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FLEXO Magazine : February 2008
40 FLEXO FEBRUARY 2008 www.flexography.org Single Angle (0º/45º) Instruments. 0º/45º (or 45º/0º) instru- ments are designed to exclude gloss from the measurement. These instruments and their data are referred to as "specular excluded." This family of devices sees color almost exactly as human observers do. To understand this, imagine looking at photographs in a magazine printed on very glossy paper. Depending on the lighting in the room, you may need to move the magazine back and forth or tilt it away to get a clear view of the photograph. What are you actually doing? In order to see the photograph, you need to remove gloss from your view. It turns out that the angle where gloss is minimized and color is most enriched is at an angle of 45º. 0º/45º instruments perform well with most surfaces and most dyes and pigments, but there are situations where this instrument fails to really capture the nature of the sample. One such case is when the sample has a mirrored, nearly perfect, polished surface, such as foil or metallized paper. In this case the light is reflected away from the instrument optics. It turns out that other geometric designs are required for metallic substrates, and will perform better for samples that are printed with metallic, pearlescent, or special effects pigments. Spherical Instruments. Figure 4 (page 41) depicts how the spherical instrument operates. A strong beam of light is flashed into the sphere, which is a near perfect, white reflector with a very low gloss, matte surface. As the light beam strikes the surface of the sphere, more than 99 percent is reflected. At the same time, the matte finish of the sphere causes the light to be scattered randomly in all directions. This process repeats itself dozens, hundreds, or thousands of times in a fraction of a sec- ond. The sphere itself becomes the light source. The result is that the surface of the sample is uniformly light- This is an ideal circumstance ective. In general, spheres are ded measurement devices. The th the "color," although the word is now a bit ambiguous. Take a closer look at the sphere. There are two ports near the top of the illustration, the "sample viewing port" and the "specular port." The receiver is placed at the viewing port, as well as the light sensitive detectors that quantify wavelength by wavelength the amount of light being reflected by the surface of the physical sample. Now for the "specular" port. Remember, the 0º/45º instru- ment excluded the gloss or specular component, allowing only "color" to be measured. Humans also choose to exclude the specular when viewing most objects. Most spherical instrument designs can attempt this as well. If the specular port is opened, light from that portion of the sphere wall is not reflected onto the surface of the sample. The open port directs light from this area into a black trap. It is lost from the sphere and lost from the measurement. When the port is closed, a white tile or plate replaces the black trap. This section of the sphere is then included in the process of lighting the sample. If the sample is glossy in nature, then this portion of the sphere sends the gloss signal directly into the viewing port. The angle of incidence equals the angle of reflection. A sphere can either include gloss in a measurement of "col- or," or it can partially exclude gloss. It is not as good as a 0º/45º because there are only 16º between the incident and reflected light paths. Nonetheless, in most cases, it is enough to provide much more detail about the sample and makes it possible to say something about the color and the appearance. A spheri- cal measurement in graphic arts would almost always be made specular excluded. COLOR DATA We can extract many types of data from either a single angle, or spherical spectrophotometer. The flexo ink formulator is go- ing to be interested in the complete set of reflectance data---all 32 points when available. This is so that he/she can formulate an ink with the same characteristics as the original we are matching. The flexo pressman, however, will find it more useful to de- fine color data using values such as L*a*b*, or L*c*h° which are used to define the standard, and compare samples to that stan- dard in a three-dimensional color space. While working for the National Bureau of Standards in the 1940s, Richard Hunter created a new color model he named "Lab," pronounced "L," "a," "b." Hunter scales his color space in an effort to achieve near uniform spacing of perceived color differences. The L axis represents lightness/darkness, with abso- lute white at 100 and absolute black at 0. This L axis represents FIGURE 2. A densitometer measuring head mounted on a flexo press. TECHNOLOGIES & TECHNIQUES