Print appearance analysis

An Introduction to Appearance Analysis)

Editor's note:
For consumers, the appearance of the product is the most direct and convincing product information, so printers and printing customers value the surface of prints. An important task that printers and designers face daily is to perform image color correction, match proofs, or use spectrophotometers, densitometers for various tests, and color management. These are familiar appearance analysis concepts, and there are also subjective ones. Intuitive judgment on.
In fact, the judgment of the appearance of a product inevitably includes various subjective and customer factors. Therefore, the analysis of the appearance of the product can be started from these factors that affect the appearance of judgment, supplemented by appropriate techniques, instruments and measurement methods for analysis. This is exactly what Richard Harold is going to introduce to you in this article. Harold has 30 years of research experience in the field of visual analysis and served for the GATF for two years. Harold first introduced the interaction between the light and the target, which is why the observer creates a visual effect. Harold will then introduce various devices and principles for analyzing appearance attributes based on this visual principle.

The appearance of an object is the result of the interaction of many complex factors, including the properties of the light irradiating the object, the visual characteristics of the object, and the observer's perception.

If a product is to achieve the expected market value, then appearance is one of the most important business attributes of the product, which largely determines whether the product will be accepted by consumers. Because from a psychological point of view, the appearance of a product is always connected with people's expected quality.

It is precisely because the appearance of the product directly affects the purchasing behavior of potential consumers, so all manufacturers are concerned about the appearance of their products, including visual factors such as color, luster, shape, texture, and transparency. Manufacturers understand that consumers will choose the best appearance when the product has the same performance, quality and price. The appearance of the product is undoubtedly the most direct and most illustrative product information.

In addition to the visual appeal of the appearance of the product, the printing customer also requires that the same product be printed in different batches. Because if the appearance and packaging of a batch of products are different from the previous ones, it is easy for consumers to suspect that they are defective products or expired products. Therefore, the consistency of product appearance is also a customer concern.

At the same time, when consumers consider the product life, the durability of the packaging is also an important factor. Although subjective judgments are doubtful, there are also some objective criteria for judgement. Lightfastness and stability can be tested through standardized tests that help consumers make more informed purchasing decisions.

Light and ink, paint, paper, fiber, plastics, metals, ceramics and other different substances have different interactions, which are determined by many physical factors. As long as appropriate instruments and methods of operation are used, the appearance properties of various substances can be measured.

First, the interaction of light and objects or substances When the light emitted by the light source is visible light, the appearance of the objects seen by the eyes is determined by the effect of the objects on the glazing of the objects themselves. The object here can be either a print or a piece of paper or an apple or any other object or substance.

Visible light has a wavelength range of 380 to 770 nm, and light of different wavelengths exhibits different colors. Since the energy of different wavelengths of light radiation is different, some wavelengths of light appear stronger than others. So the human eye responds differently to light with the same total energy and different wavelengths. It is expressed as a luminosity curve of the human eye and describes the energy of light or radiation of each wavelength.

Light can be emitted from objects heated to a white-hot state, such as incandescent light bulbs, or excited by atoms or molecules, such as light emitted when the metal coil of an electric furnace is heated to redness. Fluorescence is a special case, which arises when light travels from one spectral region to another.

A pure source of radiation, called a "black body radiator," can be used as a reference standard for identifying the color of a white-hot light source. The correlated color temperature (CCT) of the light source is the temperature at which the black body radiator looks closest to the light source. For example, the CCT of a typical incandescent lamp is about 256K.

Although the representation of matter is composed of many complex factors, it can be roughly divided into geometric properties, color properties, and specific light properties for analysis.

After light hits an object, several different reactions may occur, which produces different visual effects. If light totally reflects on the surface of the object, the object will look bright and bright. What happens to the reaction is determined by the nature of the material. Metals are usually more harmful than total reflections of other materials. The smooth surface is usually brighter than the rough surface.

Second, geometrical properties and colors are not the same. The geometric properties of an object's surface are usually associated with its surface features. It is very complex and cannot be described by some simple shapes and arrangement concepts. Fortunately, if we only focus on a relatively balanced, uniform surface, we can simplify its geometric properties.

First, some of the light that strikes the object is reflected, and some of it is transmitted. The reflected light is reflected from the surface of the object. This object becomes a light in the eyes of people who see this reflected light. Transmitted light is the light that passes through the object and is seen by the outside of the object. The reflected light and the transmitted light can be further divided into complete reflection and diffuse reflection, and the transmission and diffuse transmission will be finished. Such a division is to analyze reflected light or The geometrical properties of the transmitted light lay the foundation.

The selective absorption of light at a particular wavelength results in the perception of color by the human eye. When the majority of the absorbed light is, the color looks weaker. When all wavelengths of light are absorbed, the human eye sees black. When all wavelengths of light are reflected, white is seen.

There are four main types of motion that occur on light that strikes an object: complete reflection, diffuse reflection, complete transmission, or partial reflection of the transmission. These physical changes to light can be measured with a photometric spectrometer and an angle spectrometer.
Photometric spectrometer curves are measured in the visible spectral range at wavelengths that reflect and reflect light of various types. Therefore, the spectral curve can be associated with various colors to help identify the pigment or dye portion that produces a certain color.

The angle spectrometer curve describes how light rays reflect at the object surface or reflect through the object at different angles. These conditions are related to the geometric properties of the object. Although both photometric spectrometers and angle spectrometers cannot provide a conclusive description of the appearance of objects, they illustrate how light interacts with objects in an observation environment.

Third, the color attribute color is associated with light waves, especially the wavelength of light. Visible light refers to picking from the infrared to the end of the spectrum. Their wavelengths range from 400 to 700 nm. The selective absorption of light of different wavelengths in this range usually determines the color of the object. Light that is reflected or transmitted without being absorbed by an object can be seen by the human eye. In other words, yellow objects absorb blue light, red objects absorb green light, and so on.

In reality, the color of a material is measured and represented by a photometric spectral curve. In a typical spectrophotometric plot, the white, gray, and black curves are almost straight horizontal lines at the top, middle, and bottom of the plot. The curves representing the various colors are at the highest point in the corresponding spectral region of the color, and at the lower values ​​in the other wavelength regions.

Fourth, the psychological factor of color From a psychological point of view, color is the consciousness of the brain, generated by the signal transmitted to the brain by the human eye light receiver. The color of any kind of object is the result of the action of dyes, pigments, or other absorbing substances on the object that illuminate the light. Although the eye itself cannot perform wavelength analysis, it can synthesize the response of three color receivers (red, green, and blue light) to light waves. Therefore, different people have different color analysis abilities due to different sensitivity and proficiency in color. A skilled color analyst can estimate the color effect of a sample based on the photometric curve, which is difficult for a non-specialist to do.

V. The Color Attributes Observed by the Observer When an artist examines a work, he considers not the color luminosity spectroscopy curve or the response of the human eye to the color of the red, green and blue receivers. When he first identifies the color, Considering the hue of the color, that is, whether it is a color attribute such as red, yellow, blue, or purple, in short, what color the object is, this attribute is commonly used in art, color technology, and decorative industries. Indicated.

The second attribute of color is saturation, and the size of a certain color saturation is determined by the difference between the outer edge of the pure tone of the color and the gray axis, for example, a low color saturation is low, and a pure color saturation is high.

The third attribute of color is related to the luminous intensity of the object (usually called reflective and transmissive). This property is called many and can be called brightness or light value.

So the object color has three properties: Hue lightness and chroma. In color analysis, especially when comparing color differences, light, color, and hue (LCH) are the most used terms.

Six, luminous body (light source)
Light sources also affect people’s perception of color. In the actual examination of the color of different objects, you can either use natural daylight or artificial artificial sunlight. In natural daylight, non-direct sunlight (such as daylight entering from a window facing north) is usually selected, which is what is commonly referred to as “northern” light in art. However, there is a drawback of natural daylight. It is instability. There is a great difference in daylight in different seasons, different locations, and different times. Therefore, the development trend of industrial inspection is to use simulated sunlight, which can achieve standardization and ensure the relative stability of the spectral quality.

In order to define different artificial light sources, the International Lighting Association CIE has established a set of standard illuminants whose spectral characteristics are similar to those of natural light sources and can be reproduced in the laboratory (CIE, 1931): Light source A represents a typical The incandescent light; the light source B represents the direct sunlight; the light source C represents the average of the sunlight in the entire sky.

In 1963, CIE organized a series of light sources and was subsequently accepted. The D series light source represents sunlight more comprehensively and accurately than the B and C series, because the spectral properties of the D series cover the entire ultraviolet, visible and near-infrared wavelengths (300-830 nm).

D-Series light sources are usually distinguished by the first two digits of their correlated color temperature (CCT). For example, the D65 represents the average daylight with a correlated color temperature of 6504K. Most industries use this light source for color measurement and evaluation. With the exception of the printing industry, the D50 light source is used in the printing industry because the light source is visible throughout. Better spectral balance. For example, the low-temperature light emitted by candles is mainly long-wavelength (red and yellow). Using candles as light sources makes it difficult to judge blue and purple.

In recent years, the content of ultraviolet light has increased in visual assessment and color measurement. This is mainly due to the increase in commercial applications. It focuses on paper, fiber, and fluorescent agents. Appropriate visual evaluation and color measurement of these substances are performed. Not only do you have to control visible light, you have to consider the UV rays that invade them.

Seven, color measurement ratio / CIE standard viewer

Scientific color measurement methods are based on the three color reaction mechanisms of the human eye. The eye's light receiver reacts differently to different wavelengths of light. If you want to mimic the human eye's reactions and mechanisms to make color measurements, you first need to give the eye some value for the response to various wavelengths.

The standard description of the three color-capture reaction functions of the eye is called the 1931 CIE Standard Viewer (also known as the 2° observer). The purpose of this international standard is to observe small objects, providing a viewing angle of 1° to 4°.

In 1960, the CIE organization also introduced a 10° supplemental standard with the aim of satisfying


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