Application of Density and Chromaticity Measurement in Printing (I)

In modern printing, the measurement of color density and chromaticity is widely used in plate making, proofing, and printing, and more color accuracy and descriptiveness are pursued. So what is density? What is chroma? What is the role of density in printing? What is the role of color? What are their measuring instruments? What is the difference between their respective application areas? What kind of disadvantage does it have? These problems have plagued many of us, including many of us who often see similar problems in our winning forums. With the above questions, we begin the discussion of this article. I believe readers will have a more comprehensive understanding and differentiation of colorimetric and density measurements.
Density and chromaticity The so-called density is the logarithm of the reflectance or reciprocal of the reflectance measured on a reflective or transmissive document (for the sake of discussion we will discuss only the case of a reflective document). It sounds like the concept is very abstract, it is also reflectivity, it is reciprocal, it is logarithmic, but as long as we are careful we will find that the most direct source of measured density value is calculated by measuring the reflectivity owned. Reflectivity is also the only factor that can affect the density value. The stronger the object's ability to absorb light, the lower the reflectivity of the object, the higher the reflectivity of the object. The relationship among the three is certain.
Let's take a look at what color measurement is.
Chroma, as its name suggests, is a measure of color. Such a measure is an “objective” description of color. The objective is to add quotes because it is based on the visual physiology of the human eye. But it is the average visual perception of color for most people. This measure can finally be expressed in the form of a value. There are three kinds of colorimetric forms that we have commonly used in the normative sense: CIEXYZ, CIELAB, CIELUV. This is a bit like the length measures (such as the relationship between inches and centimeters) of the different units we use. The difference is that there is no absolute conversion between them.
From what has been discussed above, we know what density is and what color is (at least there should be an approximate impression). Let's take a look at the different measurement tools used to measure density and chromaticity.
Density and Color Measurement Tools It is clear that densitometers are used to measure density. There are two main types of measurements commonly used for densitometers. One is the spectral narrow-band color density and the other is the spectral broadband color density. The narrow band and wide band of the spectrum are mainly achieved by different filters. With a densitometer using a broadband filter, the result of color measurement is of course a spectral broadband density, and the narrow band is the opposite. The density meter we use will vary depending on the situation. For example, narrow-band measurements add sensitivity to small changes in density, and they are much less visually responsive to humans than broadband filters. Narrow-band density measurement is mainly used to measure dot gain, overprint, ink layer thickness, and ink strength. While the broadband filter density measurement does not depend on the absolute value of the spectral distribution, it depends on the relative spectral radiation distribution, which is always related to the relative density of the sensor used in the density measurement and the spectral transmittance of the filter. . Broadband measurements are mainly used to evaluate hue, grayscale, transparency, and color correction.
Now we are abandoning the issue of broadband and narrowband, in general terms the problem of the density meter. Densitometers use three different color filters for printing proofs. The most common is a complementary color filter using a hue of ink (usually a standard color ink), such as a filter that measures yellow. Blue is used (λ=430 nm), green color filters (λ=530 nm) are used for magenta measurements, and red (λ=620 nm) is used for basic cyan measurements. The result of such a measurement is obviously for the ink, not the human eye. This measurement can only tell us the relative amount of a certain ink on the printed and already printed product, that is, whether an amount of ink at the measurement site is sufficient and whether the desired density is reached. At the same time, a range of color contrasts can also be made. This contrast has little to do with the human eye's vision.
In this way we found that densitometers have a limited ability to measure and indicate hue. Densitometers are not colorimeters. Although readings of three filters are used simultaneously, hue can be indicated approximately. However, this indication of hue is quite inaccurate and cannot meet the needs of printing color measurement. For more needs (eg, analysis of paper whiteness, color analysis of originals, etc.), measurement of chromaticity has begun to receive wider attention.
There are two main colorimetric measurements. The first method is a method of color measurement using a photoelectric colorimeter. The photoelectric colorimeter is very similar in principle to a densitometer. Its appearance, operation method and even purchase price are quite similar. The photoelectric colorimeter directly displays the tristimulus values ​​x(-)(λ), y(-)(λ), z(-)(λ), and most of them also convert the tristimulus values ​​into a color space scale, for example, into CIELAB scales, but most of them have only one or two kinds of lighting, so the color measured with a colorimeter does not always represent the visual color. In addition, CIELAB is not an ideal colorimetric system for printing because it cannot CIELUV calculates the saturation of the color. The photoelectric colorimeter is sufficient for determining the color difference, so it can be used as a measurement of color difference comparison in a printing shop. Many high-end opto-electronic colorimeters are also accurate enough to measure absolute color and relative color differences, but generally speaking, people prefer to use spectrophotometers to accomplish these tasks.
The colorimeter can be thought of as a reflectometer, or a densitometer without a logarithmic converter but with a special set of color filters. Of course, this is a way to complete the color measurement. The purpose of attaching a set of color filters is to weight each wavelength of the spectrum in each channel of the colorimeter based on the CIE spectral tristimulus value. However, unlike colorimeters, colorimeters mainly deal with reflectivity rather than logarithmic problems, but reflectivity can easily be converted to density, and vice versa. The spectral composition of the colorimeter is considered to have a good linear relationship with human visual acuity. However, in reality this is not possible (the Lussian condition* problem is involved), so the photoelectric colorimeter has an error in principle.
The second method is to use a spectrophotometer to measure color. Just as the three-color filter photoelectric colorimeter can be regarded as a special reflectivity measuring instrument, the spectrophotometer can be seen as such, but unlike the photoelectric colorimeter, the spectrophotometer measures the whole of an object. Visible reflectance spectroscopy, spectrophotometer point-by-point measurement in the visible spectral domain, that is measured at some discrete points, usually measuring a point every 10 or 20nm, measuring 16 to 31 points in the range of 400 ~ 700nm. Some spectrophotometers measure the spectrum continuously, while the third filter opto-chromatometer measures only three points, so the spectrophotometer can provide much more information, at least measuring 16 points. .
Spectrophotometers measure color as a physical phenomenon that is not subject to the observer. In order to obtain a tristimulus value it can integrate the reflection spectrum and can interpret the color as a visual response. It is the most flexible color measurement instrument.
Some phenomena in the printing process, such as paper web spot coverage, ink strength, etc., are essentially physical phenomena that occur within a narrow band, and of course it is better to use narrow-band measurements for evaluation. However, it should be noted that narrow density measurements cannot be used to measure visual color, but spectrophotometry can solve this problem. Because the measurements it makes are narrow-band measurements, the sampling of the spectrum is sufficient, so it is possible to make color measurements that are consistent with the vision. For the intended type of measurement (narrowband or wideband), a calculation program can be pre-programmed for the spectrophotometer. Many new spectrophotometers include a computer, and it is appropriate to perform standard print copy quality control and narrowband measurement according to the program, but it is obviously more expensive than the density meter.
As we all know, the most basic method for measuring color is subjective visual inspection. This method visually matches the unknown color based on the colors in the chromatogram. The color data measured by the spectrophotometer is finer than that of the human eye. This is useful for analyzing the concentration of pigments, and only needs to be based on some formulas. With the calculation, you can analyze and control the amount of raw materials.
According to the spectrophotometer measurement values ​​can be calculated density and chromaticity values ​​(but the reverse calculation is incorrect); metamerism can be analyzed; new spectrophotometer can also be spectrophotometric data can be directly converted to other colors The system parameters and conversion methods are the same as for the colorimeter.