Guhan S K - Colour Space

Maths and me @ Colour Space

As a Printer what can I do by learning Color space?

Going into the final stage of printing a product, we find that the most critical factors that create COLORS in printing are in the form of ink pigments we have and supplying / controlling ink. This can be done effectively by learning to spot our printed color in visible color space. If we need to improve in quality, standard (ISO) output, testing etc. we need to know about the fundamentals of; color space - > visible spectrum -> input color gamut -> PCS -> working color space -> output gamut etc - all are path key words that you can explore in internet.

It is also good practice to study about our CMYK output profile, reference values and ISO standard - which is a must for printers who care about quality output. This helps us to compare printed copies, to test ISO standard, create reports and to maintain good quality in production. We can practice this with the help of Excel worksheet too.

Excel worksheet !! math’s and formulas helped me to;

Prepare power full worksheet to handle color that we print using formulas, and thus helped in plotting our own gamut, visualizing tonal value increase (TVI) and grey balance.
Color conversion CIELAB and CIEXYZ values, which is key factor to spot our printed color in visible colour space.
Plot TVI curve based on L*a*b* values - explored the power of formulas and programming.
Spot my colors gives vision to create / compare profile
It help us to choose paper (substrate), ink, printing unit and find position to place our testing element by practice.
Even color control strip element helped to check ISO quality and generate report.
Input L*a*b* value can be virtually altered in the worksheet to study the range that suits our production.

Note : In this article I use tag " # " symbol to point to you where and what condition CIELAB value colour gets changed.

Colour Space:

Here is the visible spectrum (Wave length range from 400 -700 nano meter). Different lighting conditions like natural Sun, D series of illuminants like D50, D65 daylights and observer angles 2° or 10°, each give # different CIELAB value for the same printed image. So for comparison we must all use same illuminant and angle of observer settings in the measuring device. As a printer I realize the importance of my reference white plate supplied with densitometer - to keep clean and safe, that too # change my CIELAB value from others.

Color models are mentioned with different shapes (in pic.) like Prophoto RGB, sRGB, CMYK, Adobe RGB etc. Here • Input digital camera and scanner commonly use sRGB, • Photoshop normally uses working color space SWOP CMYK for conversion from RGB to CMYK and finally, our • output profile (Eg. IFRA26v5.icc) is ISO standard which converts all input colours to possible matching combination colours, # that Web offset machine for coldset ink can reproduce for good print. Profile is applied at the last stage for the images, before entering to raster image processor for conversion to TIFF file, then exposed on plate which is sent to our printing machine.

What is color Gamut?

The most common usage of gamut refers to the subset of colors which can be accurately represented in each circumstance, such as within a given color space or by a certain output device. It also refers to the complete set of colors found within an image at a given time. In this context, digitizing a photograph, converting a digitized image to a different color space, or outputting it to a given medium using a certain output device generally alters its gamut, in the sense that some of the colors in the original are lost in the process. Gamut also helps to compare our output with reference so that it guides us to improve and get best quality. In short Gamut are boundaries of possible color that a device can produce. 100% of every additive and subtractive (exclude K) colors are the vertex coordination points for each profile gamuts shown below.

Here I plotted actual profile values for comparison between gamut’s - and displayed here to know the difference between them;

SWOP CMYK -> Specification for Web Offset Publications -> which publish magazines, catalogues and books -> high quality big gamut -> used mainly in USA
FOGRA42 -> High quality Improved Newsprint commercial printing West Asia -> smaller than SWOP
IFRA26v5 -> Standard Newsprint commercial printing -> Asia and Europ - medium quality -> smaller than FOGRA42

We understand that there are limitations for smaller gamuts to reproduce colors to match higher gamut counterparts. This implies that the raw materials (paper and ink) that we use and production type of our press (Heatset/coldset) will determine the correct profile we need to use for conversion.

My off-set machine can produce this size of gamut, but monitor gives a bigger and different gamut. By using profiling software, we can downgrade or reduce the monitor gamut closer to machine gamut. This reduced gamut, when displayed on monitors, can be used as soft-proof. As another example, if my television or mobile phone displays less colours compared to others, it is because the gamut of my particular device is smaller than others.

What is the difference between FOGRA42 and IFRA26v5 profile ?

FOGRA42

Printing condition : ISO 12647-2:2004 [Dt- 17/07/2009]
Web offset printing with INP (Improved Newsprint), for Heatset inks
GSM >55-70 g/m²
Screen ruling - 60 l/cm
TIC (total ink coverage) - 260%
TVI at 40%: CMY = 19%, K = 22%

TVI curve plotted using L*a*b* values from FOGRA42 characterization data set.

White point reference value XYZ - 0.58, 0.61, 0.47 and sported below in colour space .

IFRA26v5

Printing condition : ISO 12647-3:2004 [Dt- 04/05/2007]
Web offset printing with SNP (Standard Newsprint), for Coldset inks.
GSM > 45-55 g/m²
Screen ruling 40 l/cm
TIC - 240%
TVI at 40%: CMYK = 26%

TVI curve plotted using L*a*b* values from IFRA26v5 characterization data set.

White point reference value XYZ - 0.64, 0.66, 0.50 and sported below in colour space.

What is White point ?

The spectrum of a standard illuminant, like any other profile of light, can be converted into tristimulus values. The set of three tristimulus coordinates of an illuminant is called a white point. If the profile is normalized, then the white point can equivalently be expressed as a pair of chromaticity coordinates and it can be point out in visible chromaticity graph as shown in picture.

For example ; Standard illuminant and its CIEXYZ chromaticity coordinates (x,y) with temperature(in Kelvin).

D50 - x 0.34567 - y 0.35850 ; 5003 K ; Horizon Light : used in ICC profile PCS.

D55 - x 0.33242 - y 0.34743 ; 5503 K ; Mid-morning / Mid-afternoon Daylight.

D65 - x 0.31271 - y 0.32902 ; 6504 K ; Noon Daylight: used in Television, sRGB color space.

D75 - x 0.29902 - y 0.31485 ; 7504 K ; North sky Daylight.

Every profile has its own white point reference value measured in standard light at a particular angle of observer. For our output printed copy it is also important How bright my substrate (paper) is? So at two conditions, for reference value and measured printed copy (paper) white point value is important base # that decide the change in CIELAB values.

For Paper white index of Hunter says value >100 is considered blueish (>5000 K - cool colour) and <100 is Yellowish (<5000 K - warm colour) - in short, we use BLUE while washing our clothes to improve whiteness (towards cool). So white point lies in CIELAB - b* (blue - yellow) abscissa X axis and luminous in ordinate Y axis, and what point should be my substrate (paper) used for printing is also deciding factor that depends upon the reference profile value we use.

Here FOGRA42 uses improved Newsprint and heatset ink, but IFRA26v5 uses standard Newsprint and coldset ink - this determines the difference between the gamut color they are capable of reproducing.

Size and Measurments of gamut profile:

White point & Distance

On white point reference we can measure the distance between Profile and our own paper value. Here the distance between Daylight D50/2° and IFRA26v5 is 13.4 x10^-3 Metric unit, D50/2° to FOGRA42 is 8.35 x10^-3 Metric unit and example our white point is 38.23 or 15.6 x10^-3 Metric unit distance from D50/2°. So it is possible to locate our paper color in the visible spectrum and compare.

As we said above white point reference is also important factor that decides the gamut size # (area). So the shade of paper we used on the day of testing gives results in particular manner.

Gamut & Area

Apart from plotting gamut we can also measure the size of 3D gamut as in volume and area in case of 2D as mentioned here. IFRA26v5 43.6 x10^-3 sq.Metric units and example our gamut 38.78 x10^-3 sq.Metric units. We try to compare these two gamuts as follows;

If IFRA26v5 gamut (RED in above diagram), and Our machine gamut (BLUE in the above diagram) fit one over the other perfectly, that is Area 43.6 x10^-3 each, in this case we say - My machine is capable of printing all possible colours suggest by ISO standard.
If our gamut is bigger than the ISO reference then, our machine can reproduce more colours than the ISO standard.
When our gamut is small as in picture, surely we will not able to match the ISO standard. Here magenta & red OK but more attention on other colours is needed.

What is PCS (profile connection space)?

Profiles describe the color attributes of a device or viewing requirement by defining a mapping between the device source or target color space and a profile connection space (PCS). This PCS is either CIELAB (L*a*b*) or CIEXYZ. Mappings may be specified using tables (LUT), to which interpolation is applied, or through a series of parameters for transformations. It works as a translator between devices, help them to communicate or map suitable value from defined color space.

CIEXYZ

CIE 1931 - In XYZ color space defines in tristimulus values, in which they are denoted by "X", "Y", and "Z", all combinations of non-negative coordinates are meaningful, but many, such as the primary locations [1, 0, 0], [0, 1, 0], and [0, 0, 1]. Y as luminance Y = 1 or Y = 100 is the brightest white that a color display supports . Z is quasi-equal to blue, or the S cone response, and X is a mix of response curves chosen to be nonnegative. They are used as PCS for colour conversion.

CIELab

CIE 1976 - It expresses colour as three values: L* for the lightness from black (0) to white (100), a* from green (−) to red (+), and b* from blue (−) to yellow (+) . CIELAB was designed so that the same amount of numerical change in these values corresponds to roughly the same amount of visually perceived change. The CIELAB color space is typically used when graphics for print have to be converted from RGB to CMYK, as the CIELAB gamut includes both the gamuts of the RGB and CMYK color models.

We commonly use CIELAB to compare and check our printed copy with reference value.

How colours are measured?

For example we measure Red Solid dot ;

RGB = 255, 0, 0 - Computer soft-proof value - CIEL*a*b* - 53.23, 80.11, 67.22
CMYK = 0, 100, 100 ,0 - Working colour space value - CIEL*a*b* - 51.52, 93.91, 53.43
Apply CMYK output profile IFRA26v5 CMYK (0,100,100,0) - CIEL*a*b* - 53.82, 44.81, 26.04

Output on printed copy ;

SID of CMYK (0,100,100,0) = 0, 0.86, 0.93, 0 (D)
Measuring RED dot example CMYK values (D) - 0.23, 0.75, 0.85, 0.18 [Status E D50/2° polarise]
Measured RED (CMYK - 0,100,100,0) CIEL*a*b* - 50.26, 41.90, 23.53 [Status E D50/2° No polarise].

After applying our output profile, in photoshop input RGB or CMYK values remain same, but # CIELAB values get changed. So at any point we do not know the meaning of CMYK (0, 100, 100, 0) or RGB (255, 0, 0), every profiles has its own matching point for RED colour.

Find here value of a* and b* in Profile IFRA and output CIELAB value get too closer than original image at RGB format or working color space. Thus our output is depend upon printing method, raw materials used for specific outcome, so obviously our color gamut also get change in size that reproduced accordingly.

When my ISO standard suggest mid-tone (40-50%) as 26% TVI densitometric reference then our SID for CMYK match like 0.90, 0.90, 0.90, 1.10 (D), If this TVI reduced to 18% then our SID # CMYK for example be - 0.85, 0.83, 0.85, 1.05 (D)

Here, if Solid Ink Density increases or deceases, then # CIELAB gets changed and placed in particular point on color space respectively. TVI variation, due to SID change also affect clarity in image quality. At a particular solid density we will be able to match CIELAB reference to produce RED color. This is considered our printing SID value, and also ensures correct TVI reproduction to print APPLE to match with original.

This happen in accordance with our raw materials used and machine performance. If we lowering GSM of paper, we need to reduce the ink TIC (total ink coverage) value, so alternatively our # CIELAB gamut reduces further and we then try our best in alternate method to match original red APPLE in print.

And finally it is clear that we can measure / compare the distance and angles of our colour (saturation / hue) and area / volume of our gamut colour that we produce or print (like were we are? and what size we produced?) with respect to ISO reference in COLOUR SPACE.

Conclusion:

People who can explore and spot their printed color gamut in the visible colour space, can compare, modify and create their own profiles and document the results. By studying our printing machine's capability to print a particular Hue gamut and reproduce this tonal variations (TVI) saturation - we can create a profile of our own. This will happen by - Understanding / Learning / Mastering color in simple and easy way by putting our focus / attention towards target. Once mastered, every printer is considered as COLOR expert.

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