Why Red Isn’t Always Red
The human eye can differentiate among several million color
nuances. Prepress specialists and printers are now being confronted
with the task of reproducing some of these millions of nuances as
best they can. Impossible or a question of color management?
A print product's quality is always measured by how well the customer's desired color impression is actually achieved. In most cases, the customer has only seen a version on the screen or at most received a proof. Specialists in prepress and printing have the task of perfectly transforming these specifications into reality. But the devil is in the details when it comes to colors. There are too many factors influencing the print result to be able to achieve it correctly with just simple instructions. "A little bit more friendly, please" or "a colder blue" do not lead to set tone values in printing. In the issue "Color and Quality" of the Expert Guide series from Heidelberg, the basics of how to be better prepared for this challenge are explained.
A print product's quality is always measured by how well the customer's desired color impression is actually achieved. In most cases, the customer has only seen a version on the screen or at most received a proof. Specialists in prepress and printing have the task of perfectly transforming these specifications into reality. But the devil is in the details when it comes to colors. There are too many factors influencing the print result to be able to achieve it correctly with just simple instructions. "A little bit more friendly, please" or "a colder blue" do not lead to set tone values in printing. In the issue "Color and Quality" of the Expert Guide series from Heidelberg, the basics of how to be better prepared for this challenge are explained.
Why is a red book not always red?
Colors aren't characteristics of an object like the specific weight or its shape. Color originates in our brain. The object has the characteristic of absorbing or reflecting light with certain wavelengths. If light rays hit the red book, parts of the white light are absorbed and the rest is reflected. The wavelength of these colors determines which color tone we see. Of course, the wavelength of the illuminating light plays an essential role as well. The red book thus looks different under the office lamp than under a blue sky.
Colors aren't characteristics of an object like the specific weight or its shape. Color originates in our brain. The object has the characteristic of absorbing or reflecting light with certain wavelengths. If light rays hit the red book, parts of the white light are absorbed and the rest is reflected. The wavelength of these colors determines which color tone we see. Of course, the wavelength of the illuminating light plays an essential role as well. The red book thus looks different under the office lamp than under a blue sky.
Why do we perceive color differently?
When the customer and printer look at a print product, they sometimes perceive different nuances in color. This is because the color we see depends on the stimulus level of our eye's receptors; specifically on the three types of cones in the retina which each react to different wavelength ranges in light: to red, green, and blue light. According to the individual color perception, some people perceive bordeaux red, for example, as violet.
When the customer and printer look at a print product, they sometimes perceive different nuances in color. This is because the color we see depends on the stimulus level of our eye's receptors; specifically on the three types of cones in the retina which each react to different wavelength ranges in light: to red, green, and blue light. According to the individual color perception, some people perceive bordeaux red, for example, as violet.
Details
How can you measure colors?
Since colors aren't a physical characteristic of an object, but rather, dependent on the individual sight perception, the "Commission Internationale d'Eclairage" (International Commission on Illumination) developed a standardized color space, CIELab. The color space is based on tests with people who perceive color normally, in order to create a color description system which reflects the sensitivity of the three color receptors in the human eye. Accordingly, the CIELab color space is made up of three coordinates in line with the color cones in the human eye: red, green, and blue, supplemented with a third dimension, the lightness axis. With the help of these coordinates, a color's location, and thus all color nuances which can be created with an additive mix of spectral colors, can be defined. The prepress or print specialist then knows where his or her print job is actually located in color space (actual value), and how far off he or she is from the desired value.
Why doesn't a densitometer always vouch for the quality of colors?
A densitometer determines the density from the difference in lightness between absorbed and reflected light. It thus differentiates only between light and dark and is only applicable for determining the density of black. In order for the technique to function for the standardized process colors cyan, magenta and yellow as well, densitometers have color filters for the complementary colors. Red filters for cyan measurements, green filters for magenta and blue filters for yellow. Since the densities behave essentially the same as thickness of ink layers, the printer can use these values to initiate a change in the color. However, the same density value doesn't mean that the color impression is the same for the viewer. Because of the diversity of special colors, there are no defined filters for them. In this case, you make do by choosing the color filter that is the furthest away from the test sample - for example, a blue filter for measuring light green or orange. In so doing, it sometimes happens that identical density values are given for both these colors.
Why does the spectrophotometer ensure a higher color quality?
A spectrophotometer separates the reflected light from the test sample into spectrums. This is then used to calculate the colorimetric values L*a*b* on the one hand, and the corresponding density and tonal values on the other. Spectrophotometric color measurement systems on printing presses also fulfill a further purpose. Their color calculator uses the difference between the desired value (for example in the proof or a color sample) and the actual value on the sheet to determine the necessary changes needed in the ink zones and forwards this on to the printing press online. The number of pulls or reprints needed is entirely dependent on the difference between the desired and actual values. The goal of the color management processes is to achieve a first pull which is as close to the desired value as possible. Then only a single standard step is often needed to reach the OK sheet. For this purpose, a sample is printed according to standards and spectrally measured. The determined values are used in creating an ICC color profile. This profile ensures that when retouching on screen, the color is correct and the proof shows the printing result reliably.
The whole purpose of color measurement is to know in advance what will be printed afterwards. This prevents time-consuming lithography at the printing press, lowers costs, reduces waste, and the red book is just as red as the customer wants.
Since colors aren't a physical characteristic of an object, but rather, dependent on the individual sight perception, the "Commission Internationale d'Eclairage" (International Commission on Illumination) developed a standardized color space, CIELab. The color space is based on tests with people who perceive color normally, in order to create a color description system which reflects the sensitivity of the three color receptors in the human eye. Accordingly, the CIELab color space is made up of three coordinates in line with the color cones in the human eye: red, green, and blue, supplemented with a third dimension, the lightness axis. With the help of these coordinates, a color's location, and thus all color nuances which can be created with an additive mix of spectral colors, can be defined. The prepress or print specialist then knows where his or her print job is actually located in color space (actual value), and how far off he or she is from the desired value.
Why doesn't a densitometer always vouch for the quality of colors?
A densitometer determines the density from the difference in lightness between absorbed and reflected light. It thus differentiates only between light and dark and is only applicable for determining the density of black. In order for the technique to function for the standardized process colors cyan, magenta and yellow as well, densitometers have color filters for the complementary colors. Red filters for cyan measurements, green filters for magenta and blue filters for yellow. Since the densities behave essentially the same as thickness of ink layers, the printer can use these values to initiate a change in the color. However, the same density value doesn't mean that the color impression is the same for the viewer. Because of the diversity of special colors, there are no defined filters for them. In this case, you make do by choosing the color filter that is the furthest away from the test sample - for example, a blue filter for measuring light green or orange. In so doing, it sometimes happens that identical density values are given for both these colors.
Why does the spectrophotometer ensure a higher color quality?
A spectrophotometer separates the reflected light from the test sample into spectrums. This is then used to calculate the colorimetric values L*a*b* on the one hand, and the corresponding density and tonal values on the other. Spectrophotometric color measurement systems on printing presses also fulfill a further purpose. Their color calculator uses the difference between the desired value (for example in the proof or a color sample) and the actual value on the sheet to determine the necessary changes needed in the ink zones and forwards this on to the printing press online. The number of pulls or reprints needed is entirely dependent on the difference between the desired and actual values. The goal of the color management processes is to achieve a first pull which is as close to the desired value as possible. Then only a single standard step is often needed to reach the OK sheet. For this purpose, a sample is printed according to standards and spectrally measured. The determined values are used in creating an ICC color profile. This profile ensures that when retouching on screen, the color is correct and the proof shows the printing result reliably.
The whole purpose of color measurement is to know in advance what will be printed afterwards. This prevents time-consuming lithography at the printing press, lowers costs, reduces waste, and the red book is just as red as the customer wants.
Download "Color & Quality"
Should to wish to get more information about the topics color and color management, please click here in order to download the issue "Color & Quality" from Heidelberg's "Expert Guide" series.
