Some Definitions used in Color Characterisation

CIE L*a*b* (CIELAB)
A 3-D color space mathematically derived from CIE xyz chromaticity coordinates resulting in greater perceptual uniformity.
L* = neutral light-dark axis, a* = red/green axis and b* = blue/yellow axis.

dE Color Difference
CIELab  is perceptually uniform color space and the beauty is that it enable us to compute mathematically the distance between 2 colors that represents how close they are in  in the color space. In general, and subjectively speaking,
1 dE: threshold of what is just perceptible to the human eye.
3 dE: detectable by color professionals or an trained eye.
6 dE: often considered acceptable for printed material.

CMYK

Theoretically, black can be produced by mixing magenta, cyan, and yellow which correspond to the three subtractive primaries. To achieve higher quality, CMYK additionally uses black ink to create all colors.
Cyan (also called process blue), Magenta (process red), Yellow (process yellow), Black


lpi
Lines per inch is the number of lines or dots per linear inch in a halftone screen. A 300-600 DPI laser printer can usually only print at an LPI of 50-65, newspapers typically use 85 LPI whereas image-setters and plate-setters print at much higher resolutions and can print up to 200 LPI.

Neutral Gray

A gray which when measured with a calibrated spectrophotometer has zero values for a* and b*, whether or not it looks “neutral” to the eye. In RGB mode, equal amounts of RGB will yield a neutral gray. CMYK has no real solution for an 18% gray other than setting a solid black to 18%. If you plan to print the project using only gray, then the printer color matching becomes critical.  If the grays were balanced properly, the print was naturally accurate because the reproduction process was not adding or taking away color from the original. Otherwise color cast would be introduced to the reproduction of the original.

NPDC

NPDC is a target curve defined along the entire tonal range. G7 uses gray balance as the primary control measurement in that it establishes a  NPDC as a target for the reproduction of highlights and shadows. When neutrality is defined and maintained, color will usually look correct and stay consistent for a print job.

Reference:
GRACoL® 7 Specification for Commercial Printing on  #1 Coated Sheet

Hard Proofing: 120-patch Chart

These 120 patches were randomly selected with BabelColor from the standard 1617 patches in GRACoL2006_Coated1, only those within the srgb gamut are chosen. They are then compacted into a single 4R photo size. The printed test charts were measured with DTP41 and results were compared for color differences with the GRACoL2006_Coated1 standard data set.

120-patch Chart

Generic vs Custom - Visual Check

Generic vs Custom - compared analytically

Soft Proofing with Gamutvision



Standard View



HSV (polar)

GRACoL2006_Coated1 standard consists of 1617 patches, 1447 of which is within srgb gumut.

 

1617 patches arranged in a single A4 page





S9045 Canned vs Customed profiles -Percepture

 

S9045 Canned vs Customed profiles - Colorimetric

One short coming about the above test is that the user has no control over the dE scale. Visual comparison can be difficult if the scale for the 2 profiles under test yields different maximum dE.

Color Rendering Intents

Rendering Intent is a transformation that describes how colors both inside and outside the gamut of the printer are mapped into the colors available on the printer. In general, the effect of rendering intents has more impact on colors that fall outside the gamut of the target output device than those colors inside the gamut.

http://www.normankoren.com/Gamut_mapping.jpg
Gamut Mapping is carried out by one of the following rendering intents. There is no standard algorithm on how the rendering intent is handled by the application creator.

1) Perceptual (Picture or Maintain Full Gamut)

The source gamut is compressed to fit into the destination gamut. It affects all colors and not just out-of-gamut colors. It aims to reduce out-of-range colors that might result in banding artifacts, and to produce pleasing colors with adequate gray balance.

2) Relative Colorimetric (Proof or Preserve Identical Color and White Point)

White point will be mapped to the theoretical white point (L=100, a=0, b=0)  and all other colors will be mapped relative to the white point, hence resulting in more precise color rendering for in-gamut colors, as they remain unchanged.

Rendered from the same image and displayed with the same scale
Top: Relative Colorimetric     Bottom: Absolute Colorimetric



 3) Absolute Colorimetric (Match or Preserve Identical Colors)

Absolute Colorimetric rendering behaves the same as Relative Colorimetric rendering except that no compression or stretching of the lightness occurs to fit the gamut of the printing material. In other words, white point will be mapped to the measured white point or paper tint  whereas relative colorimetric maps the white point to the theoretical white point.

If the source image contains colors that are lighter than the printing material, they will be clipped to the color of the printing material. If the source image's white is darker than the printing material, it will be printed with some ink in order to simulate the input white as precisely as possible. It is the most effective rendering intent for hard-copy proofing of spot colors.

4) Saturation (Graphic or Preserve Saturation)

Color is rendered to its most saturated equivalent within the gamut of the output profile. It produces appealing colors that are saturated but with poor color match, and is mostly used for business graphics (illustrations, charts, graphs, etc).

References:
www.normankoren.com/color_management.html

www.gamutvision.com/docs/gamutvision.html

UV vs Non-UV Spectrometers

Opinions on how color measurement should be performed with or without a UV-cut filter on a spectrophotometer differ greatly in the market.

Optical Brightener Agent (OBA)
Paper manufacturers add UV brighteners or fluorescent materials to their paper to increase the apparent brightness of the media and the range of colors that can be produced when ink is applied to it.

Fluorescent materials have the unique characteristic in that it absorbs invisible ultraviolet light and emits the light into the visible region. When measured with a spectrophotometer, the effect is that some wavelengths will be unusually high—in some cases resulting in more than 100% reflectance, which is higher than expected. Although fluorescent brighteners may improve the appearance of printed materials, they present a problem for color measurement and color matching. The light source (usually tungsten) used in color measurement devices such as spectrophotometer does not contain the same amount of ultraviolet light as the light source used to view the printed material (fluorescent light bulbs simulating D50). This introduces a discrepancy between measured color and viewed color, even under laboratory conditions.

Spectrometer with UV-cut Filter

The use of UV-cut filters in the spectrometer is to remove the UV component of the light source in the instrument so that the effect of the fluorescent brighteners or colorants is excluded from the color measurements. The above charts show the effect of measuring a fluorescent media both with and without UV filtration. The two measurements shown are 4.34 dE2000 apart. The UV-filtered measurement data suggests that the paper is more neutral; the unfiltered measurement data suggests that the paper is cyan-bluish in hue. When viewed in a light booth with the proper light source, the paper appears bright and neutral, not cyan-bluish.

Color Calibration
Fluorescent brighteners present a challenge for color calibration. Some fluorescent brighteners can become unstable over time and with exposure to light, if stored improperly. Careful storage maintains the level of fluorescent brighteners and ensures measurements for color calibration are accurate.

If the fluorescent brightener shifts over time, the longer the paper sits on the shelf, the darker it becomes (brighteners can lose their effectiveness with time). So, if your color measurement device does not have a UV filter, it will measure the colors of the output device as darker. The software may try to lower the amount of ink in order to compensate for this—or produce an erroneous “out-of-tolerance” message as a result.

If the fluorescent brightener shifts from batch to batch (or with different lots of paper) this would affect calibration as well. Without UV filtration in the color measurements, some batches may measure brighter than others. Again, the software may try to compensate for this even though the output device has not shifted.

Compensation for UV Brighteners
There are two ways to compensate for UV brighteners and get a normal-looking profile. One is to attach a UV filter to the spectrophotometer. The i1, DTP41, DTP45, and Pulse spectrophotometer are available with a UV filter, but they must be ordered with this feature and cannot be changed by the user. The Spectrolino spectrophotometer has an accessory UV filter that can be changed by the user. The DTP70 and iSis both have user switchable UV options.

Alternatively, effect of UV brighteners can be corrected by using a profiling program with built-in UV compensation feature. Both ProfileMaker 5 and i1Match 2 include software compensation for UV brighteners. With these programs it is not necessary to use a UV filter.

Inks & Ink Drying process

Inks drying is primarily by evaporation and absorption. Heavy ink coverage on uncoated paper or matte stock take longer to dry. A print job also dries more slowly on a humid day.

Drying can be accelerated by placing the finished prints in a 25°C to 30°C environment with continual air movement. Depending on the inks and paper combination, drying time may be more than 24 hours.

Solvent-Based Inks
They are very resoluble and are often favored in graphic printing on paper or board. Solvents in printing inks evaporate as the inks dry, releasing Volatile Organic Compounds (VOC’s) into the atmosphere.


Water-Based Inks
These inks are based on a combination of acidic resin held in solution by amine and water-borne resin dispersions known as emulsions. Solution resins aid print quality, and the emulsion is used to provide resistance properties in the dried ink. During drying, on absorbent surfaces such as paper and board, the ink components will start to penetrate into the substrate. Evaporation of water and amine takes place at the same time, often aided by dryers on the press.

The liquids (water, oil & solvent) act as a carrier of a dye or pigment onto a given substrate. The jetting system in the printing mechanism defines the types of inks that can be used, e.g. Thermal Ink Jet-> water-based only, Piezo Ink Jet-> water, oil, solvent, etc.. and in turn, decides the specific substrates that are required for the inks.

Dye-based vs Pigment-based
Dye-based inks are not generally waterproof and can fade quickly in direct sunlight, hence normally recommend to be used for indoor graphic only.
Pigment-based inks use insoluble colourants which are highly water and fade resistant. They are used for outdoor as well as indoor graphics. They may however offer slightly muted colours.

IDEAlliance Printer Proofing Certification

IDEAlliance stands for Internal Digital Enterprise Alliance, an membership organisation dedicated to the fields of digital imaging, printing and publishing. Under which, the GRACol (General Requirement and Allications for Commercial offset Lithography) is responsible for the guidelines for commercial printing.

An extensive discussion is available from http://www.babelcolor.com/ article AN-3a Using PatchTool for IDEAlliance Printer proofing certification.

In short, a certified proofing printer process allows an overall assessment of what the final print will look like before going to the actual printout, this saving time and expense.

Color Managed Work Flow

Photographers and prepress professionals are better informed with the importance of color managed work flow this day. Icc profile is device specific and each device ( scanner, camera, monitor or printer) must be profiled in order to achieve its ultimate performance. In fact, each device can be profiled independently to meet the International Color Consortium, or ICC requirement. A well color managed solution is one that minimise the color differences and conform to the de facto standard. An ideal scenario is that you can edit your images and be confident about the results that you will be getting from your printout.

Why use profiling service?

Very often we come across people who already equipped with basic printer profiling tool but are not getting satisfactorily color output. It is important to understand that color profiling can be error prone--either due to unskilled operator or equipment misbehaved, which are difficult to detect and rectify immediately. With the assistance of profile analytical software tools and most importantly, our expertise, will assist you in creating the custom printer icc profile that will provide the best possible color reproduction when trying to match the original.

Custom Printer profiles

Every color device, regardless of monitor, camera, scanner or printer, each requires a color profile that can accurately describes the characteristic of that device, in order to produce the best color output. The generic, canned or default icc color profile that comes with the printer drivers from the manufacturer may not always provide you with the most accurate color performance. The custom profile will take into account the ink, media, printer and RIP you use to generate the profile. When created properly, custom profile can result in significantly improvement in color reproduction accuracy, enhancing greater detail in shadow and highlight areas.

Lots of generic printer profiles are readily available from numerous sources. Often, these profiles work fine for you and just as often, give you unsatisfactory results. Moreover, not all printers are built identically, hence custom profile becomes the solely choice if accurate color reproduction is critical for your application.

Our Goal

Our goal is to offer the service to the photography community and home users who wish to reproduce color as precisely as possible with their printing devices.

Color Managing .....