QTH filament collapse: an investigation into the collapse of filaments of FEl QTH lamps used as irradiance standards.

Business and science communities rely on spectral irradiance standards to underpin a vast range of measurements. For example, for Earth Observation (climate change, solar UV, ozone) [Hunt & Harrison (1999) and references therein]; for testing Infrared and UV lamps used in photomedicine and photobiology; UV lamps used for sun tanning to ensure safe levels of radiation are being used [Hawk (1999); Moseley (1988) and references therein] and also scientific studies requiring defined irradiance levels in specific spectral regions for growing biological samples and cultures – and so on. Quartz Tungsten Halogen (QTH) FEL (1000W/120V) lamps are currently the irradiance transfer standard of choice of the international community, following the introduction of a modified version by the USA National Institute of Standards and Technology (NIST) [Ohno (1997); Ohno & Jackson (1996)]. Whilst these lamps are relatively cheap and portable there are now recognized problems with a) transportation b) filament collapse [Hunt & Harrison (1999) p24-27 & references therein]. In addition, manufacturers claim that any burning orientation is acceptable*, see Appendix 10.1 for an example specification. However, whilst researchers have indicated that in order for an FEL to be used in the horizontal position, it should also be calibrated in that position [Early & Thompson (1996)] they did not refer to filament sag; a problem that has been confirmed by our own experience. Refer to figure 1.1 which is of a lamp used in the horizontal position (as shown). In this report, there will be a full description of the measurements made at NPL and the changes in spectral irradiance or total flux from new lamps and lamps pre and post collapse. Work has also been undertaken to develop a simple model to explain the relationship between changes in FEL lamp coil configurations and changes in irradiance. This model is based on the manufacturer’s calculations of projected output. We are indebted to Dr Milan Vukcevich for providing us with a copy of his book on incandescence, which is not generally available; we have used his work extensively to develop our model. This work will highlight the fact that it is difficult to keep up with work in other fields that might materially affect your own. Filament design, manufacture, and associated problems/failures are not new to lamp manufacturers. [Langmuir (1912); Moon (1936); Baker (1975) Covington (1975) Coaton (1971 & 1978) and others.] However, their emphasis has been on the consequences of filament failure with respect to lamp life and therefore saleability of the product. Our concerns as metrologists/radiometrists are with the changes in output (in our case, this is irradiance) of lamps we use as a metrological tool. This work will show that FEL filaments collapse, even when they are used in the vertical position; especially the unsupported, modified, FELs currently used as transfer standards. This collapse or sag, even in the very early stages, results in large changes in lamp output (irradiance).