Medición de textura superficial 2D/3D

1. Medición de color tal y como se ve

El spectro2profiler utiliza una iluminación circunferencial a 45° desde seis direcciones y una visualización de 0°. La probada e innovadora tecnología BYK LED garantiza un rendimiento excepcional: la estabilidad a corto y largo plazo y la temperatura se controlan con la mayor precisión posible. El punto de medición extragrande con iluminación homogénea garantiza lecturas altamente repetibles y representativas. En conjunto, se garantiza la máxima precisión y concordancia entre instrumentos y se permite el uso de estándares digitales, la clave para la gestión global del color.

Figure 1 Medición de color con Configuración a 45°c:0

2. Brillómetro convencional para reflectividad y brillo a 60°

Por razones históricas, el spectro2profiler tiene incorporado un brillómetro de 60. La reflectividad y el brillo se basan en la interacción de la luz con las propiedades físicas de la superficie de la muestra. La intensidad depende del material y del ángulo de iluminación. Los resultados de medida de un brillómetro convencional están relacionados con la cantidad de luz reflejada de un estándar negro con un índice de refracción definido. Los instrumentos de medición actuales son muy precisos y ampliamente utilizados en la industria, pero tienen puntos débiles en la medición de superficies estructuradas. Las sombras proyectadas y las áreas invisibles al detector de medición pueden falsear la medición

Figure 2 Proyección de sombras con el uso de medición de brillo tradicional 60°

3. Medición de reflectividad 2D resuelta espacialmente

Además, la percepción del brillo no solo depende del brillo especular, sino también del contraste observado entre los reflejos especulares y las áreas superficiales de reflexión difusa. (1) Un medidor de brillo convencional no es capaz de capturar un comportamiento reflectante más complejo, como reflejos distribuidos espacialmente, por ejemplo, picos brillantes junto a valles mates que se producen en estructuras similares al cuero.
Para superar esta limitación, el spectro2profiler ofrece una nueva tecnología basada en cámaras para capturar la distribución espacial de la reflectividad. Una configuración de iluminación en línea elimina las sombras proyectadas, las áreas invisibles y las distorsiones de perspectiva para que la medición sea independiente de la orientación. La cámara adquiere imágenes de reflectividad 2D. Las figuras 3 y 4 muestran el principio de medición del spectro2profiler y un ejemplo de un mapa de reflectividad en escala de grises en el que cada píxel representa un valor de reflectividad que permite un análisis más detallado de las distribuciones de reflectividad de una superficie.
 

Figure 3 Configuración de la medida de reflectividad espacialmente resuelta

Figure 4 Mapa de reflectividad de una pintura en polvo con spectro2profiler

4. 3D Análisis Topográfico con técnica fotométrica estereo

Hasta ahora, la evaluación visual era la única forma de emitir un juicio completo de una superficie texturizada. Es por esta razón que los microscopios 3D se utilizan para proporcionar información muy detallada de la estructura de la superficie en el laboratorio con fines de investigación, pero no son adecuados para un análisis rápido y sencillo de la calidad de la producción.
El spectro2profiler utiliza la técnica estéreo fotométrica para estimar las superficies normales con el fin de calcular una topografía 3D de esa superficie. La técnica fue introducida originalmente por Woodham en 1980. (1) Las superficies normales se calculan observando un objeto desde diferentes direcciones de iluminación. Con cada dirección, el objeto proyecta diferentes sombras sobre la superficie y una cámara adquiere imágenes para cada iluminación. Usando la forma del sombreado, se estima la curvatura de la superficie y se puede calcular el mapa de altura del objeto. El resultado es una topografía 3D real de la superficie del objeto medido. La unidad P-µm es la altura percibida.
 

Figure 5

Figure 6

5. Watershed analysis to define structure cell sizes

Topographies such as leather grains or coarse powder coated structures can be characterized by their structure cells. To divide the topography into cells, the watershed algorithm is used, a region-based segmentation approach. One can imagine that the algorithm gradually floods the valleys of the topography, building rivers until hill areas are surrounded. (3) These areas will be defined as cells, marked as green lines in Figure 7.
Characteristic features of the surface can be calculated based on the watershed segmentation results to compare different structures or grains. Spatial length scales result from camera calibration and are traceable to SI-units. The calculated average cell size correlates to our visual impression of coarseness. The distribution of individual cell sizes is an indication for the uniformity of the surface structure. For example, a natural leather structure varies in uniformity depending on the part of the cow skin. A textured paint can form agglomerations during the wet paint application if the application parameters vary resulting in an inhomogeneous appearance. The normalized cell size deviation is calculated by dividing the cell size distribution with the mean cell size. It is an objective measure to compare the uniformity of different structures independent of its absolute cell size.

Figure 7 3D topography data of a leather grain

Figure 8 Watershed segmentation of topography data

6. Combination of 3D topography data with 2D reflectivity data

To assess the overall appearance of an object, it is necessary to measure surface structure and reflectivity in parallel, as they are mutually interdependent, but are combined for an overall visual assessment. (4) Because our eyes are only capable to acquire 2D information, the human visual system reconstructs 3D information of objects in our brain using shading and reflections. (5) That means, the perceived depth of a structure is dependent on the reflection behaviour on the hills and valleys. Since the spectro2profiler uses the same camera and lens system for the acquisition of 3D topography and 2D reflectivity data, it is possible to combine the data of both measurement principles (Figure 8 and Figure 9). Thus, the reflection of hills and valleys can be separated. The difference between reflection of hills and valleys, describes the contrast and perceived depth of a structured surface.

Figure 9 3D topography data set: Height is gray scale coded

Figure 10 2D reflectivity data set: Intensity of reflectivity is gray scale coded

7. Practical example from the automotive industry

Many automotive interior components have a leather-like look and are manufactured by different suppliers with different processes and made of various materials. The appearance of the products surface is analyzed in the different development phases, e.g., at the very beginning by the design department in the grain development to approve suppliers and at the very end by quality control in production. Leather grain structures can appear different in contrast although color and 60° gloss are the same (Figure 10).  This can be caused due to different reflectivity levels of the surface on hills and valleys. Up till now this had to be evaluated visually which is subjective and non-repeatable. The measurement results in the table display how the reflectivity contrast Rc can distinguish the samples despite having the same color and 60° gloss. Moreover, the results of the reflectivity for hills and valleys provide details about what causes the different reflectivity contrasts.

The new measurement parameter reflectivity contrast is an ideal measure for production QC of injection or slush moulded parts.

Figure 11 Four dashboard slush skins of same material with different levels of contrast

8. Coarse paint - powder coated samples

In this example power coated panels of the same color with a fine to coarse structure are evaluated. Visually the samples differ due to different cell sizes. This specific difference is caused by variations in film thickness, but also additives or temperature changes can have an impact on surface texture.
In the smart-chart data table (Figure 12) one can clearly see that the four samples have the same color and 60° gloss values. A differentiation can be clearly done by the mean cell size.

Figure 12 Four powder coated panels with different structure

Figure 13 Measurement results displayed in smart-chart

9. Eroded plastic parts or fine structured paint

Eroded plastic parts or fine structured paint as shown in Figure 13 have structures too small to segment into visible cells. Therefore, another approach is necessary to evaluate the topography data.
Local maxima and minima are detected and the Micro Peak Distance µPd (µm) is calculated as the peak distance between adjacent peaks on the topography (Figure 14). It correlates with the visually perceived roughness of these fine structures. The higher the value, the rougher the structure appears. The effect of roughness is often enforced, indicated or illustrated by the amplitude of the structure peaks which is measured by the Micro Local Amplitude µA (P-µm).
The results in the smart-chart data table (Figure 15) show the rougher the sample appears, the higher is the micro peak distance and the micro mean amplitude, respectively.
In addition to roughness the visual perception is also influenced by the reflectivity of the surface. This “glossy appearance” is mainly dominated by the contrast between sparkling spots and non-sparkling spots. The spectro2profiler captures the effect with the measure Micro Reflectivity Contrast µRc using the 2D spatial reflectivity information from the camera image.

Figure 14 Three panels with fine structured paint

Figure 15 Calculation of Micro Peak Distance µPd (µm)

Figure 16 Measurement results displayed in smart-chart

10. Summary

spectro2profiler is a game changer and marks a turning point in the analysis of structured surfaces. The combination of 45°c: 0° color measurement, 60° specular gloss, 3D topography and 2D reflectivity in one easy to use instrument is a milestone in the objective measurement control of textured surfaces. At this moment, the spectro2profiler incorporates four algorithms for surface structure analysis - leather-like structures, inverted leather-like structures, coarse paint textures and fine paint or plastic textures.  Due to its excellent technical performance regarding repeatability and inter-instrument agreement, digital standards can be used as a reference, allowing a flawless communication within a global supply chain.
From now on, our visual perception of colour, gloss and structure can be evaluated in a holistic and objective approach, color and appearance harmony when combining different components can be optimized and all this is possible in the laboratory as well as on the production line with the portable spectro2profiler.