, the freezing point of pure copper at 1084.62 °C is the highest fixed point. Using high-temperature blackbodies, NRC-INMS metrologists, along with international colleagues, are working to extend the fixed point temperature scale using metal-carbon eutectics, highly ordered mixtures of a metal and carbon that melt at up to 3200 °C.

Photometry/Radiometry and Temperature Standards are only two of eight diverse fields of research in physical and chemical metrology conducted at NRC-INMS. The physical metrology programs develop, maintain, improve and disseminate standards for the base quantities of mass, length, time, electricity, temperature and luminous intensity as well as a number of derived measurement standards. The chemical metrology program develops and maintains world-class capabilities in selected areas of organic and inorganic trace analysis, and provides certified reference materials.

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