PART I - CHAPTER IV
INSTRUMENTS FOR THE
MEASUREMENT OF COLORS

How to measure a color?

         The measurement of the colors was forsaken for a long time by the representatives of the graphic chain. Subjective qualities of observation of the chromist took a dominating share in the final result. Currently, the means of colorimetric measurement are accessible to all and allows the uninitiated persons a facilitated use. The quality of work and especially reliability throughout the graphic chain, are the principle technical interests. It is possible several to measure the color. The desired level of quality is in direct relationship to the type of work to be realized and the type of measuring apparatus to be chosen.

          It is quite obvious that the instruments for the measurement of colors allows a clarified communication and more effective between the various graphic chain links. In all, it is necessary to distinguish four types of measuring apparatus, that are the densitometers, colorimeters (and spectrocolorimeters), spectrophotometers and spectroradiometers. The principle properties of each of these products are explained below by proposing their qualitative aspects in order to clarify it for what they are intended. These tools are used to decipher a color by managing numerical values. The densitometer is an instrument which is used since many years in graphic and photographic industry. Its role is to measure the opacity of a film or the absorption of the incidental light by a reflective object. The density of films like inks of impression or the pigments are thus measured precisely. Thus, the densitometers are also called as transmission and those by reflexion.

The densitometer



Diagram 9: Principle of measurement of a densitometer by transmission







Diagram 10: Principle of measurement of a densitometer per reflexion

Densitometer measures the light reflected by surface, proportionally with the light sent by the lamp of the instrument. This measurement of reflection is converted towards a logarithmic scale. The formula is D = - logR (D = density is R = reflection). A densitometer cannot give colorimetric information. It gives information on the value of opacity.

Colorimeters

          General information on the colorimeters

          The colorimeter is an instrument for measuring color, its principle is based on the tristimulus values XYZ. The numerical data provided by a colorimeter are inadequacy with the standards defined by C.I.E.

          The sensitivity of a colorimeter is thus based on that of the human eye. However parameters being able to influence the vision of the colors (source of light, methods of measurement, etc.) remain stable of a measurement with another, contrary to the vision by the eye. The results obtained are thus more precise. Thus, the visualization of measurements can be carried out in one of colorimetric spaces described previously (Yxy spaces, L*a*b *, L*C*h, etc.). Nonobservable differences



Figure 7: Colorimeter of calibration and characterization of data-processing screens




Figure 6: Kleo colorimeter


The colorimeter of screen Kleo of Qubyx has four sensors different for measurement from the colors. It carries out the calibration of the monitor (TC, gamma, etc.) and characterization by creating ICC profiles compatible with Colorsync 3 (or ICM for world PC).

with the naked eye can be detected by the colorimeter, precisely calculated and determined by the known concept of ?E*ab or other formulations of color variations.

          Principle operation of a colorimeter

          A colorimeter consists of a sensor associated with filters and a microprocessor. The system of detection (equivalent to the retina of the human eye) is composed of three or more interferential filters associated with sensor. The properties of these filters is similar to those of the peaks of the spectral curve of the human eye. The trichromatic response of the eye is simulated, in the case of a measurement has been carried out under a source of daylight. The metamerism of the colors (phenomenon explained in the preceding chapter) cannot be highlighted owing to the fact that measurement is taken under only one illuminating. The sources of light



Figure 8: Detail of a colorimeter screen (connectors and sensors)

employed are is of xenon type, maybe of the lamps halogen-tungsten. Specifically filters adapted to phosphorus are integrated into the measuring apparatus in the colorimeters being used to gauge the monitors,

Spectrocolorimeters

         The spectrocolorimeters have, as for them, more detectors than in the case of a standard colorimeter (for example 40 for the spectrocolorimeter Minolta CM-2002), to measure the spectral reflectance of an object for each wavelength. The microprocessor in the apparatus calculates the tristimulus values from the data of spectral reflectance by integration. It can analyze the spectrum more precisely on a group wavelengths. In this case, we obtain a more precise spectral curve.

Spectrophotometers

          General

          A spectrophotometer analyzes wavelength per wavelength (that is to say with an interval wavelengths of some nanometers) luminous energy in reflexion or transmission of an object. After measurement, we have obtained a sample spectral curve on an interval wavelength corresponding to the visible spectrum (in general between 380-780 Nm). The spectrophotometer makes it possible to calculate values XYZ for the given illuminating sample, according to the selected type of observation (2° or 10°). Thus it is possible to determine the metamerism of the colors precisely, which is not possible with the use of a colorimeter.

Observation 2° and 10°

The measuring apparatus such as the spectrophotometers Xenon or Spectro Table of Qubyx has the possibility of carrying out measurements under an angle of 2° or 10°. These angles of measurement came from a standard defined by the C.I.E, in 1931. This is based on the properties of sensitivity of the eye for the colors. This sensitivity varies according to the angle under which the color is observed. In 1931, the rule on the standard observer 2° was installed by the CIE, followed in 1964 by that on the additional standard observer 10°.

So now we compare the fields of vision 2° and 10°, at a distance of reading of 50 cm, we will have:
-for the field of vision 2° a circle 1.7 cm in diameter.
-for the field of vision 10° a circle 8.8 cm in diameter.




          Principle of operation

          In its principle of operation, the spectrophotometer differs from a colorimeter with its system of detection. The type of source of light is same for the two kinds of apparatuses. Currently lamp with Xenon is very much used because it has advantages in term of quality, stability in time and lifespan.



Figure 9: Spectrophotometer Xenon QUBYX


The spectrophotometer Xenon of Qubyx makes it possible to take measurements all the 5 Nm, on a wide beach of 380 with 750nm. It is controlled by the software Kleo Print Pro which uses traditional color spaces CIE Yxy, CIELAB, CIELUV, CIELCH, etc. The illuminats present are A, C, E, D50, D55, D65, D75. It can, moreover, used to measure a source of light (e.g. monitor).




          The spectrophotometer has a dispersive and selective system of the light called monochromator. There are two types of monochromators: progressive interferential filters and diffraction patterns. The geometry of illumination and reading is inadequacy with the standards of C.I.E. Indeed, in each software controlling a spectrophotometer has a significant number of illuminating is stored in memory, with an aim of enriching the types of measurement. After the measurements taken for all 3, 5 or 10 Nm, the software interprets the results and rebuilds the spectral curve of the measured sample. Certain spectrophotometer have more than one UV filter, because UV rays emitted by bleaching particles of paper can distort measurements.

         The spectrophotometer SpectroTable of Qubyx measures using a diffraction pattern (grid) associated a barette of diodes, on an interval wavelengths of 80 to 780 Nm. The interval of measurement is of 3 Nm, and the apparatus allows the analysis of transmission and reflexion. The spectroradiometers makes it possible to measure the spectral distribution of a source light. It also uses the system light diffraction. This system of measurement of the light is very expensive (utilisable to measure screens, and other primary sources). Its utility is primarily reserved for research and the design.




The software eye one allows to measure a color and to transform these LAB values in another colorimetric space. It also provided a previsualisation in the way in which this color LAB

- PART I - CHAPTER IV - INSTRUMENTS FOR THE MEASUREMENT OF COLORS -
 
Publicity