4. How to find the best CIE L*a*b* values

Transcription

1 Only five print sheets With this test form you easily print five sheets containing ten variations of cyan, magenta, black and blue. Five variations of yellow and a lot of red and green. Within the variations you find the CIE L*a*b* values that give the lowest Delta E ( E) and Delta H ( H). At the same time you have the values to use in the production. The printing process should not take more than one hour and therefore this test form is excellent for testing new papers or printing inks. The test form is also excellent for education. In step by step you see how it works. A test form to find the best CIE L*a*b* values and densities to use in the production. 1

2 Low Cyan range High The point The point of this test sheet is to print graduations in. Magenta range Within the variations you find the range of CIE L*a*b* values that fit the ISO standard. Yellow range Black range The point is to find the range of densities inside the ISO standard. 2

3 Low E 5 Cyan range Best match E 1.5 E 5 High Two of a kind You can achieve the same E value with two different densities. E does not tell if the color is light or dark compared to the target. It only says that there is a difference! The ISO standard does not use values. The standard only refers to the CIE L*a*b* values, but in practice you can use the for the daily work. You just have to know the values that give the best range in E. To achieve E under 5 you have a wide range for each single printing ink: cyan, magenta, yellow and black. The range is reduced because the ISO standard says that the primary colors should be less than E 2.5 in 2/3 of the sheets printed! Simple illustration of the same E with low and high of cyan. 3

4 Low Cyan range High The secondary colors In the secondary colors it gets even more difficult! The combination of cyan and magenta to create blue (c+m) also reduces the range. Magenta range And you also have the combinations of green (c+y) and red (m+y) to take care of! Blue (c+m) range Luckily the ISO standard is only informative on the secondary colors. Practical printing range for cyan and magenta In practical production you often have to compromise, and remember that the primary colors are the most important according to the ISO standard. In practice the range is getting smaller in cyan and magenta when you also want good results in the secondary blue color. 4

5 Low Cyan range Magenta range High Push and pull Sometimes you cannot get all the secondary colors under E 5 when you go for the best match in the primary colors. If the combination of paper and printing ink does not fit the ISO standard it might be impossible. Yellow range Red (m+y) range Green (c+y) range The best match of CIE L*a*b* values in the primary colors are not neccessarily the values you should go for in the production! Perhaps you should push and pull a bit to get better results in the secondary colors of course still having E and H within the ISO standard in the primary colors. Blue (c+m) range Finally it is up to you to decide whether you want to go for the absolutely best match values in the primary colors or you want to push and pull to help the secondary colors. The sample shows that the practical range in yellow and magenta is very small when you also want to have good values in red. 5

6 One sheet two formats Get the test forms on under downloads. 13. CMYK GUIDE - Delta test form 1020x720.pdf Zone width 32.5 mm 14. CMYK GUIDE - Delta test form 720x520.pdf Zone width 30 mm Zone You can adjust the width of the forms to fit the zones of the printing press. This test form looks a bit complicated, and needs an instruction. Please read the following pages and you will get the idea! The design of the test forms makes sure that each zone uses all four printing inks, with an area of 40% of the sheet. 6

7 1 E Red R E Green G E Blue B E CMY E Cyan C E Magenta M E Yellow Y E Black K Density Density Density Density Illuminant: D50 Observer: 2 Geometry: 45/0 or 0/45 Filter: No These eight patches are the colors you need to measure to find the best values. The primary colors: Cyan C Magenta M Yellow Y Black K The secondary colors: Red R = M+Y Green G = C+Y Blue B = C+M The overprint C+M+Y is for the technical staff. These color charts are the colors that you need. The rest of the sheet on page 6 is only for the printing process to make sure that a certain amount of printing ink is being used. 7

8 Graduation of To find the best CIE L*a*b* values and the range of low and high, you simply print the test form with a graduation of. It is very important that the graduation is created in the printing process, not a simulation in prepress. This image is a simulation. If you concentrate on the black part you see low on the left-hand side of the sheet. 8

9 Very low Low Midpoint High Very high Black graduation of Density: Very low -6% to -10% Low -3% to -5% Midpoint High +3% to +5% Very high +6% to +10% Simulation of the black printing form from low to high. 9

10 Very low Low Midpoint High Very high Cyan graduation of Density: Very low -6% to -10% Low -3% to -5% Midpoint High +3% to +5% Very high +6% to +10% Simulation of the cyan printing form. 10

11 Very high High Midpoint Low Very low Magenta graduation of Density: Very low -6% to -10% Low -3% to -5% Midpoint High +3% to +5% Very high +6% to +10% Simulation of the magenta printing form. Note that the now is low on the right-hand side opposite cyan! 11

12 Same all over the sheet Yellow graduation of in rounds The yellow form requires the same all over the sheet, and has to be printed in five rounds. Density in rounds: 1. Very low -6% to -10% 2. Low -3% to -5% 3. Midpoint 4. High +3% to +5% 5. Very high +6% to +10% Simulation of the yellow printing form. Note that yellow has the same all over the sheet. 12

13 Cyan, magenta and blue The two colors cyan and magenta in opposite graduation give you ten variations of cyan, ten variations of magenta and ten variations of blue. If you see the printing form without black and yellow you have the two colors cyan and magenta in opposite graduation. 13

14 Five rounds of yellow On the form with the two colors cyan and magenta you print the yellow form which also creates red and green. It is the rounds that create the three dimensional color space. You cannot only print one sheet and get all the color variations. You get a lot of variations of red and green and only five of yellow! In fact you get 50 variations of red and green, but in practice you do not have to measure all the red and green variations! If you want more than five variations of yellow, you simply print more rounds with smaller steps in. On the cyan and magenta part (page 13) you print five different densities of yellow. The simulation here shows low in yellow. 14

15 Less numbers of yellow Experience tells that you can often start with the midpoint in yellow and from there print the rounds in higher and higher densities. In practical production you normally do not need low and very low densities in yellow. Often you need higher densities higher than the best match in yellow! (To achieve better results in red and green). For education it is advisable to print the full scale to learn how it works. When you print the test, the best result is given when the printing process is done in a steady flow without stopping the printing press. This requires that the rounds of yellow are built up manually from low to high during the printing process. High in yellow. 15

16 The densities of cyan, magenta, black and blue (c+m) are the same and therefore you can have an average to get fine measuring results! Ten variations of cyan, magenta, black, blue and five in yellow. First you have to measure all the primary color patches to find the best match and the range. Then you measure blue (c+m) on the charts with the best match in cyan and magenta. Note: This test form creates ten variations of blue (c+m) and the combinations of cyan and magenta might not be optimal. If you want more variations, you have to print the test form again. See more on page Cut out the five stripes of color charts and mark them just to have an overview. The of yellow is the same on each round and you can have an average of ten measurings. 16

17 When measuring you quickly find the colors that do not fit, and you will end up with a few only. 50 variations of red and green The total numbers of red and green are 2 x 50 variations, and when you start measuring the charts with the best values in the primary colors, you only have to measure a few. If red and green are not satisfactory (on the charts with the best match in the primary colors) you can find better values on other charts being aware that you are now leaving the best match in the primary colors and that you still have to be within the ISO standard! When you have the combinations that fit the best, you measure the of the primary colors. Now you know what to go for in the production. See more in 5. CMYK GUIDE Best CIE Lab values Case.pdf on under downloads a case study of how it works. 17

18 1. 2. Round 1. Very low in yellow. The five rounds of yellow The densities of cyan, magenta and black are the same on all five sheets only yellow goes from very low to very high in the five rounds. Round 2. Low in yellow. 3. Round 3. Midpoint in yellow. 4. Round 4. High in yellow. 5. Round 5. Very high in yellow. Simple illustration of the five rounds of yellow. 18

19 1. 2. Round 1. Little low in yellow. Second test When you print the test form again, you have very fine midpoints to reduce the number of rounds or to print smaller variations in low and high. Round 2. New midpoint in yellow. 3. Round 3. Little high in yellow. 19

20 Round 1. Little low in yellow. Round 2. New midpoint in yellow. Round 3. Little high in yellow. Second test An alternative is to print cyan and magenta in gradients on half of the sheet. On the left-hand side of the sheet the of cyan is the same. On the right-hand side the cyan goes from midpoint to high. On the right-hand side of the sheet the of magenta is the same. On the left-hand side the magenta goes from midpoint to high. The point is to find the best combination of cyan and magenta to create blue. Cyan midpoint on this part of the sheet Cyan from midpoint to little high on this part of the sheet Magenta from midpoint to little high on this part of the sheet Magenta midpoint on this part of the sheet 20

21 Round 1. Very low in cyan. Round 2. Low in cyan. Round 3. Midpoint in cyan. Change the colors of rounds When you print the test form again you have a fine midpoint and you can change the gradients of the colors and print cyan in five rounds. Then you get 50 variations of blue and green, ten variations of red and only five of cyan! If magenta is printed in five rounds you get 50 variations of red and blue, ten variations of green and five of magenta! With a little experience you can create very delicate variations. Round 4. High in cyan. 5. Round 5. Very high in cyan. If you already know that the range of cyan is narrow, you do not have to print a lot of variations outside the range. 21

22 Low High Cyan sample 1 This sample of cyan shows that with this specific printing ink on this specific paper it is impossible to fulfill the ISO standard. Best match is E 3.45, and it should be less than E 2.5! L* a* b* E When you look at the way the L* a* b* values act from low to high, you find that the a* values change very little. It is more or less -33,50. This tells that this specific cyan printing ink is not green enough! See more on page 4 in the 2. CMYK GUIDE Delta E and Delta H How it works on under downloads. Target is L* 55, a* -37 and b*

23 Low High Cyan sample 2 This sample of cyan shows that this printing ink is excellent for the ISO standard. L* a* b* E Target is L* 55, a* -37 and b*

24 Low High Seven samples of printing inks The samples show the range of E values from low to high printed with seven different printing inks. Ink 1 Ink The test is printed by one printer, on the same paper and printing press. Only the printing inks are different from seven manufacturers. Ink Ink 5 and 7 are excellent for the ISO standard. Ink 2 is also fine. Ink Ink 1, 3, 4, and 6 are not satisfactory for the ISO standard. Ink 5 Ink The samples clearly show that it is very important to use printing inks, that can handle the ISO standard. Ink Numbers in cyan are the. Black numbers are the E values. 24