Pro Lite Technology

Intense UV Lamp Spectroradiometry

 NPL Spectralon Integrating Sphere

Spectralon® sphere lining improves integrating sphere performance
in the UV

Integrating spheres have been used for meauring the output of lamps for over a century. Over the years, a variety of integrating sphere coatings have been used, including magnesium oxide and, more recently, barium sulphate. The ideal integrating sphere coating must possess high reflectance, exhibit diffuse (Lambertian) reflectance and should remain stable over long periods in order to maintain the calibration of the system. For many applications, barium sulphate-based formulations are ideal, however the testing of intense ultra violet (UV) lamps presents something of a challenge to ordinary sphere coatings. The output of very intense UV sources can cause the reflectance of barium sulphate sphere coatings to drop, rendering the system calibration quickly invalid. This was something that scientists from the National Physical Laboratory (NPL) were keen to find a solution to when they undertook a study to investigate the ageing of materials under intense UV radiation*. Their research concluded that Labsphere's Spectralon® diffuse reflectance material offered the highest stability to UV sources. Spectralon is Labsphere's award-winning diffuse reflectance coating that exhibits the highest reflectance in the 250-2500nm band, is hydrophobic, chemical inert, thermally stable and resistant to the ageing effects of UV radiation.

NPL approached Labsphere with a view to having their 2m diameter integrating sphere upgraded to a Spectralon coating. Unfortunately, the largest sphere that can be machined from a single piece of Spectralon is 30cm. Not to be deterred, Labsphere set about designing an innovative, geodesic lining for the large sphere that would comprise over 250 individual, small panels of Spectralon machined as "ship-lapped" hexagons and triangles. These tiles would be held in place within the NPL sphere by captive bolts slotted into the Spectralon.

A team of Labsphere technicians installed the Spectralon tiles on-site at NPL, with special care being taken to ensure that the tiles created a seamless join when the two hemispheres were closed. We think you'll agree that the finished sphere is as much a work of art as an engineering triumph.

* "Ageing of Materials Under Intense UV Radiation" by Gibbs, D R, Duncan, F J, Lambe, R P, Goodman, T M published in Metrologia, 1996, 32, (6), 601-607. Click here for a link to the publications area on the NPL web site.

Pro-Lite gratefully acknowledges the permission of NPL to publish this article. For further information on integrating spheres, click here.

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