2.1.5Visual appearance of HPL surfaces

The visual appearance of a surface depends on the material, structure, roughness, gloss, color, light, and the observer. These individual factors overlap and influence each other.

• The visual appearance of a surface depends on the material, structure, roughness, gloss, color, light, and the observer. These individual factors overlap and influence each other.
• The material (type, purity, refractive index, homogeneity, etc.) influences the structure, roughness, gloss, and color.
• The structure, such as smoothness, waviness, etc., influences the appearance in terms of light reflection and shadow casting, as well as the function of the surface.
• Roughness, on a smaller scale than structure, influences the light reflection, gloss, and color of a surface.
• Gloss results from the material and roughness, influences light reflection, and depends on the observer.
• The color depends on the material (substrate, pigments, dyes, distribution, purity), roughness, gloss, type of incident light, and the viewer.
• The type of light, its spectral composition, influences the gloss, the color, and thus also the viewer.
• The viewer, in turn, has individual physiological and psychological characteristics that determine their perception of colors and gloss.

The perception of the appearance of HPL surfaces is therefore individual, although there is a certain consensus among observers.

Technically speaking, differences in the low single-digit micrometer range can influence the appearance of HPL surfaces measuring several square meters.

Light
Light can be interpreted as the visible part of electromagnetic transverse waves. An interpretation as a stream of particles (photons) is particularly suitable when considering the interaction of light with matter. The description in quantum electrodynamics combines these two aspects.

Light type, light temperature, light color
The light type is determined by the intensity and composition of the individual wavelengths of a light source and is described by the spectral radiance. The sun has a specific spectrum that is emitted and perceived on Earth, influenced by the position of the sun, the atmosphere, and clouds. For technical purposes, the CIE standard light types have been defined, such as D65 (~6500 K, overcast sky), D50 (~5000 K, morning and evening sun), A (~2850 K, incandescent lamp), and others.

Reflection of light
Reflection refers to the reflection of light at an interface between materials of different optical densities (refractive index). The incident light can be reflected back in a directed or scattered manner, or it can be transmitted or absorbed. If the incident light is reflected at a smooth surface such as a mirror, the reflection is directed with the angle of incidence equal to the angle of reflection. On a rough surface, the incident light is diffusely scattered, i.e., reflected back in many different directions. On glass panes, for example, part of the light is reflected and part is transmitted, with a portion usually being absorbed and converted into heat. Smooth surfaces appear shiny and rough (finely structured) surfaces appear matte.

White surfaces reflect the entire wavelength range of light visible to the human eye. Since this never happens perfectly, different shades of white are produced, such as blue-white or yellow-white. Black surfaces absorb the incident light.

Scattering
The light beam incident from a light source is reflected diffusely and without direction. This requires a surface or interface with a high degree of roughness in relation to the wavelength of the incident light. Such a surface appears matt and has a lower degree of gloss.

Reflection
In the special case of flat mirrors, the observer sees a virtual image of an object in front of the mirror. The virtual image is the same distance from the mirror plane as the object. The image is true to length and angle, but reversed – a “mirror image.” Such a surface appears shiny and has a high degree of gloss.

Reflectance, reflectance values, and average luminance factors
The reflectance value is required for office furniture surfaces to ensure that they do not cause excessive glare (EN 13721, accident insurance). Measuring the reflectance value is a complex process. Alternatively, reflection value charts are available that allow for an approximate approximation. With sufficient accuracy, the light reference value HBW or the luminance Y calculated from CIEL*a*b* color data corresponds to the reflection values. For office furniture, the reflection value should be between 0.15 and 0.75 across countries.

Light Reflection Value (LRV)
The Light Reflection Value (LRV) is a measure of how light a color is in comparison to an ideal white surface with the same lighting. Here, LRV = Y = 100*((L*+16) /116)3. Black corresponds to 0 and 100 to a relative, ideal white reflector. The light reflectance value (LRV) is used in the door and facade cladding industries. It serves as an approximate measure of the temperature that an outdoor surface can reach (VST data sheet no. 002, BFS data sheet no. 18):

• DunDark tint with HBW 0 - 30 → 65 - 80 °C
• Medium tint with HBW 30 - 50 → 50 - 65 °C
• Light tint with HBW 50 - 100 → 40 - 50 °C

Gloss
The gloss of a surface is determined by the ratio of directional and diffuse scattered luminous flux (lm). If a large proportion of the light is reflected directionally, the surface appears glossy. Conversely, the more light is scattered diffusely, the more matt the surface appears. The structure of a surface, e.g., finely or coarsely structured, polished, has a major influence on gloss, as does the material. Subjective perceptions, physiological and psychological, have a significant influence on the assessment. To compare gloss technically, it can be determined using gloss meters (reflectometers).

Methodology
The method used to test surfaces is essentially based on EN 13722. The standard for HPL EN 438-3 refers to EN 13722. The reflectometer is defined as a measuring device in accordance with EN ISO 2813. In principle, a light beam is directed at the surface at a defined angle. Since the angle of incidence is equal to the angle of reflection, the intensity of the incoming light is measured at the corresponding angle of reflection. The degree of gloss is defined as the ratio of the intensities of the emitted and measured light. The value resulting from the measurement is defined without SI units as GE (gloss units, sometimes GU) in whole numbers. The decimal places on some measuring devices are negligible. For matt surfaces, the light is irradiated at 85°, for high-gloss surfaces at 20° and for all other surfaces at 60°.
Representation of the areas of the different measurement geometries. In practice, it has proven useful to measure the specification using only a 60° angle. The measurement angle and device type must be specified.

Measurement of gloss level
The surface of the test specimen must be cleaned with a soft, clean, lint-free cloth before testing. The calibration of the reflectometer depends on the texture of the surfaces. Textured and other surfaces must be determined at a minimum of 4 measuring points in accordance with the directions specified in EN 13722. Measurements of textured surfaces may only be compared with measurements of surfaces with the same texture. Care must be taken to ensure that measurements are only taken on flat surfaces with a level support and without tilting the measuring device. A comparison of measurement results is only permissible if they were determined under consistent measurement geometry with the same color and structure of the test specimen.

Values according to EN 438:

Color perception
The eye has different types of receptors that are sensitive to different wavelengths of visible light. The receptors send impulses to the brain, where they are processed, resulting in color perception. This is determined by the parameter pairs black–white (light value), red–green, and blue–yellow, and not by the original signals from the receptors. Color perception is influenced by individual cognitive and psychological factors, such as feelings associated with a color. As a result, no two people ever “see” a color in exactly the same way.

Color systems, color models
Color systems are designed to bring order to the seemingly endless variety of colors. Over time, a multitude of color systems have been developed to meet different needs. There are discrete and continuous color systems, each with different advantages and disadvantages. Put simply, continuous color systems represent an area within which calculations can be made freely. Discrete color systems would then only be the more or less large nodes in a network. They allow the production of color charts.

• Discrete color catalogs: RAL Classic, Natural Color System (NCS), RAL Design, Pantone, etc.
• Continuous color spaces: RGB, CIE L*a*b*, CYMK, etc.

RAL Classic and NCS are most commonly used in construction. The paper industry mainly relies on CIE L*a*b*.

CIEL*a*b* color space in brief
The CIEL*a*b* color model describes all perceivable colors in a space under standardized lighting conditions and independently of the device. This color model is standardized in EN ISO 11664-4. Each color is defined using Cartesian coordinates L* (usually the z-axis) and a* and b* (x- and y-axes). The brightness of colors or grayscale (a* = 0, b* = 0) is described using the L* axis, where L* = 0 defines black and L* = 100 defines white. The a* axis represents values (saturations) of the complementary colors green (-170) and red (100), while the b* axis represents blue (-100) and yellow components (150).
Color differences or color distances can be calculated using the color distance Delta E (or dE or ΔE): Delta E = ((L1* - L2*)2 + (a1* - a2*)2+ (b1* - b2*)2)0.5. The following table serves as a rough guide for evaluating delta E.

Color measurement
In principle, color measurement is performed using either a colorimeter or a spectrophotometer. Colorimeters are mainly used to calibrate monitors (RGB). Spectrophotometers are suitable for measuring the colors of almost all types of materials and substances. They scan through the wavelength range of visible light at intervals. The smaller the intervals, the more accurate the device.

There are various ways of irradiating and detecting light. One difference lies in the inclusion or exclusion of gloss. Other important factors are the observer (viewing angle 2° or 10°) and the standard illuminant used (e.g., D65, D50, etc.). The measurement results are output in color spaces such as L*a*b*. In order to compare measurements with each other, the parameters must be selected identically. Color measuring devices must be calibrated with each other. Color determination with a smartphone or similar device does not achieve the same quality and cannot be compared with color measurement using a spectrophotometer.

HPL Standard EN 438
Excerpt on assessing appearance: HPL must be examined for surface appearance under standardized lighting and observation conditions. In addition to normal vision, this requires that clean panels be examined in their delivery condition. The horizontally positioned HPL is illuminated with diffuse light with an illuminance of 1200 +/- 400 lx. The light source can be diffuse daylight or an artificial light source with the same properties (D50 - D65). Testing is also carried out with standard light type A (tungsten filament). The distance between the observer and the HPL should be 0.75–1.5 m. A slight deviation of the test specimens
compared to the corresponding color sample available from the manufacturer is permissible. For applications with critical color and surface finishes, it is recommended that the HPL be checked for compatibility in this regard before further processing or installation.
minimizes the generation of electrostatic charge through contact changes or friction with other materials and does not need to be grounded. The surface resistance is 109–1012 Ω and the charging capacity according to DIN EN 61340-4-1 is < 2 kV. HPL is therefore antistatic. Through modifications, HPL can be made conductive with 105 – 109 Ω. The magnitude of this value depends significantly on the relative humidity.