Instrumental Measurement:

Most color measurement instruments have not been designed to measure the effects of fluorescence. Color instruments found in most textile companies are spectrophotometers that are either 450 or sphere diffuse 8 geometry instruments. They are what we refer to as single monchromator instruments where light reflected or transmitted from the sample is broken into its respective wavelengths and presented to the analyzer. The analyzer calculates percentages of this light at its designated bandwidths and displays the color curve. The light from the sample is referred to as polychromatic (white light) and the instrument as a polychromatic illuminating spectrophotometer. This is an efficient, accurate, and repeatable method of measuring non-fluorescent samples.

As we know, fluorescent samples absorb light in other regions of the spectrum and then re-emit in the visible area of the spectrum. Ideal instruments for measuring this effect would be those that have the ability to separate the base reflectance from the amount of reflectance caused by the fluorescence. Such instruments do exist but they are expensive and not common to the industry. They are dual (2) monchromator a system where the first monchromator breaks the illuminant into its individual wavelengths and the second monchromator receives the light from the sample (at its specific wavelength) and presents it to the analyzer. Output data can separate the base reflectance from the fluorescent reflectance and allow better analysis of the effects of optical brightners and fluorescent dyes and/or pigments. Again, these instruments are expensive, slow in measurement time, and not common to the other applications within the textile industry.

When using conventional spectrophotometers for fluorescent measurements and comparing their results to other instruments, the following conditions should be considered:

 

• Geometry of Instrument.
• Measurement Area
• Light Source of Instrument
• Illumination Filtered to D65
• Calibration of the UV Component

 

Instrument Geometry: Measurement Area:

Two types of geometry found in conventional spectrophotometers are sphere (8) and 45/0 or 0/45. Both geometries can be used to measure fluorescent samples and if comparing data from two (2) instruments it is important to insure that they have identical geometry. If not, measurement data may be different and may not be compatible. This is true not only with fluorescent samples but also with any measured samples.

Another variable required for consistency is that of aperture or area of measurement on the instrument. As an example, an 8mm circular aperture will provide reflectance energy from an 8mm area of the sample. When measuring fluorescent standards/samples the data collected is a result of the measuring area. If the measuring area is changed to 25.4mm the data collected may not correlate to the data collected from the smaller measuring area. Reflectance from the fluorescent sample may be higher or lower in UV content relative to the area of measurement.

 

Light Source: Illuminant D65:

Two important specifications are necessary relative to the light source of the spectrophotometer. One, in order to meet CIE specifications for the measurement of fluorescent samples it is necessary for the illuminant to contain ample levels of UV energy. Secondly, it is necessary for the light source illuminating the sample to be filtered to D65. Some instruments use quartz or tungsten light sources that lack ample UV energy to excite the whitening or fluorescent agents within the sample. Whatever UV energy they do contain begins to diminish early into the life cycle of the lamp. Because of the low amount of UV energy emitted by these type lamps they are more difficult to filter to D65.

Instruments that have pulsed-xenon light sources contain a high amount of UV energy and they are easier to filter to D65. Therefore, this type source provides the UV power to simulate D65 daylight.