by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
FLEXO Magazine : May 2009
TECHNOLOGIES & TECHNIQUES into various types according to the wavelength; these types include: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. Of these, radio waves have the longest wavelengths and gamma rays have the shortest. Do not feel bad if you did not fully understand those sentenc- es—I am not sure anybody does. What is important is that a small window of the many electromagnetic radiation frequencies is sensed by the eye and we call that portion the visible spectrum, or light. A color fi lter absorbs certain frequencies of light and allows other frequencies of light to transmit through the fi lter. To be very specifi c, when we describe a fi lter we must describe the spectral absorption curve of that fi lter. As you will see, just saying a red fi lter is not enough. Figure 3 shows spectral absorption curves for the red, green and blue fi lters of a densitometer (Status T) and a colorimeter. While the diff erences are apparent, this in and of itself is not useful. It is in the origin of these spectral curves that we start to get a true understanding. The spectral absorption curves for the densitometer are simply measurements through some arbitrary fi lter set (and there are other just as arbitrary fi lter sets; Status E, Status G, etc.). The spectral absorption curves for the colorimeter represent the red, green and blue responses of the human perceptual system. These curves were derived from a and blue fi lters, those red green and blue fi lters are somewhat arbitrarily determined. This is a diff erence, but it is not the major diff erence. The major diff erence comes down to how each device quantifi es the amount of red, green and blue light measured by the detector. We will start with the densitometer. The fundamental equation for densitometry is that density equals the log of one over the refl ectance (D = log 1/R). For example, if 10 percent of the light is refl ected the density is 1 (10 percent is 0.1, one over 0.1 is 10, and the log of 10 is 1). If 1 percent of the light is refl ected the density is 2 (1 percent is 0.01, one over 0.01 is 100, and the log of 100 is 2). If 0.1 percent of the light is refl ected the density is 3, I think you get the picture. This equation has a fascinating history and begins with Ernst Heinrich Weber (1795-1878) at Leipzig University. Weber was a physician and was the fi rst person to study a human’s response to a physical stimulus in a quantitative fashion. In his most famous experiment, Weber gradually increased the weight that a blindfolded man was holding and asked him to respond when he fi rst felt the increase. Weber found that the smallest noticeable diff erence in weight was proportional to the starting value of the weight. In other words, if the starting weight was ten pounds, an increase of an ounce would not be noticed. But if the weight were an ounce then an increase of an ounce would easily be noticed. This intuitively makes sense and can One could say that a colorimeter is a better device for measuring color, but even that is an over simplifi cation. With today’s instruments, a densitometer is just a colorimeter with some of the software functions disabled. series of experiments done in the late 1920s by W. David Wright and John Guild1,2,3 . The experiment was brilliant in its simplicity. An observer was presented with a viewing booth with a partition in the middle. Half the booth was illuminated with a light of known spectral properties and the other half was illuminated by the combination of red, green and blue lights. The observer was asked to adjust the intensities of the red, green and blue lights until the color matched the spectral light. The intensity levels assigned to each of the primaries for each of the spectral colors became the color matching functions for that observer for those primaries. The results from many observers were averaged and this resulted in the spectral absorption curves for the standard observer. WARNING: MATH AHEAD! We have just seen the fi rst diff erence between a densitometer and a colorimeter. The fi lters in a colorimeter are based on a more precise model of how humans see color. While the densitometer is based on how we see color; it does use red, green be mathematically stated as a perceptual change (∆P) is proportional to the change in stimulus (∆S) divided by the initial stimulus (S). ∆P = k * ∆S/S Applying the rules of integral calculus we learn that the relationship between stimulus and perception is logarithmic. Weber’s observation remained virtually unrecognized for almost 40 years until he collaborated with Gustav Theodor Fechner (1801-1887). In 1834, Fechner was appointed professor of physics at Leipzig University. Unfortunately, in 1839 he contracted an eye disorder while studying color and vision, after much suff ering he resigned his position at the university. He subsequently recovered and turned his attention to studying the mind and its relations with the body. In 1860, he published Elemente der Psychophysik which inspired much of the science and philosophy in the 20th century. It was the popularity of that book that made Weber’s observations more widely recognized. Today, the law is known as the Weber-Fechner law. With respect www. f le xography. org MAY 2009 FLEXO 41