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Calibration of whiteness scale
The goal of calibration is the standardization of an instrument according to certain convention and norms. Implicit is the idea is also to correct for small
imperfections, unavoidable with commercial instruments in operation under hostile conditions.
Proper calibration of instruments intended to measure whiteness must be conducted carefully and maintained in regular
intervals of time. A full calibration consists of several steps as follows:
Calibration of reflectance scale: whiteness values are strongly dependent on the primary standard used for calibration of reflectance values, for this reason special attention must be paid to values and their back traceability of the secondary white standard used during calibration. The black trap used for black calibration must be also controlled periodically, remember that its values are assumed to be zero for the whole wavelength region under consideration; normally this is a good assumption (typical maximum values are around 0.15%).
In general instrument manufacturers today offer different back traceability for their standards, but older instruments are poorly calibrated or the values are just false. In general the delivered white
calibration tile is tied to the instrument by its serial number; reflectance values of the tile are stored in an EPROM within the instrument or the values of its certificate have been typed into the operating
software. If for any reason the tile is lost or broken, the values of the new certified tile must be stored accordingly otherwise wrong results are anticipated. Even under these conditions reflectance values of the
tile are not invariant, in fact they change with time and handling such that its values must be checked on a periodical basis against a primary standard. Small variations in the values are not critical when
measuring color (especially dark ones) but they can lead to large errors for light colored or white samples.
A good practice is to have a primary standard calibrated at regular intervals by a certified
institution or get certified secondary samples (use one of the internationally recognized calibration services like STFI, Sweden or KCL, Finland); the
secondary standard is thus regularly calibrated and used with fresh reflectance values for daily calibration.
(More details on certified primary standards material can be found in W. Erb, PTB-Mitteilungen
1994, 104, 20-22)
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Calibration of whiteness scale: calibrating whiteness scale involves in general two steps:
While calibration of the UV intensity is mandatory, calibration of scale is only required for the Ganz scale, since the CIE scale has constant parameters.
For conducting calibration of whiteness scale certified calibration samples are required, these are samples containing a defined amount of fluorescence that are accompanied by exact calibrated whiteness data. In
this respect two different methods are available:
Two-point calibration:
two paper samples are provided with CIE whiteness values. One is just bleached cellulose without any fluorescence, the second sample contains a certain amount of FWA and shows a level of fluorescence (CIE whiteness of typically 130 points). The first sample is given with certified reflectance values that can be used to calibrate the reflectance scale and also produce own secondary standards (see previous section); certified CIE whiteness is given for the second sample. Data is traceable back to international institutes. As a first step reflectance data for the first sample is entered into the instrument or its software; the instrument is calibrated with this sample at a given position of the calibration filter. Then the second sample is measured and the whiteness number compared to the certified value. Both values should agree within one CIE whiteness unit for the instrument to be regarded as calibrated (some instruments equipped with a step motor for moving the filter have too coarse step, choose the position giving the nearest value, preferably higher rather than lower). Depending on the results the filter must be positioned accordingly (push it in if values are higher or pull it out if smaller) and the procedure repeated until agreement; the instrument must be recalibrated after every change in filter position. Tint values are irrelevant in CIE whiteness, they can just be used to determine the degree of incongruence between the instrument and the master instrument. Samples can be obtained from STFI or Technidyne.
Multiple-point calibration:
if more than one fluorescent sample are present the procedure will target a best fit for all whiteness values available in the calibration set. Hohenstein Institut, Germany offers a four-step set and Textiforschungsinstitut Thüringen-Vogtland, Germany (TITV) offers a five-step set. The latter gives also full
reflectance data i.e. reflectance values of the non-fluorescent sample can be used to calibrate the reflectance scale (see previous section). Samples are made of bleached textiles that contain different amounts of
FWA, up to typically 160 CIE whiteness. Data is also traceable back to international institutes. Samples have a guaranteed shelf life of about three months, they are however required only once for a single
instrument calibration. Both calibration sets contain also full data for calibration of Ganz whiteness scale. The calibration involves the simultaneous determination of position of the filter and the calibration of
the scale itself through the procedure of parameter variation. The procedure consists in measuring every step in the set and running the software routine to find the best fit; calibration state of the instrument is
characterized by the value of the parameter (dW/dS) that is related to the amount of UV present in the sample illumination of the instrument. The goal is to find a filter position that leads to the value (dW/dS)=
4000, that corresponds to an amount of UV similar to the one present in daylight. Analogous as before the procedure involves changing the position of the filter (values higher than 4000 correspond to low UV, lower
than 4000 to high UV, move the filter accordingly), recalibrating the instrument and running the fitting procedure. While this appears complicated the procedure is quite simple and straightforward, even for totally
new and non-calibrated instruments, full calibration is fully accomplished in three to four rounds. Tint values are also fully calibrated, the inter-instrumental agreement of measurements after proper calibration is
very high, although the parameter values are specific to the instrument and reporting them along with results is not meaningful. Instruments of the new generation equipped with a numerical UV control require the
whole scale to be measured just once; operating software find conditions under which the requirement (dW/dS)= 4000 is fulfilled.
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Calibration of fluorescence control tile: it is advisable right after the instrument has been calibrated with a scale calibration set (specially after multiple-point procedure) to measure a fluorescence checking tile (it is part of the equipment delivered with most modern instruments).
The measured value is written down and belongs for the time after to that particular temporal standard and can be used to check the level of UV.
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Daily calibration of the instrument: the instrument should be switched on at least 30 minutes before use to allow it to reach a thermal equilibrium before calibration. Since the flash lamp is fired only when used, it is advisable to let the instrument switched on all the time, when often used; if used in heavy industrial environment (for example at paper mills) it should be connected to a current filter and voltage regulator to compensate for line variations. Instruments must be calibrated daily.
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Check of the UV level: when regularly used a check of the UV level should be conducted at least once a week. This assures a constant and proper level of UV with the sample illumination. Just measure the fluorescence checking tile and compare the value with the one written down right after full calibration was done (see above). If numbers agree within two Ganz whiteness points (one CIE whiteness point), the UV level is appropriate for reliable measurements. With time the amount of UV decreases until to the point that a redetermination of the position of the filter must be carried out. For this just reposition the filter, recalibrate the instrument and measure the fluorescence checking tile again, until the numbers agree. A full calibration is not necessary, unless the position of the filter has reached the upper limit; in this case the lamp must be replace and a full calibration carried out.
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