Appearance of a Class A Surface Orange Peel and Brilliance

1 Percezione visiva dell'aspetto di una superficie di classe A

Normalmente valutiamo una superficie mettendo a fuoco il nostro occhio sull’immagine riflessa di una sorgente luminosa, oppure sulla superficie stessa. Quando osserviamo l’immagine riflessa di una sorgente luminosa, giudichiamo la qualità di formazione dell’immagine, ossia la capacità di riflettere gli oggetti. La sorgente luminosa può apparire brillante o opaca (gloss, brillantezza). Quando si riflette un profilo l’area scura può apparire più chiara (velatura, haze) e il bordo può apparire più sfocato o più distinto (DOI).  (Fig 1) 
Quando focalizziamo i nostri occhi sulla superficie, otteniamo ulteriori informazioni sulle dimensioni e sulla forma della struttura. Vediamo queste strutture come un modulo ondulato di aree chiare e scure. Questa ondulazione è spesso indicata come buccia d'arancia o difetti di distensione/livellamento. (Fig 2)

Entrambe le modalità di osservazione, esaminate individualmente contribuiscono alla percezione globale dell’aspetto della superficie. 

In order to guarantee reliable and practical quality assurance, it is necessary to define appearance with objective and measurable criteria. Accurate characterization of appearance does not only help to control quality but also supports to optimize the manufacturing process and thus leads to an improved quality of the product.
The total appearance and the visibility of structures depend on the structure size, the observing distance and the image forming quality.

Image 1 Focus on reflected image

Image 2 Focus on surface

2 Waviness and Structure Size

The waviness of automotive paints is in a range of approx. 0.1 to 30 mm lateral wavelength. These phenomena are often visually evaluated and subjective terms like degree of orange peel or texture are used as descriptions. Orange peel can be seen on high gloss surfaces as a wavy pattern of light and dark areas. Depending on the slope of the structure element the light is reflected in various directions (Fig. 3). Only the elements reflecting the light in the direction of our eyes are perceived as light areas. 

Visibility of structures is dependent on the observing distance. The greater the distance, the smaller objects will appear. Structures with a size of 10 to 30 mm can best be seen at a distance of approximately 3 m. Fine structures in a range of 0.1 to 1 mm can only be recognized at a close distance. (Fig. 4)

The resolvable structure size is also dependent on the observing distance. Very fine structures that are below the human eye’s resolution (smaller than 0.1 mm) can no longer be recognized as a light / dark pattern, even at a close distance. The result is a reduction of the image forming quality (IFQ). At three meter distance, structures between 1 - 3 mm can hardly be resolved as a waviness but influence the appearance (Fig. 5)

Image 3 Visual perception of orange peel

Image 4 Appearance changes with observing distance

Image 5 Resolution of the human eye

3 Image Forming Quality (IFQ)

The higher contrast and sharpness of a reflected object, e.g. the edges of black and white lines, the better the image forming quality will be. Fine structures disturb the reflected image, consequently edges become blurry and are no longer sharp.
Image Forming Quality at a close distance: Distinctness of Image (DOI)
DOI can also be described with terms like brilliance, sharpness or clarity. The DOI is diminished reduced by very fine structures close to the human eye’s resolution (smaller than 0.3 mm). (Fig. 6)
Image Forming Quality at a far distance: Wet Look
At a distance of three meters, the image forming quality is mainly influenced by structures between 1 - 3 mm. This effect is also referred to as Wet Look. 

Image 6 Brilliance – DOI

4 Objective Measurement of Waviness with wave-scan

The wave-scan simulates our visual perception. Like our eyes, the instrument optically scans the wavy light / dark pattern. A laser point light source illuminates the specimen at a 60° angle and a detector measures the reflected light intensity at the equal but opposite angle. The orange peel meter is rolled across the surface and measures point by point the optical profile of the surface across a defined distance. The wave-scan analyzes the structures according to their size. In order to simulate the human eye’s resolution at various distances, the measurement signal is divided into several ranges using mathematical filter functions (Fig. 7):

Image 7 wave-scan measurement principle

5 Objective Measurement of Image Forming Quality

5.1 Limitations of gloss measurement

A glossmeter measures the specular reflection which is reflected at the equal but opposite angle of illumination. The light intensity is registered over a small range of the reflection angle. On flat surfaces, the incident light is directly reflected in the main direction of reflection and completely measured by the detector. On curved samples the reflected light beam is more or less scattered in directions outside of the detector range (Fig. 8). Thus, the measured gloss values cannot be compared to those of flat surfaces. Additionally, the measurements are not very repeatable due to the curvature influence.

In addition, gloss readings are dependent on the refractive index of the coating material. Figure 9 shows a 1K and 2K system. Visually, no gloss differences can be recognized. Yet, the measurement results show a gloss difference as the two paint systems have different refractive indices. As a consequence, traditional gloss measurement cannot be used to compare the gloss of different materials.

5.2 Objective measurement of Image Forming Quality with wave-scan

Structures smaller than 0.1 mm influence our visual perception, therefore the wave-scan uses a CCD camera to measure the diffused light caused by these fine structures. A green LED illuminates the surface at 20°. A CCD camera analyzes the reflected image of the light source`s aperture (Fig. 10). If there are no fine-micro textures, all light will be detected within the image of the aperture (= max). Otherwise light will be detected outside (= scatter value). The ratio of these two components is expressed in a new value: “Dullness” (structure < 0.1) Dullness measurement is independent of the refractive index (Fig. 11) and the curvature of the surface as it is not an absolute, but relative measurement.

Image 8 Gloss measurement on curved surface

Image 9 Refractive index versus 20° Gloss

Image 10 measurement principle dullness

Image 11 Refractive index versus Dullness

6 wave-scan Scales

6.1 Structure Spectrum

The values of dullness and Wa to We form a so-called “structure spectrum”. This allows a detailed analysis of the appearance of a Class A surface and its influencing factors, being material and / or application parameters. (Fig. 12)

6.2 Customer specific wave-scan scales

The detailed information of the structure spectrum as well as LW and SW became the basis to correlate customer specific appearance perceptions resulting in a variety of scales. In addition, visual perception studies were done and correlated with wave-scan measurement data as well as the obsolete Hunter Dorigon measurement data to correlate with the DOI as described in ASTM E430:

Image 12 wave-scan structure spectrum

7 Typical Applications for Trouble Shooting

7.1 Influence of baking position

In general, horizontal surfaces have a better flow and leveling behavior than vertical surfaces due to the influence of gravity, i.e. resulting in lower long wave values (Wc, Wd, We). The shorter waves are hardly influenced by the baking position. (Fig. 13)

7.2 Influence of film thickness

The structure spectrum can help optimize the appearance, e.g. in determining the optimum film thickness. In general, increasing clear coat thickness will improve flow and leveling. In figure 14 this can be seen in decreasing Wc and Wd values.

7.3 Substrate influence

In figure 15, the substrate influence is analyzed. The substrate roughness can telegraph through the clear coat and reduce the brilliance of the topcoat. Sample D is a laser-textured panel with a specific texture resulting in lower SW values than on standard car body steel panel.

Image 13 Influence of Baking Position

Image 14 Influence of Film Thickness

Image 15 Substrate Influence

8 wave-scan Measurement on High to Semi Gloss Surfaces

Appearance control is no longer limited to final topcoat inspection. The wave-scan dual, orange peel meter scans the optical profile of high gloss surfaces using a laser light source. An additional, infrared - high energy LED allows measuring the same structure spectrum (0.1 - 30 mm) on medium gloss surfaces. The dullness measurement is recorded with state-of-the-art CCD camera technology. It gives information on the image forming qualities of the surface caused by structures < 0.1mm.

Close the loop by checking the quality of each process step
Thus, the surface quality after each paint process step can be objectively evaluated (Fig. 16). No more guessing which substrate layer is influencing the final appearance. The wave-scan dual will help you to objectively analyze appearance problems and reduce the time necessary for trouble shooting.

Example: Influence of Steel Quality on Final Appearance
Step 1: Appearance Control after E-coat
Same E-coat system was applied on rough and smooth steel. The influence of rougher steel can be seen in increased Wb and Wc-values (Fig. 17).
Step 2: Appearance Control after Primer Surfacer
The primer surfacer was applied on both panels. The roughness of the steel quality can still be detected in increased Wb and Wc- values. This primer system could not completely cover the steel influence (Fig. 18).
Step 3: Appearance Control after Topcoat
The final appearance shows higher shortwave values on the rougher steel panel. Therefore, the smooth panel will appear more brilliant (Fig. 19).
wave-scan dual – a diagnostic tool for trouble shooting and optimizing appearance
Now, you can establish appearance specifications for each paint layer to ensure the final appearance is always on target.

Image 16 Paint process steps

Image 17 Appearance Control after E-coat

Image 18 Appearance Control after Primer Surfacer

Image 19 Appearance Control after Topcoat

9 Summary

The appearance of a Class A surface can be influenced by many different parameters. To keep the quality of your product smooth and brilliant with uniformity between adjacent panels the wave-scan objectively measures waviness and distinctness of image. The measurement results are correlated to visual perception studies resulting in customer specific scales which can be used for quality control to ensure a stable process. In order to optimize appearance and improve harmony between body and add-on parts the detailed structure spectrum data will guide you in the right direction to either improve process or material properties.