7 minutes

Introduction

When it comes to color measurement, selecting the right instrument geometry is crucial to obtaining accurate and consistent results. Among the options available, two commonly used optical geometries are the Sphere and 45/0°. These geometries play a pivotal role in capturing color information, and understanding when to use each one can significantly impact the desirable color measurement outcome.

In this article, we will review the benefits, challenges, and real-world applications of each geometry, helping you make an informed choice for your color measurement needs.

Understanding the General Optical Arrangement

Overview of the 45/0° Optics

The 45/0° geometry involves a controlled light source positioned at a 45° angle relative to the sample’s surface. The color sensor, on the other hand, is positioned at 0° from the sample surface, mimicking the perspective of the human eye when viewing the sample directly from above. By arranging the light source and the observation in this way, the observer does not see any specular reflection of the light, otherwise known as glare. This geometry is well-suited for capturing color reflections on smooth and matte surfaces, resembling human visual perception in real-world environments. In simpler terms, imagine looking directly down onto a sample with light shining down onto the surface at a 45° angle.

Figure 1:

Overview of Sphere Optics

In contrast, the sphere geometry employs a diffused light source within an optical sphere. The sphere’s interior surface is coated with a highly reflective material, and the light source is positioned on the sphere’s side wall. This design ensures light scatters within the sphere before illuminating the sample, providing a diffuse illumination condition. The sample is then observed at an 8° angle from the perpendicular.

Figure 2:

Specular Included and Excluded in Sphere Spectrophotometers

An additional attribute of spherical spectrophotometers is that they offer color measurement through two distinct measurement methods: specular included and specular excluded. Specular included readings encompass the gloss or specular component, while specular excluded readings eliminate this gloss component. A simplified explanation is that measuring under Specular Included would identify the actual color regardless of surface effects while measuring under Specular Excluded would be most similar to visual/perceived color by your human eye. For more information, please read “What’s the Difference Between Spectro 1 and Spectro 1 Pro”.

Nearly identical reflectance curves emerge when measuring a smooth, matte surface using both specular included and excluded. However, Figure 3 illustrates two reflectance curves of a high-gloss, powder-coated sample (RAL5021). Notably, specular included measurements yield higher reflectance values since it’s reading the actual color. The specular excluded measurement will measure a lower reflectance value as the gloss makes the sample appear slightly darker because the specular reflection has been removed.

Figure 3:

It’s important to note that specular excluded readings from a sphere instrument may deviate from those of a 45°/0° instrument, which also measures in Specular Excluded. Among their technical differences, the more obvious difference is the diffused illumination of the sphere versus the direct 45° light source hitting the surface.

Benefits and Challenges of 45/0° Geometry

The 45/0° geometry is particularly advantageous when measuring color on smooth and matte surfaces. It captures color in a manner that closely matches human visual perception, making it an ideal choice for applications focused on inspection and quality control. However, this geometry may face challenges when used on textured or irregular surfaces, where the light reflection varies significantly at different angles.

Example:

Consider a scenario in the consumer packaged goods industry. A manufacturer produces cereal boxes with pre-determined brand colors that must be produced under a defined tolerance. To ensure uniformity across production runs, they employ a 45/0° color measurement device. This device enables them to verify that the color of the first to the twenty-thousandth production run remains within tolerance to the reference color, ultimately enhancing the visual appeal and brand reputation of the product.

Benefits and Challenges of Sphere Geometry

The sphere geometry, with its diffuse illumination, is well-suited for measuring a wide range of surfaces, including different gloss levels and textured samples. It accommodates both specular included and excluded readings, providing flexibility in analyzing appearance and actual color variations. However, sphere geometries typically have a larger aperture, making it more difficult to measure smaller measurement sample sizes.

Example:

Imagine a retail paint store that offers a wide array of paint options. To accurately match a customer’s preferred color, the store employs a sphere-based color measurement instrument. This enables them to measure color on different surfaces, including textured samples like fabric swatches, ensuring customers receive paint that closely aligns with their expectations.

Aligning Optical Geometries to Reference Data

Regardless of whether you choose the 45/0° or sphere geometry, aligning your optical geometry to reference laboratory data is essential for consistent color measurement. Matching the optics of your Variable instrument to any reference instrument at your organization provides a more consistent method to measure color, which is vital for industries reliant on precise color consistency.

Conclusion

In color measurement, optical geometries are essential tools that impact how we perceive and measure color. The 45/0° and sphere geometries offer unique benefits and face specific challenges. The choice between them depends on your specific needs and the surfaces you frequently measure. You can confidently and consistently measure color by aligning your optical geometry with reference data. As technology advances and industries evolve, accurately measuring color remains critical to quality control and customer satisfaction.