Using a 4K Digital Microscope in RGB Measurements

Product quality is affected by subtle changes in color. For example, product colors varying from one lot to another and some exterior parts having different colors from the others negatively affect the quality of the product appearance and also reduce product reliability. With items such as functional films, different parts having different colors and different types being mixed in but not being visibly detectable may negatively affect the functions and performance of the product and may lead to defective products being released to the public.
This section introduces basic knowledge for color measurement methods and for color systems as well as examples of RGB measurement using our 4K digital microscope.

Color Measurement Methods

Using a 4K Digital Microscope in RGB Measurements

When people view an object, they experience its colors differently depending on factors such as the ambient light, brightness, and viewing angle. Colors are also experienced differently from one observer to the next. However, in mass production in modern industries, these differences lead to variations in products, parts, and can even decrease product quality.
To prevent these defects, it is important to accurately measure colors and to identify colors from these measured values. Hence, colorimeters and spectrophotometers are used at many manufacturing sites to measure and quantify colors. The characteristics of and differences between these devices are explained below.

What is a colorimeter?

In a human eye, the retina separates the light reflected from a viewed object (visible light with a wavelength of 400 to 700 nm) into red (R), green (G), and blue (B) and transmits this stimulus to the brain where it is judged as color. In the same manner, a colorimeter measures these three types of light stimuli, quantifying them as tristimulus values: X, Y, and Z.
This measurement method is referred to as the stimulus value direct reading type. This type of colorimeter is relatively inexpensive, compact, and easy to handle, so it is widely used in color inspections and similar operations at manufacturing sites. On the other hand, the numeric values vary depending on the light source, so this device is not suited to the advanced analyses performed with spectrophotometers, explained next.

What is a spectrophotometer?

A spectrophotometer measures reflectance by using a sensor with multiple receivers to disperse into multiple wavelengths the light emitted from the light source and reflected from the target. In addition to calculating tristimulus values X, Y, and Z, this device is also capable of color (wavelength) analysis via spectral reflectance with a graph or something similar.
Differing from the stimulus value direct reading type colorimeter, a spectrophotometer can use the data of various light sources to investigate differences in viewing methods attributable to the light source (color rendering), the phenomenon that occurs when two colors appear to match under one lighting condition (metamerism), and differences in target surface conditions. Although they are more expensive than colorimeters, spectrophotometers enable more advanced color analysis and are therefore mainly used in research and development.

CIE Color Systems and Their Types

Measuring and quantitatively evaluating the colors of a target requires a common definition for color, which is an ambiguous concept. Hence, the International Commission on Illumination (abbreviated as CIE for its French name Commission internationale de l’eclairage), which has established various international standards on science and technology in the fields of light and lighting, has defined color systems to allow people to accurately distinguish colors in a common manner. They are known as CIE color systems and are widely used in diverse industrial fields. Contained therein, the RGB color system, XYZ color system, and L*a*b* color system are typical color systems and are explained below.

RGB color system

This is the first color system established by CIE. This system expresses color as a mixing ratio (an additive mixture of colors) of the three primary colors (also known as tristimulus, reference color stimuli, and color stimuli) R (red), G (green), and B (blue) that actually exist. The spectrum of the three primary colors is R = 700 nm, G= 546.1 nm, and B = 453.8 nm. However, there are colors that cannot be expressed as an additive mixture of colors in the RGB color system. For example, bright cyan cannot be created with any combination of the three primary colors.

RGB color system

Because color combinations in the RGB color system are also used to display colors on LCD monitors, this system is also known as the monitor color system.
Each of the three primary colors is expressed as gradations of intensity ranging from 0 to 255. By combining these gradations, it is possible to express 256 to the power of 3 = 16,777,216 different colors. White is displayed at the point where the three primary colors overlap.

XYZ color system

The XYZ color system is widely used in various industrial fields. Using X, Y, and Z to express colors, this system was designed to mathematically avoid the problem of the RGB color system not being able to accurately reproduce monochromatic light of the color gamut.
R, G, and B are the spectrum that actually exists and are called true colors. On the other hand, X, Y, and Z in this color system are the colors of mathematically converted light. Because some of these colors do not actually exist, X, Y, and Z are called false colors. Instead of systemizing colors perceived by people, using false colors with the purpose of displaying colors in a quantified manner makes it possible to express all colors as X, Y, and Z values.

XYZ color system

The three axes of the XYZ color system are assigned as follows.
X: Amount of red (does not contain brightness)
Y: Amount of green (the only value that contains brightness)
Z: Amount of blue (does not contain brightness)
X, Y, and Z have a three-dimensional interrelationship, but the figure on the right expresses a two-dimensional graph in which Z has intentionally been omitted. This graph is known as an xy chromaticity diagram. X is plotted on the horizontal axis and y on the vertical one, and the graph shows a horseshoe shape, indicating just the primary wavelength corresponding to the hue and the excitation purity corresponding to the saturation. The brightness is not indicated.
The point close to the center is called the white point, indicating that this is where the color is white. Also, from the positions of cyan (C), magenta (M), and yellow (Y), it can be seen that the saturations of colors such as printing ink and paint are low on the xy chromaticity diagram, indicating how wide a range is covered by the XYZ color system.

L*a*b* color system

L*a*b* color system

This color system was defined by CIE in 1976. L*a*b* is read as L star, a star, b star.
This section describes the conditions indicated by the positive and negative values of each axis together with the following figure.
L* axis: Axis that indicates the lightness. Positive values indicate a whiter (brighter) color, and negative values indicate a blacker (darker) color.
a* axis: Axis that indicates the hue from green to red. Negative values indicate a stronger green hue, and positive values indicate a stronger red hue.
b* axis: Axis that indicates the hue from blue to yellow. Negative values indicate a stronger blue hue, and positive values indicate a stronger yellow hue.

The color difference (ΔE) can be found by using the values obtained from these three axes in the color difference formula. Colorimeters, widely used in quality inspections, also calculate color differences with this method.
Setting this ΔE as a control index is useful in improving quality by enabling quantification in color difference management and comparative judgment against reference colors in industrial fields.

Examples of Optimizing Color Measurements with a 4K Microscope

In quality management at manufacturing sites, handheld colorimeters can be used to easily increase the number of measurements but are not suited to highly accurate measurements. Spectrophotometers can perform accurate measurements and are suited to a wide range of measurements. However, neither of these devices is suited to color judgement in product research or quality assurance when the evaluation point is so small that it must be magnified to be measured.
KEYENCE’s ultra-high accuracy VHX Series 4K digital microscope can acquire 4K magnified images with high resolution, enabling accurate measurement of RGB values. Color management is now possible for microscopic targets and measurement points.
The VHX Series uses sRGB (standard RGB),* which is widely used in various industries, including the electronics field. Measured values can be easily converted to XYZ values with an Excel sheet.

Tips:What is sRGB?
This is an international standard established by the International Electrotechnical Commission (IEC) in 1999. A wide range of products including monitors, printers, and digital cameras conform to this standard. It has excellent versatility and is highly compatible with monitors and other color modes. Because it makes advanced color management easy, it is also used in fields such as the capturing, editing, and printing of images.

Color difference evaluation by film RGB measurement

The VHX Series 4K digital microscope is equipped with an advanced optical system and a 4K CMOS image sensor that combines a large depth of field with high resolution imaging capabilities.
The VHX Series can easily and quickly acquire clear, magnified images of films, which have various surface conditions. For example, even if the surface is rough, the large depth of field allows for automatic focusing throughout the entire field of view. It is difficult to determine the lighting conditions for glossy films, but the Multi-lighting function, which automatically acquires multiple images under omnidirectional lighting at the press of a button, makes this work easy.
The high-resolution 4K images acquired with these simple operations can be used in highly accurate RGB measurements and color difference evaluations. Additionally, when a past image is selected, the conditions used to capture that image are fully reproduced, enabling fast RGB measurements and quantitative color difference evaluations under the same conditions even for a different sample of the same type of product.

Ring illumination + RGB measurement (300x)

Ring illumination + RGB measurement (300x)

Color difference evaluation of a film using the VHX Series 4K digital microscope

Identification of different film type by RGB measurement

The VHX Series 4K digital microscope can capture clear 4K images and perform highly accurate RGB measurements. These features make it useful in identifying different types of films, which are difficult to determine visually.
In addition to accurate differences between RGB measured values, the VHX Series can clearly capture subtle texture differences—which are normally difficult to check for due to their low contrast—arising from differences in materials and processing.
Furthermore, it is easy to identify film product types by comparing their images side-by-side on a large, 27-inch color LCD monitor specially designed to display images faithful to the target.
This advanced core performance not only simplifies the operations of measuring film RGB values, investigating color differences, and differentiating between product types, but also the advanced observation and analysis of microscopic flaws and defects on film surfaces.

Ring illumination + RGB measurement (200x)

Ring illumination + RGB measurement (200x)

RGB measurement and identification of a different film type using the VHX Series 4K digital microscope

RGB value and XYZ value conversions and automatic report creation

Examples of conversions between XYZ values and RGB values conforming to sRGB are shown in the following table. W.P. is an abbreviation of white point.

RGB system Three primary colors & W.P. XYZ ← RGB RGB ← XYZ
R (0.64, 0.33)
X = 0.4124R + 0.3576G + 0.1805B
R = 3.2410X − 1.5374Y − 0.4986Z
G (0.30, 0.60)
Y = 0.2126R + 0.7152G + 0.0722B
G = −0.9692X + 1.8760Y + 0.0416Z
B (0.15, 0.06)
Z = 0.0193R + 0.1192G + 0.9505B
B = 0.0556X − 0.2040Y + 1.0507Z
W (0.3127, 0.3290)

Excel can be installed directly on the VHX Series 4K digital microscope, making it possible to easily convert measured values and automatically create reports with this one device.

RGB measurement, conversion to XYZ values, and automatic report creation with the VHX Series

  • All image capture settings can be reproduced from past images, so it is easy to capture images under the same lighting and camera conditions.
  • Accurate sRGB values can be acquired just by reducing ambient light as much as possible and by setting the white balance appropriately.
  • Excel can be installed directly on the VHX Series, allowing for sRGB measurements, conversions to XYZ values, and automatic report creation with just this one device.

Using One Device to Optimize RGB Measurement and Other Work Requiring a Microscope

The VHX Series 4K digital microscope can be used to quantitatively measure RGB values of films (as shown in the above example) as well as of various other targets. Additionally, because Excel can be installed directly on this product, this one device can greatly improve the efficiency of all the work related to reports by allowing data output to templates, automatic conversion to XYZ values, and automatic creation of reports.
Furthermore, automatic control prevents the overspecialization of work by allowing even users unfamiliar with microscopes to operate them easily.
The accurate RGB measurement of the VHX Series is supported by its high performance and functionality as a microscope. Consequently, this one device can be used in multiple types of work such as observation, 2D and 3D measurement, and automatic area measurement/count in the research and development as well as quality assurance in various industries.
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