Calibration of the UV Component:

It has been found that the Xenon light source provides an excess of UV energy and that the UV content of these lamps degrades with use. This was discovered in the early seventies by work performed by Griesser and Gartner relating to the UV content of new xenon lamps and xenon lamps after a period of aging. Their work resulted in the creation of the UV calibrator used within some makes of spectrophotometers to monitor and provide consistent amounts of UV energy to the sample.

The basic issue is that xenon flash lamps are not consistent in regard to the amounts of UV energy they output. We are aware that they contain an excess amount of UV energy but from lamp to lamp they may contain uneven amounts of this energy. We have also indicated that the UV energy contained in these lamps will degrade as the lamp ages and changes occur within the instrument due to aging. The effects of these changes upon the sample indicate that measurements taken of a fluorescent standard over a period of time will show differed measured results even though the fluorescent standard has not changed. These changes occur on the same instrument, the same type of instrument, and with a light source that has ample UV energy and excellent simulation to D65 daylight. Take into consideration the differences that can occur when results of these measurements are compared to measurements of the same sample taken on instruments with different geometry containing insufficient UV energy with the illumination source to the sample. What can occur are measurements taken with too low or too high UV energy as opposed to measurements taken with the right amount of UV energy to simulate D65 daylight.

In summary, the best spectrophotometer to use for measurement of fluorescent samples is one that has a pulsed-xenon light source that is properly filtered to daylight D65 and that has the ability to filter or calibrate the UV component to a fluorescent standard. Calibration of the UV component was based on results obtained by measuring fluorescent plastic standards obtainable from Ciba-Geigy. These fluorescent standards are no longer available from Ciba but are now available from Frederick T. Simon (FTS, Inc.) of Clemson, SC.