Instruments for measuring whiteness

Measurements of fluorescent samples are tied to the spectral distribution of the actual light source of the instrument. Contrary to color measurements, where the reflectance spectrum is independent on the light source of the instrument, reflectance spectra of fluorescent samples are in general unique and can not be converted to other illuminants by software manipulation. Here lies the reason that while there is a large number of instruments designed to measure color, they are not suited to handle samples showing fluorescence. In fact the correct way of measuring fluorescent samples is through the use of the two-monochromator technique: samples are illuminated with a monochromatic ray and the light from the sample (containing reflected of the monochromatic ray and the induced fluorescence) is analyzed by a second monochromator. Until recently such instruments were literally hand-made and existing only at National Laboratories; meanwhile two models have made their entrance into the market, the BFC 450 of Labsphere and the CM-3800 of Minolta. While differing in design and geometry, they are both suited for measuring fluorescent samples in general and whiteness in particular.
Traditionally industry has measured whiteness using modified color instruments, due to the fact that fluorescence from FWA is confined to a narrow region of spectrum, reflectance values have been found to be in minor error only. It must be recognized however, that measured color coordinates are not exactly correct, but through application of a standard calibration method they correlate quite good with visual assessment.
An instrument can be used to measure whiteness if it meets two different conditions

i) the light source must contain enough intensity in the UV region such to excite the FWA, and
ii) it must have a device (mechanical or numerical) to regulate the intensity of the UV level such to allow calibration.

Instruments fulfilling both conditions are capable (with proper software) to produce accurate and reliable whiteness data. Models suited for whiteness measurement are listed below:

Conrex: This company commercializes the Model AutoElrepho, a highly interesting instrument. It consists of two different parts: a three-filter colorimeter and a filter-wheel spectrometer. The light source consists of different lamps that allow (according to manufacturer) the simulation of different spectral distributions. Since the UV is adjustable it is quite suited for whiteness measurements. An interesting feature is the low intensity of the light onto the source that allows measurement under intensity levels comparable as encountered in normal room illumination. It has a d/0° geometry and its design complies with ISO norms.

Datacolor International: DCI offers a large number of instruments with different geometries. Under those suited for whiteness measurements are the SpectraFlash 600 plus (in the close tolerance version) and the Elrepho 3000, the former has d/8° and the latter d/0° geometry. The SF-600 is particularly suited for whiteness measurement due to the extended spectral detection range that starts at 360 nm, reflectance values at 360 nm can be used as a basis for quantification of the FWA present in the sample, thus a highly valuable piece of information. The Elrepho 3000 (similar at its predecessor E-2000) can be operated as stand-alone instrument through a small liquid crystal based display; it comes completely factory-calibrated and certified for whiteness measurements. Both instruments are delivered with calibration plates for periodical control of the UV level.

Gretag-McBeth: The legendary MS-2020 finds its proper successor in the Color Eye 7000 series, its excellent optic and rugged construction makes it a suited desktop lab instrument. The somehow fastidious manual adjustment of the calibration filter has been eliminated in the motor-driven version 7000A. The series has d/8° geometry.

Hunter: The model UltraScan XE is well suited for performing whiteness measurements. It has a rugged structure, d/8° geometry and is controlled by external software.

Lenzing Technik: the model Vibrochrom 400 has a flash-lamp without UV for color measurements, and optionally a second version is offered with a second flash-lamp with UV; certainly only the version with the second lamp is suitable for measuring fluorescent whiteness.

Lorentzen & Wettre: L&W is a well-known company of testing equipment for paper and pulp industry, on the optical testing sector this company offers the L&W Elrepho. The instrument is manufactured by Datacolor Intl. exclusively for L&W and it shares most of the optical design and components with the Elrepho model 3000; some changes were introduced into the instrument to achieve a better agreement with results coming from the old Elrepho 2000 and in this respect the L&W Elrepho is different from the Elrepho 3000. The L&W Elrepho is operated exclusively with L&W software running on an independent computer and is suited for whiteness measurements.

Minolta: Two instruments are offered, the CM-3700d and the CM-3600 series. The CM-3700d has d/8° geometry and a pulsed Xenon flash lamp that can be switched between high and low intensities. Whiteness measurements should be conducted in the low intensity position. Optical performance is excellent and in quite agreement with the McBeth instrument. The CM-3600 series offers models with d/8° and d/0° geometries; main characteristic of the series is the numerical UV control, the instrument fires a total of three flashes coming from three different lamps, leading to two sets of spectra corresponding to measurements with and without gloss and with high and low UV intensities. Proper calibration and software allows the calculation of reflectance spectra under conditions with and without gloss. The speed of the measurement and the high amount of information obtained will certainly change the way whiteness measurements are conducted and interpreted in the near future.
A somehow different approach is represented by the modified CM-3600 series operating with the software package Polaris - White Star of Axiphos GmbH. The system achieves a separation of the different contributions to perceived whiteness allowing a total assessment and control of whiteness; furthermore the calculation of metameric indices is possible.

Optronik: This German company offers a neat instrument under the denomination Colorflash; similar to the CM-3600 series of Minolta measurements are performed by flashing two different sources with different UV content.

Technidyne: The Color Touch is particularly suited for routine measurements at plant labs. A computer is integrated within the instrument making it independent of external software; a beautiful feature is the touch screen that allows easy and fast operation of the instrument. The instrument comes full calibrated to CIE whiteness scale from the factory, samples can be purchased on regular basis from the manufacturer for recalibration; this ensures proper and long-term whiteness data. In order to excite the FWA to proper levels a Xenon light flash should be preferred over the version with a QHT light source.

Wenzhou Jiayi Measurement and Control Technology Co.: The model JY9800 resembles the old E-2000 but it is actually a three-filter colorimeter. It has a d/0° geometry and is quite reliable; a manual regulation of the UV content enables it for whiteness measurements. It is a stand-alone instrument, it has an integrated thermal printer and it is money wise quite attractive.

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