Optical Geometry of Color Measurement Instruments

       The optical geometry of a color measurement instrument is one of the key factors that determine measurement accuracy and applicability, especially in colorimeters and spectrophotometers. This type of geometry affects the angle at which the light source illuminates the sample and the angle at which the sample is observed. Common optical geometries in color detection instruments include: 45°/0° , 0°/45°, d/8° , 8°/ d. In the description of the instrument geometry, the first number is the illumination angle or method, and the second number is the observation angle or method. They are all relative to the perpendicular line of the surface of the sample being measured.

The production of color requires three key factors: object, light source, and observer. Changes in any one factor will affect the final color perception, so the three must work together to accurately present and perceive color. As a tool for accurately measuring and quantifying color, color detection instruments must ensure that other factors other than the sample remain consistent during detection. The geometric structure of the color detection instrument is to illuminate the sample and receive its reflected light through specific lighting angles and observation angles to detect its color. Different geometric structures affect how light reaches the observation position in the defined color space, which in turn affects the measurement results. Therefore, it is very important to choose a suitable geometric structure as a standard to accurately detect each sample.

                 Illumination angle of optical geometry

       The illumination angle refers to the angle at which the light source illuminates the sample surface, which determines how to eliminate or control the gloss and texture effects on the sample surface. The illumination angle in optical geometry refers to the angle at which the light source illuminates the surface of an object. Generally speaking, the standard illumination angles in color measurement include 45° illumination angle and 0° illumination angle . The 45° illumination angle is mainly used to reduce the impact of specular reflection on measurement and is suitable for samples with smooth surfaces, while the 0° illumination angle is more suitable for some specific applications.

Observation angle of optical geometry

Observation angle in color measurement refers to the angle at which the sensor (or detector) receives the light reflected from the sample. That is, from which direction the light reflected or transmitted by the sample is observed and measured. Different observation angles affect the measurement results because the surface characteristics of the object (such as gloss and texture) will produce different visual effects depending on the observation direction.

Single-item lighting system in color measurement instruments

The single-item lighting system is a common geometric structure in color measurement. This structure usually uses a 45°/0° or 0°/45° lighting and observation angle combination, which can better measure the surface reflected light and is widely used in color detection of textiles, plastics and other opaque materials.

1. 45°/0° structure : The light source illuminates the sample at a 45° angle, and the sensor receives the reflected light at a 0° angle. This structure can effectively eliminate the interference of surface texture or gloss, and is suitable for color measurement of uniform or flat surfaces, such as printed materials, plastic parts, etc.

2. 0°/45° configuration : The light source illuminates the sample at a 0° angle, while the sensor receives the reflected light at a 45° angle. This geometry is similar to the 45°/0° configuration and can also reduce the effect of surface reflections. It is often used to measure the color of product surfaces, such as coatings, plastics, and other glossy materials.

Integrating sphere structure in color measurement instrument

The Integrating Sphere structure is an advanced optical system that creates uniform illumination through a d/8 or 8/d geometry. This geometry is more tolerant of the angle of the sample being measured and is suitable for a variety of surface types and color measurement applications.

1. d/8° (integrating sphere configuration) : This configuration uses a diffuse light source (d), usually in conjunction with an integrating sphere, to evenly illuminate the sample at multiple angles, with the sensor receiving reflected light at an angle of 8°. The d/8° configuration is less sensitive to surface gloss and texture, so it can more accurately measure samples with high gloss or significant texture. This configuration is widely used for measuring various materials, such as textiles, plastics, paper, and metals.

2. 8/d structure : In this structure, the sensor receives the reflected light from the sample at an angle of 8°, and the light source is evenly distributed on the sample surface through an integrating sphere. Since the 8/d structure can reduce the influence of surface gloss, it is very suitable for measuring high gloss or gloss-sensitive materials such as coatings, metals and plastics.

Optical structure of 3nh spectrophotometer

What is the difference between 45°/0° and d/0°

Different geometric structures have their own characteristics in terms of measurement results, surface adaptability and accuracy. The 45°/0° structure is suitable for reducing specular light interference, while the d/8 structure can capture the surface light scattering characteristics more widely. Understanding these differences will help you choose a measurement system more rationally.

45°/0°: This geometry is particularly advantageous when matching or evaluating color samples, as it closely aligns with visual assessments. However, a notable limitation is its inability to measure transparent or translucent samples. In this setup, the specimen is illuminated directly at a 45° angle, while the detector measures the light reflected at a 0° angle (or vice versa). This configuration effectively excludes gloss, resulting in a measurement that better matches the visual impression perceived by the human eye. It's similar to adjusting your viewing angle to reduce glare when reading a glossy magazine in direct sunlight. Unlike sphere-based measurements, which attempt to mimic this effect with a gloss separator, angle geometry reveals the “true” visual difference between matte and glossy surfaces (eg, a matte color swatch versus glossy screen ink). Angle-based geometry is particularly suitable for smooth or minimally textured surfaces.

d/8°: An advantage of this setup is the option to attach a gloss separator to an additional opening in the sphere, which prevents illumination of the specimen at an 8° angle, thereby eliminating gloss. Measurements with gloss separators are beneficial primarily for high-gloss specimens. When using this sphere geometry, the surface structure of the specimen has minimal impact on the results, meaning that measurement values remain relatively consistent across specimens with varying surface textures (eg, textiles, rough plastics, etc.).

How to Choose the Right Color Measurement Instrument Geometry

Choosing the right optical geometry is crucial for accurate color measurement. The applicability of different geometries in color measurement depends mainly on the properties of the material surface, measurement requirements, and industry standards. The following are the specific steps and precautions for choosing the right geometry matter:

1. Choose the geometry based on the material surface properties

· Smooth surfaces (such as plastics, metals, and paints) : 45°/0° or 0°/45° geometry is recommended . This combination effectively reduces the interference of specular reflections through an inclined light source and a vertical observation angle, making the measurement more consistent and suitable for materials with smooth reflective surfaces.

YS4510 45/0 Grating Spectrophotometer

· Rough or irregular surfaces (such as textiles and leather) : d/8 or 8/d geometry is recommended . This diffuse reflection geometry ensures that the light is evenly distributed in the integrating sphere, thereby obtaining the average reflection value at all angles of the sample surface, which is suitable for samples with diverse textures.

3nh Spectrophotometer ST-700d

2. Consider the influence of gloss

· High gloss materials (e.g. polished metal, mirror paint) : Using 45°/0° geometry works well because it tilts the light source to capture only color reflections and avoid excessive specular light reflections, providing more stable measurement data.

· Matte or low gloss materials : d/8 and 8/d geometries are more suitable because the diffuse reflectance integrating sphere can reduce the angle effect, making the measurement of matte materials more uniform and accurate.

3. Based on the specific needs of the application scenario

· Textile industry : Textiles usually have irregular surfaces and complex textures. It is recommended to use the d/8 geometry, which can better capture the color information at different angles on average.

· Plastics Industry : There are many types of plastic samples with large variations in gloss and transparency, so it is necessary to select 45°/0° , 0°/45° or d/8 geometry according to the specific properties of the sample.

· Paint industry : High gloss paints often use a 45°/0° structure to measure their true color, while low gloss paints are more suitable for using a d/8 structure.

4. According to international standards and customer requirements

· International standards (such as CIE, ASTM, etc.) usually specify the geometry of color measurement instruments. It is key to refer to these standards and consider customer requirements when developing quality control standards. For example, in many industries, 45°/0° and d/8 structures are used as common standards to ensure that measurement results are consistent worldwide.

5. Weighing costs and measurement accuracy

· Some complex geometric structures (such as the integrating sphere d/8 structure) are more expensive, but can provide a wider range of sample applicability; while the 45°/0° structure is relatively low-cost and more suitable for large-scale standardized applications. Therefore, you can make a reasonable choice based on your budget and accuracy requirements.

6. Test and validate the chosen geometry

· In order to ensure that the selected geometry is most suitable for a specific application scenario, it is recommended to conduct a comparative test on the measurement results of several geometries. By comparing the measurement consistency and accuracy of different geometries, it can be further verified whether the selected geometry meets the measurement requirements.