“Develop success from failures. Discouragement and failure are two of the surest stepping stones to success.”
UNIT – 6
COLOR MEASURING INSTRUMENTS
What is spectrophotometry?
Spectrophotometry is the measurement of the reflectance or transmittance of a sample at discrete wavelengths. Spectrophotometers usually provide illumination of the sample by white light and then contain a diffraction grating to refract the reflected light and enable measurement of the amount of light reflected at discrete wavelengths.
What is a colorimeter?
The word colorimeter is normally used for a device which uses three or more filters to produce a response similar to that of the eye, as opposed to a spectrophotometer which measures the amount of light reflected or transmitted at each wavelength. Both colorimeters and spectrophotometers can give the same tristimulus values though the spectral method is usually more accurate.
What is the CIE system of colorimetry?
A. Colour is the sensation achieved when light falls on the retina of the eye. In the retina colour sensitive receptors are ‘triggered’ to produce electro-chemical signals, which are sent to the brain to produce the sensation of colour. The light reaching the eye is the product of the light reflected at each wavelength by the sample and that of the illumination source shining on it.
The three types of receptor each peak in sensitivity at different wavelengths - one at short wavelengths, one medium wavelengths and one at slightly longer wavelengths. This means that any colour can be reproduced by just 3 coloured dyes, pigments or coloured luminous stimuli - so long as their peak absorption or emission wavelengths are also separated. It also means that colours can be seen to match despite having different spectral composition - a phenomenon known as metamerism. Such a match will generally fail when the light source shining on the sample is changed.
Colour (whether coloured light or print) is traditionally measured by specifying the amounts of Red, Green and Blue lights which would be needed to match it. Based on experiments in which observers were asked to match various colours by mixing three coloured lights, the international colour standards body International Commission on Illumination (CIE) defined a ?standard observer? as the average of these observers for a specific set of ?lights?. They then defined a system of measurement units and measurement procedures which enable any colour to be specified in terms of the amount of the three standard lights that would be needed to match it. These are the CIE XYZ values, and other quantities such as CIELAB are calculated from them.
Implementing color management
Color Management is based mainly on international standardization
Commission Internationaled’Éclairage(CIE)
- Specification of the Lab color space 1976
- Device-independent,
- Based on human color perception
International Color Consortium (ICC)
- Specification of the ICC profile format 1994
- Computer-independent,
- Manufacturer-independent
PROFILES FOR MONITOR, SCANNER AND PRINTER
ICC profile:
ICC profiles help you to get the correct colour reproduction when you input images from a scanner or camera and display them on a monitor or print them. They define the relationship between the digital counts your device receives or transmits and a standard colour space defined by ICC and based on a measurement system defined internationally by CIE. Thus, if you have a profile for each of your scanner, camera, display and printer, the fact that they refer to a standard colour space lets you combine them so that you obtain the correct colour as you get images from the scanner or camera and print or display them.
An ICC profile is one that conforms to the ICC specification. By conforming to this specification profiles may be exchanged and correctly interpreted by other users. The two main types of profiles are source (input) and destination (output) profiles and essentially consist of tables of data that relate the device co-ordinates to those of the standard colour space defined by ICC.
How do I make ICC profiles?
The main requirement is a software application that will generate profiles from measurement data. For output profiles, you also need a measurement instrument to measure your prints or display.
What is a rendering intent?
A rendering intent defines how the gamut of colours which can be achieved on one media is modified when reproduced on a media with a different colour gamut. Each profile contains three of these rendering intents and which should be used depends on the colour gamuts of the original and reproduction media.
Rendering intents
Scanned natural photographic images reproduced on prints or displays will usually use a perceptual rendering. This takes account of the fact that the range (gamut) of colours on a print or display is often lower than the original ? although for high gamut printing a colorimetric rendering (which attempts to produce an exact colour match) may be appropriate.
However, many other cases (such as proofing - simulating one device on another such as a print on a display) require a colorimetric intent when there are no colour gamut mis-matches. The saturation rendering intent is often used for business graphics and produces a maximum colourfulness on the print.
Integration into the workflow
Every device used in an open, digital color management workflow portrays color in its own specificmanner. Monitors for instance use different light sources and CCDs; and proof printers use different inks, laminates and papers. Monitors and scanners work in the RGB color space, proof printers work in the CMYK color space.
In order to integrate all these devices into a properly functioning color management system, it is necessary to ‘fingerprint’ each of them so the system can know how each device ‘sees’ or portrays color. This ‘fingerprint’ is also called the individual device profile – this is a table, which shows the actual values of the device that differ to the theoretical nominal values. With each portrayal or color space transformation, the fingerprint of each device is needed for print simulation whilst taking the
print standard (output profile) into account. A variety of manufactures can provide measuring devices and ICC compatible software for generating fingerprints.
Monitors
There are several different approaches for generating monitor profiles.
Scanners
Manufacturers of high-end and mid-price scanners offer the possibility to scan in an IT8 reflective test chart for the calibration, and the scanner profile is calculated by comparing the ascertained colorimetric actual values with the theoretical nominal data of the test chart. If the scanner is also suitable for transparent copy, the same procedure can be followed with an IT8 transparent testchart. External scanning programs or plug-ins that permit calibration via IT8 are also available for many scanners. Prior to generating the scan profile, the operating sequence and the scan parameters must be determined (e.g. linear scanning, highlight and shadow settings). The purpose of a profiled scan is always to reproduce the scanned copy as identically as possible.
Proof printers
In principle, device profiles of proof printers are generated in exactly the same manner as output profiles for print standards. However, fingerprinting proofers is very complicated since digital proof printers seldom work in a linear fashion. It helps when the proof software for the basic calibration permits linearization of the proofer via gradation curves and also allows maximum inking to be produce better profiling results. If there are any last-minute changes to the whiteness of the substrate to be printed, these changes to the existing ICC profiles can be optimized to a certain extent by profile editors. This does not require a test chart to be printed or measured.
IMAGE REPRODUCTION PROCESS
How to implement ICC colour management?
To apply colour management, you need a profile for each of your scanner and/or digital camera and another for your monitor and/or printing device. Each of these relates the device colour data to the standard colour space which allows them to be combined to produce an overall transformation.
To combine profiles you need a Colour Management Module (CMM). At its most basic this is nothing more than an interpolation engine for combining LUTs. ICC do not specifically recommend a single CMM as some CMMs attempt to ‘add value’ for specific applications by picking up private tag information in the profile.
Many colour management-aware applications such as high-end RIPs and Adobe Photoshop contain an internal CMM. CMMs are also built in to the OS on the Mac (ColorSync) and Windows (ICM and WCS).
THE “THREE Cs” OF COLOR MANAGEMENT
Many people use the term “calibration” to mean all steps necessary to achieve accurate color during the production process, perhaps implying that reproduced colors are “calibrated” to match the original. “Color management” is a more meaningful term for matching color on different input and output devices, since the calibration of each device is only the first of three steps necessary to achieve accurate and consistent color throughout the reproduction process.
Calibration ensures that all devices (scanner, monitor, and printer) perform to a known specification, be it RGB illuminance, CMYK density, or CMYK dot area.
Characterization is a way of measuring and quantifying the color space, color gamut, or color behavior of a particular device under known conditions. It is a way of determining how an input device captures color or an output device records color when it is calibrated.
Conversion (also known as color transformation or color correction) refers to translating a color image from the color space of one device to that of another under known conditions. Color conversion can be done by manually correcting the image or automatically by using color management software.
To achieve the goals of color management, calibration, characterization, and conversion must be done in this sequence. Calibrating a device to specification serves as a foundation for characterization and conversion, and a device must be characterized before color data can be converted for accurate rendering.
CALIBRATION
Color management is based on the assurance that all devices in a color reproduction system are performing to specification. Calibration alone does not guarantee color matching; it simply ensures that the scanner, monitor, and printer are performing to their respective specifications, and provides a way of ensuring they will be consistent over time.
Scanner calibration means that when a specific light level is measured from a film or paper target, the scanner consistently records a corresponding digital value in the image file for that spot on the original. Monitor calibration means that the display card consistently displays a pixel corresponding to the specific digital value received from the file. Other items that require calibration include the color printer/proofer and the platesetter.
CHARACTERIZATION
After devices are calibrated, they must be characterized. Characterization defines the color gamut, or set of reproducible colors, that an input device can capture or an output device can record. Device characterizations are stored as profiles, digital files of data describing the color gamut of a device. In page-layout software, color management systems keep track of the input, display, and output devices the user has specified using tags or data appended to color files.
A variety of models can be used to characterize input and output devices, including RGB color space, CMYK color space, and CIE color space, which includes two models based on the dimensions of hue, chroma, and value. These are the CIExyY and CIELAB color spaces. In both models, hues are arranged around the perimeter of the color space, saturation increases from center to edge, and value varies along the third color space axis.
Scanners are characterized by software that measures the values in a scanned ITS.7 target and compares them to corresponding values in a reference file. The ITS.7 target is the internationally standard input target developed by the ITS subcommittee of the Committee for Graphic Arts Technologies Standards. The basis of the ITS.7 target is the Q60, a series of photographic film and paper test images for characterizing the gamut of input devices developed by the Eastman Kodak Co.
The printer must also be characterized. Output targets are measured with a spectrophotometer in CIExyY and/ or CIELAB color space to characterize the color gamut of an output device. As with scanners, the characterizations are stored as device profiles. Profiles for commonly available monitors are offered by the developers of color management software, although they are valid only when the monitor is performing to manufacturer’s specifications. Some software allows users to characterize their own monitors; other systems have built-in calibration.
CONVERSION
Conversion refers to translating color-image data from the color space of one device to that of another under known conditions. Color conversion is necessary so that a scanned image reproduces as a believable representation of the original on both the screen and the printer. Since output devices typically have smaller color gamuts than originals, scanners, and monitors, colors in the original must be fit into the gamut of the device, a process known as gamut compression.
Color management software converts or translates color from one space to another: from scanner to monitor, from monitor to printer, and from scanner to printer.
Once color management profiling software has been used to characterize the scanner, monitor, and printer, it is necessary to apply the profiles to the image according to the desired “matching” objectives.
Three methods of color conversion are used-one for photographs, one for spot colors, and another for business graphics. Perceptual rendering, used for continuous-tone photographs, maintains the relative range of colors in a photograph. It causes the white portions of an image to have no ink on the paper, and the black portions to have the darkest color that the device can print.
Colorimetric rendering, most effective for spot colors, maintains an absolute color match. It renders colors that are within the device’s gamut identically, and brings colors outside the gamut to the closest color the device can print.
Saturation rendering is appropriate for bright saturated illustrations and graphs like those used in business presentations. This rendering style produces pure, saturated colors in print according to the printing device’s limitations. It does not try to precisely match printed colors to those on the monitor.
Posted October 07, 2019 by Tim Mouw
Colorimeters and spectrophotometers are color measurement devices that are used to capture, communicate, and evaluate color. From cardboard packaging to food, laundry soap, carpeting and small plastic parts, color measurement devices help ensure the color being produced matches the color that was originally specified. They’re used behind the scenes in just about every industry where color is important, including plastics, textiles, paints, coatings, print and packaging.
There are basically two types of color measurement instruments: colorimeters and spectrophotometers.
A colorimeter is a device that mimics the way humans perceive color.
Using an internal light source, a colorimeter shines light down onto the surface of the sample. As the light reflects back up to the device, it passes through three filters: red, green and blue. These filters distill tri-stimulus (RGB) values that match how our eyes see color.
Colorimeters can be used to calibrate computer monitors to ensure the colors you see on screen are accurate. They’re also used to specify color. Designers can carry them around to capture inspiration colors for use in their designs. However, colorimeters are limited in their capabilities. Since they are blind to metamerism (a common phenomenon that occurs when two colors appear to match under one lighting condition but not when the light changes), they aren’t ideal for all production applications, especially formulation.
Spectrophotometers allow for more sophisticated color measurements and can capture more data related to color.
A spectrophotometer works almost same way, except for one main difference – the filters. Instead of using three filters to determine the RGB values of the color like a colorimeter, modern day spectrophotometers typically have 31 filters to measure the full color spectrum. These filters measure light in each of 31 different wavelengths to determine the color of the sample.
Spectrophotometers come in many shapes and sizes, from small handheld devices to large benchtop workhorses, to in-line devices used during manufacturing production. Since they can evaluate how color will look under different light sources, spectrophotometers are ideal for identifying metamerism.
Spectrophotometers can measure just about anything, including liquids, plastics, paper, metal and fabrics. There are three primary types:
Many spectrophotometers can measure in transmission mode to quantify the opacity and haze of a sample. They can also compensate for optical brightening agents, chemicals that manufacturers add to products like paper, plastics, and textiles to make them appear whiter and brighter,
Today’s spectrophotometers are built to support a connected color management strategy while delivering the most reliable, repeatable, and accurate measurements between operators and across manufacturing sites.
It all depends on your application and price range. The higher the resolution of the instrument, the better job it can do. Colorimeters are a great way to capture color and do basic evaluation for applications that don’t require tight color control. Since spectrophotometers measure the entire spectrum instead of just red, green and blue, they provide more accurate color data; making them useful for a broad range of applications in R&D, color formulation, and quality control.
If you’re ready to enter the world of color measurement, get in touch. Our color experts would love the opportunity to help you decide which device is best for your needs.
i1Profiler software comes with the i1Basic Pro 3 and i1Basic Pro 3 Plus measurement devices and allows you to:
Which device is right for you?