Color is not an intrinsic property of the material, but a result of radiation (light/source) interacting with the object, detected by the Detector (observer) and taking into account the way the light interacts with the object (geometry).
In this way, color is defined as a psychophysical sensation, that is, a mental response to physical stimuli. Humans perceive color through the reception of light by the eyes and its projection on the retina, which has specialized cells that detect different wavelengths in the visible region of the electromagnetic spectrum.
Among the psychophysical phenomena of color perception, the following points need to be considered:
light source
That is radiation of the electromagnetic spectrum from 380nm to 780nm, characterized by its spectral distribution of relative power for different wavelengths and its correlated color temperature (CCT). Light sources are defined by their spectral distribution and color temperature. For color measurements, a standard light source or the same theoretical light source is used, characterized by a defined spectral power distribution and corresponding color temperature. These types of light sources provide the basis for comparing images or colors under different lighting conditions. In colorimetry, common light sources are as follows:
Light source A: represents 100W tungsten lamp, its color temperature is measured in Kelvin, T=2854K.
Light source C: represents the measurement of sunlight, and the color temperature is T=6774K.
D65 light source: represents the average light of a moderately cloudy day in the southern hemisphere, with a color temperature of T=6504K.
Measure geometry
That is, the way light interacts with objects. Geometry is an important element when designing a color measurement device because it takes into account the location of the light source and Detector, as well as the location of the sample to be measured. In this way, the geometry of light falling on the sample and the geometry of the Detector collecting light reflected or transmitted by the sample is known. For situations where samples of opaque materials need to be measured, the CIE has defined four standard geometries. Therefore, measurements made with similar equipment with the same optics provided similar results:
CIE 0/45 & 45/0, in the case of 45/0 light falls on the sample at an angle of 45º relative to the normal and is detected at 0º; and in the case of 0/45 the light falls on the sample at Falls on the sample at an angle of 0º to the normal and is detected at 45º.
CIE 0/T & CIE T/0, light falls on the sample and diffuse reflection is detected. In this case, the Specular Component Included (SCI) or Specular Component Excluded (SCE) measurement mode is possible.
On the other hand, for cases where samples of translucent materials need to be measured, direct transmission with 0/0 geometry or diffuse transmission with 0/d geometry can be evaluated.
observer
i.e. who detects the color. Without an observer, color cannot be detected and therefore measured. The CIE has standardized the types of observers that represent a given field of view, specifically: 2º and 10º. 10º observers are common, although it always depends on the type of company and department.
object
Because light radiation is changed by it through the physical processes of reflection, transmission, absorption, diffusion, interference and diffraction. Depending on the properties of each material, these processes exist in relative proportions, and objects can be classified according to how they behave relative to the light that reaches them:
Transparent objects: These are objects through which light passes through, through which other objects can be seen.
Translucent objects: These are objects through which light passes, but other objects cannot be seen through them. Perceived color is characterized by the transmission of light, whether in translucent or transparent objects or objects.
Opaque Objects: Objects through which light cannot pass. In this type of object or objects, the perceived color is characterized by the reflection of light.
Colorimetry and Color Measurement
The purpose of colorimetry is to numerically represent the color of an object. In this way, colorimetry is responsible for studying color and quantifying it, for which it develops the necessary methods and techniques. With this, it is possible to obtain a quantitative, objective and repeatable measurement to assess color.
Color can be assessed instrumentally in opaque or translucent materials by using Spectrophotometer s and Colorimeters; likewise, color in solids and liquids can be assessed instrumentally.
Color Measurement of Opaque Solids
Determination of color coordinates CIE, geometry 0º:di, 0º:de, 0º:45ºa
Determine the color difference according to the CIELAB system, geometric 0º:di; 0º:de, 0º:45ºa.
Follow different methods and solutions to determine and evaluate the color of any type of material or product, such as ceramic materials, packaging plastics, automotive plastics, eyeglasses, food liquids, oils, food and pharmaceutical film packaging, packaging, textiles, etc.
Color coordinates and colors in tristimulus values can be determined from reflectance measurements of different geometry types, specifically Diffuse with specular component, Diffuse with specular component excluded and Geometry 0/45.
Color measurement of translucent solids and liquids
为了对半透明固体和液体进行颜色测量，According to材料的光谱透射率数据进行颜色评估测试。According to正在评估其颜色的材料，可以执行常规或漫反射透射率测量，包括和排除镜面反射分量。
色坐标CIE的确定，在几何 0º:0º、0º:di 中。
According to CIELAB 系统确定几何形状 0º:0º、0º:di 中的色差。
ISO 11664-1 比色法。
ASTM E308:2008。使用 CIE 系统计算物体颜色的标准实践。
ASTM D2244。从仪器测量的颜色坐标计算颜色容差和颜色差异的标准实践。
ASTM E313。从仪器测量的颜色坐标计算黄度和白度指数的标准做法。
ISO 7724.油漆和清漆 - 比色法 - 第1部分：原理。
颜色加德纳
According to标准使用 Color Gardner 量表测量透明液体：
ASTM D1544， 透明液体颜色的标准测试方法（加德纳色标）。
ASTM D6045， 通过自动三刺激法测定石油产品颜色的标准测试方法。
ASTM D6166， 松树化学品和相关产品颜色的标准测试方法（加德纳颜色的仪器测定）
色差测量
关于色差的测量，可以在以下几种情况下进行：
两种产品之间的颜色差异。在这种情况下，According to在相同条件下测量的两种产品的比色值，评估颜色差异。
在加速老化过程之前和之后。在这种情况下，产品或材料会经历加速老化（紫外线、盐雾、湿擦等），并在此过程之前和之后评估其颜色。目的是评估这种材料或产品的劣化，或老化对其的影响。
可以评估以下参数：
ΔL*：明暗值之差
Δa*：红/绿轴上的差异
Δb*：黄/蓝轴上的差异
ΔC*：色度差异
ΔH*：色调差异
ΔE*：总色差值（CIE 1976）
光泽度测量
当光线照射到表面时，会发生漫反射和镜面反射。因此，在漫反射中，光线沿不同方向反射，而在镜面反射中，来自入射光线的光沿相同方向反射，或者说是相同的，其角度等于入射角。这样，一种反射类型，镜面反射，负责光泽的效果。
可以According to UNE-EN ISO 2813 标准对涂料、油漆和清漆及相关产品进行测试以测量 20º、60º 和 85º 的 镜面光泽度。
Tests can be performed according to ASTM D523 Standard Test Method for Specular Gloss to evaluate gloss for three geometries (incidence and observation angles) of 20º, 60º and 85º.
Gloss evaluation test for geometry 75º according to standard UNE-EN ISO 8254-1:2009 "Paper and board. Specular gloss measurement".
