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FLEXO Magazine : May 2009
TECHNOLOGIES & TECHNIQUES Density 1.15 Light Source Filter Optics Sample FIGURE 1. Schematic of a refl ection densitometer. Receiver 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 FIGURE 2. Density versus L value. L Value 95 85 75 65 55 45 35 25 15 Densitometry vs. Colorimetry By Mark R. Mazur W hat is the diff erence between a densitometer and a colorimeter? The common answer to this question is that a densitometer measures color in one dimension; while a colorimeter measures color in three dimensions. Consequently, a colorimeter gives a more accurate numerical representation of color. That answer is technically incorrect. A refl ection densitometer is a device that quantifi es the amount of red, green and blue light refl ected from an object. Similarly, a colorimeter is a device that quantifi es the amount of red, green and blue light refl ected from an object. The fact that we normally do not report the density of all three fi lters when we measure density does not make the densitometer incapable of measuring color in three dimensions. So superfi - cially, these two devices perform the same function, to understand the true diff erence we need to explain the details of how they work. Figure 1 is a simple schematic representation of how these devices operate. The light source illuminates the object and a 40 FLEXO MAY 2009 detector determines the amount of light refl ected from the object. Before the detector is a fi lter, which allows the instrument to be more specifi c as to what portion of the electromagnetic spectrum is measured and consequently to be color specifi c. For instance, cyan ink absorbs red light. By placing a red fi lter before the detector only red light is measured. The more cyan ink, the less red light is refl ected. This description has used very non-specifi c terms. In particu- lar, the terms fi lter and the quantifi ers more or less. It is in the specifi cs of the fi lters and the mathematics of the quantifi ers that the diff erence between densitometers and colorimeters becomes apparent. FILTERS Electromagnetic radiation is energy created by electric and magnetic fi elds, which oscillate in phase perpendicular to each other and to the direction of energy propagation. The electromagnetic spectrum is the range of all possible electromagnetic radiation frequencies. Electromagnetic radiation is classifi ed www. f le xography. org BEGINNER FLEXOGRAPHER 45°