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FLEXO Magazine : February 2008
All of the colors of the visual spectrum are found in white light; ranging from the violet to the blue to the green to the yellow to the red. The wavelengths, which include the visual spectrum, are from about 400nm at the violet end to 700nm at the red. DENSITOMETRY Densitometers measure the amount of light absorption in a very specific region of the visible spectrum. This informa- tion correlates to the quantity of pigment applied to the sheet. As such they are very effective QC (quality control) devices for monitoring the application of process col- or inks (cyan, magenta, yellow, and black). This information can be used to monitor and adjust the extension rates of our ink press-side. The filters used in densitometers are designed specifically for measuring pro- cess colors. Unexpected results can occur when using a densitometer to measure non-process inks such as PMS 185 red, or a reflex blue. Densitometers come in several forms: hand-held units for taking manual mea- surements, scanning instruments for measuring sheets off press, and the tech- nology exists today for on press in-line density measuring systems. In very simple terms, when the density value of a color increases, the amount of colorant on the sheet has also increased. When the value increases beyond our tol- erance, we need to make a change. In the flexo world, this would typically be done by increasing the amount of extender in the ink. It might also be that we are not using the proper anilox roll. SPECTROPHOTOMETRY Like a densitometer, a spectrophotom- eter contains a light source, an aperture for positioning the instrument on the sample to be read, filters, and a receiver. However, the spectrophotometer contains many filters for sensing the reflection from many different portions of the vis- ible spectrum. The result is many more pieces of data. Today's instruments tend to have 16 or 32 data points. From the violets, through the blues and greens, to the yellows, oranges, and reds, varying the wavelength from 400nm to 700nm changes the color of light. A spectrophotometer measures the re- flected light at many different points to capture the fingerprint, or signature, of the sample. This information is presented as a reflectance curve. A densitometer cannot do so because there is not enough raw data to work with. INSTRUMENT GEOMETRIES A spectrophotometer provides reflectance data that can fin- gerprint a color's unique spectral response. If all surfaces were flat, matte, and smooth, a single type of instrument would likely suffice. However, in practice, the flexo printer often prints on metallic surfaces. TECHNOLOGIES & TECHNIQUES