The result that you want to achieve is the right balance between the visible amount of UV present and the amount of UV emitted by the source in the instrument. To achieve this you need to have a light source, like the pulsed-xenon, that emits more UV energy than you need. You need to be careful because some instruments that use a pulsed-xenon light source may provide too high an intensity of pulsed light and this can cause an effect that we refer to as the triplet effect. Triplet effect results in false readings that are low at some wavelengths that will lead to numeric values that do not correlate to the eye. In the past this was a problem but most manufacturers of pulsed-xenon instruments are aware of this effect and have taken steps to correct this problem within their instruments.

I think we can assume that most instrumental measurements of fluorescent samples are provided by the traditional spectrophotometers found in the QC or formulation lab. These instruments are used for many different textile color measurement applications and most likely no special calibration routine is provided relating to the measurement of fluorescent samples. In order to improve the accuracy and repeatability of your color measurements I recommend that you consider the following procedures:

 

• Insure that the spectrophotometer used for this measurement contain a pulsed-xenon light source.
• Make sure that the instrument provides for a method of calibrating the UV component.
• Obtain a set of plastic fluorescent standards and regularly measure these standards to calibrate the UV source according to instrument procedures.
• Provide proper maintenance to the optics of the instrument in regards to dirt and dust and provide regularly scheduled factory service to the instrument.

 

Conclusion:

We have dealt mainly with the hardware issue relating to color measurement of fluorescent samples. This was our purpose. We need to be aware that the software issue can be just as critical. It has been stated that the best procedure for measuring whiteness is use of the whiteness index. A whiteness index maintains a the perfect white standard to which all your white samples are compared against. This standard has the value of 100 as the perfect white. It is up to the user to determine which whiteness index is best for their application. Some indexes use two dimensions of color space while some use three. Some formulas place more value on lightness while some place more value on yellowness as a deterrent to whiteness.

Regardless of the approach one may take relative to the mathematics of calculating the formats of your measurement, proper instrument and instrument calibration is crucial.

 

About the Author:

 

The author is associated with Minolta Corporation, Instrument Systems Division.

 

 

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