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  1. Do I have to measure the optical brightener (OBA)?

    Optical brightener will influence the color measurement. The more UV-radiation is in the illumination, the whiter the substrate appears. If the color of the product is measured without UV-radiation, it will never correlate to the visual assessment at daylight illumination. If the sample is measured with an unfiltered xenon flash-lamp, the UV-radiation is much too high, the sample will be measured whiter (=bluer) than it looks in reality.

    When a sample includes OBA in a quantity that can be visualized in a light booth with UV-radiation, it is absolutely necessary to measure the OBA separately. This is the only way to have a reliable color measurement which corresponds to the visual assessment and with the measurements of the customers.

  2. Pulp Measurement in the Laminated Paper Industry

    There is not a simple correlation between the wet pulp and the finished product in the laminated paper industry, but if there is a change in color from one batch to the next there will be a color change in the finished product.

    The task of the instrument in this application is not to compare the color of the pulp with the color of the laminated paper, but to see if a correction goes in the right direction. If there is a color difference after a small strip of paper is produced on the machine and laminated on a substrate, a correction has to be made. The pulp measurement helps to see if the correction goes in the right direction. For example, if a brown is produced, but the chroma should be a bit stronger, a yellow and red have to be added, but not necessarily the same amount of each. The pulp measurement shows immediately, if the correction goes in the right direction, and it gives you an idea of the extent of the correction even before the paper has been produced and laminated . This helps to save time and machine costs.

    Another subject is the addition of waste. If a production is running, sometimes waste is added. The pulp measurement shows exactly how much waste can be added without changing the color significantly.

    Last but not least, if a change in the process is made like adding new colorants, the pulp will be pumped from one chest to another one to mix it. The pulp measurement shows exactly how long it takes until the pulp becomes homogeneous.

  3. How to clean white calibration standards

    The absolute repeatability of a spectrophotometer depends also on the quality of the calibration. During the calibration the white standard shows the instrument a defined color, in our case a white with a reflectance of about 98%.

    The reflectance of a dirty white standard will go down to example 90%. When this dirty white standard is used, the instrument will “learn” that this 90% reflectance will be by definition 98%. From now on it will measure all samples lighter than they are.Therefore it is necessary to frequently calibrate a spectrophotometer with a clean standard.

    We recommend cleaning the standard with a super-microfiber cloth. If the standard is really dirty, alcohol or soap can be used to get a basic cleaning. The finish should be made by breathing against the glass and wiping the mist off. When the surface of the internal white standard is shiny (image of a light source is viewed sharp) the standard is clean.

    We can also recommend a fluid called “Optical Wonder Reinigungsflüssigkeit” and “Optical Wonder Cloth” which can be ordered at www.baader-planetarium.de