H51: Gloss Perception & Measurement

H51: Gloss, Perception and Measurement

Gloss or glossiness is an aspect of appearance that we all recognise and the degree of glossiness is described by phrases such as:

High gloss finish
Gloss finish
Matt Finish

High gloss is usually seen as a desirable attribute for some objects as in the remark "polish until you can see your face in it". Figure 1 shows three vases and most people would agree that the surface of the vase on the right-hand-side of Figure 1 is the glossiest and the surface of the vase on the left-hand-side is the least glossy finish.

Figure 1: Vases with three different levels of gloss

A similar impression of gloss level is obtained from the ring shown in Figure 2 and in Figure 3.

Figure 2: Ring with a matt surface finish

Figure 3: Ring with a glossy surface finish

The human visual system takes account of several types of appearance characteristics simultaneously and integrates them into a single estimate of glossiness. This is why different observers may reach different conclusions concerning the gloss level of the same object.

Study of Figures 1 to 3 shows that the quality of the image of neighbouring objects reflected by the surface must have an important contribution to the impression of gloss. A definition of gloss is:

"the degree by which reflected highlights or reflected images of objects
are seen as superimposed on the appearance of a surface"

At the simple level the gloss can be regarded as the capacity of a surface to reflect light, and was defined in the 1930's by Hunter as:

"the degree to which a surface simulates the reflecting power of a perfect mirror"

Distinctness of image gloss is a function of the sharpness of a reflected image; it is associated with high gloss surfaces in which mirror images are seen.

Example: The image of a window is clearly seen in the light reflection from the surface of the ring shown in Figure 4.

Specular gloss is a measure of the brightness of the reflected image and is often associated with the bright patches of light on a surface that indicate the location of the illuminating light source. This type of gloss is clearly seen with high gloss surfaces and with medium gloss surfaces of paint, plastic etc.

Example: The surface of the black ball shown in Figure 5 has highlights where the light from the lamps above the ball is directly reflected towards the observer.

is a measure of the difference in brightness between the specular gloss areas (described previously) and other areas of the surfaces. It is associated with medium to low gloss surfaces of paint, plastic textile cloth etc.

Example: The upper surface of the vase in Figure 6 shows brighter areas where the light from the lamps is reflected more strongly towards the observer.

Figure 4: Example of
distinctness of image gloss

Figure 5: Example of
specular-gloss

Figure 6 Example of
contrast-gloss

At the surface of an object, the incident light is partially reflected (back from the boundary) and partially refracted (into the body of the material). The process is illustrated in Figure 7.

Figure 7: Cross-section of a glossy paint layer showing reflection at the boundary and by pigments within the layer

Figure 8: Angular distribution of light reflected from the boundary and from within a pigmented layer

Consider the interaction of light with a conventional type of glossy paint, the light reflected by the boundary will be distributed around the specular angle, whereas the light reflected by the pigments within the layer will be, to a good approximation, distributed evenly into all angles, Figure 8.

The impression of the gloss of a surface is determined by the way the light shining onto the surface is reflected at the air-to-material boundary. For example, at a boundary between air and a sheet of glass, some of the light is reflected back from the glass surface and some is transmitted through the sheet.

The result of the action of an air-to-glass boundary in causing reflection and transmission of light is illustrated by the appearance of a shop window on a street in New York, as shown in Figure 9.

Glancing angles of view:

The reflection of the street is clearly visible in the glass sheet. The objects within the shop cannot be seen through the window.

Figure 9: Shop window on a Street in New York

The reflection of the light from the street is much reduced. The objects within this part of the shop are clearly seen through the window.

The gloss-light, the light reflected at the boundary, has the same colour properties as the light shining onto the source, as it has not entered the material at all. For example, in the street scene, the reflected image retains the colours of the original scene

In the case of the glossy ring, Figure 3, the gloss-light has the same "white" light characteristics as the light shining onto the object. In the gloss region, the white gloss-light has the effect of "diluting" the intensity of the coloured light that is being reflected from within the boundary of the material. In the high-gloss regions the additional white light dominates and the underlying colour of the object is no longer apparent.

At the boundary

At a boundary between two materials that do not have the same refractive index, the incident light is partially reflected and partially refracted into the body of the material as illustrated in Figure 10.

Dependence on refractive index

The refractive index (n) determines how a beam of light is reflected and refracted as it travels through a material. It is the ratio of the speed of the light in vacuum to the speed in the material.

Figure 10 Surface reflection and refraction

Reflection and refraction of the light at the boundary will occur when there is a difference in refractive index between the material of the surface (glass has a refractive index of n = 1.5) and the surroundings (air has a refractive index of n = 1.0).

The amount of light reflected at a boundary changes with the angle the light makes with the boundary, for an air-to-glass boundary, as shown in Figure 11:

Background science

The proportion of light that is reflected rather than transmitted is determined by the ratio of the refractive indices (m) of the two materials that form the boundary and the angle of incidence (θi in Figure 10) the light beam makes with the boundary (Fresnel's law). For an unpolarised beam of light, the fractional amount reflected is

The minimum amount of reflection is for light shining onto the surface at right angles or normal to the surface.

and

Where

Figure 11: Reflected intensity of light at an air to glass boundary at different angles of incidence

The fraction of light transmitted through the boundary (T ) is given by T = 1 - R

The roughness of a surface and the refractive index of the materials determine the directions and the amounts of light reflected at the boundary between two materials. When the surface is not perfectly smooth, then the boundary reflected light is directed in to a range of angles around the angle of specular reflection and the impression of gloss is changed. This is illustrated in Figure 10.

Figure 10: Angular dependence of the boundary reflected light for surfaces with different levels of gloss

Imagine that a beam of light is shone at a particular angle onto a very smooth surface. The boundary reflected light would all be along a narrow set of directions. An observer viewing such a surface will see, at certain viewing angles, reflected images of the surroundings. This gives rise to the visual impression of high gloss.

The boundary of a very rough surface will tend to reflect light at many different angles, because the light meets the surface at many different angles. The boundary reflected light is diffuse in direction so that the observer cannot make out images of the surroundings. Consequently, the visual impression is that of a low gloss (matt) surface.

The wavelength of light (380nm to 730nm) determines the length scale for measuring the roughness of a surface in relation to its influence on the gloss of a material. In general the following terms are often used:

The influence of the roughness of a surface on the boundary reflecting properties was first studied by Lord Rayleigh (1901). Rayleigh wished to determine the maximum height (h) of step shaped defects (Figure 11) that could be present and still maintain an acceptable performance as a plane reflector (mirror). Rayleigh suggested that the maximum height of a step for light incident from an angle θ (Figure 12) is:

Equation 1:

Figure 11: Step height of surface defect

Figure 12: Angle θ

The maximum step heights at the three common angles of light incidence that are used for instruments that measure specular gloss are shown in Table 1.

The values in Table 1 show that the level of gloss at an assessment angle of 20° is over ten times more sensitive to surface defects than gloss assessment at an 85° angle of incidence.

Table 1: Maximum step height
(λ = 550nm)

Angle of incidence / (deg)	Step height /(µm)
20	0.073
60	0.138
85	0.789

The relationship between the angle of viewing, the surface roughness and the quality of the reflected image is illustrated by the values shown in Table 1, which suggests that:

Gloss is an appearance attribute that depends on our perception and interpretation of the quality of an image reflected from a surface. The quality of the gloss of an object is a mental comparison of the fidelity of the reflected image from the surface to an "imagined" image from a mirror-like surface.

The comparison material that is used to define 100 gloss units is optically smooth, black glass.

Two of the standards for the measurement of specular gloss are ISO 2813 and ASTM D523

A collimated beam of light from a tungsten filament lamp (CIE Illuminant A) is shone onto the surface at a defined angle. Light is collected from the surface and, by using optical baffles and lenses, only the specular reflected light is passed to the detector. An optical filter corresponding to the CIE Luminosity Function is placed in front of the detector so that the response simulates the "brightness" sensitivity of the visual system

Figure 13: Optical design of a specular gloss meter

The distinctness-of-image gloss measurement is based on determining the spread of the light beam beyond the specular angle (ASTM5767-95) "Standard Test Methods for Instrumental Measurement of Distinctness-of-Image Gloss of Coating Surfaces".

In a DOI gloss measurement, the collimated beam of incident light is shone onto the surface at a particular angle, often 30 degrees.

The specular reflected light (Rs ) is collected in a narrow band of angles that are equal but opposite to the incident angle (Figure 14).

Figure 14: Optical principle of a distinctness-of-image gloss meter

The off-specular reflected light (R0.3 ) is also collected at angles slightly away from the incident angle to determine the degree of angular spread of the reflected light. The off-specular reflectance is determined from the amount of light reflected at, and beyond, the angles (specular + 0.3°) and (specular - 0.3°). The distinctness of image gloss value is determined from:

Contrast gloss is used to assess materials with a lower degree of gloss than both DOI gloss and specular gloss. There are not any established standards for the measurement of contrast gloss,

Most methods of assessment use the principle of comparing the amount of diffuse reflectance (RD.) from a panel with the total reflectance (RT ), the sum of diffuse and specular reflectance.

A reflectance spectrophotometer with an integrating sphere type of optical system can provide estimates of two types of reflectance value. When the instrument measures in the specular light included mode (SPIN) then the Y tristimulus value can be used as an estimate of the visually weighted total reflectance RT. When the instrument measures in the specular light excluded mode (SPEX), the Y tristimulus value can be used as an estimate of the visually weighted diffuse reflectance RD.

This measure of contrast gloss is zero for a matt panel and increases as the level of contrast gloss increases.

Richard S. Hunter and Richard W. Harold, The Measurement of Appearance, New York: John Wiley and Sons, 1987.

ASTM D5767-95; Standard Test Methods for Instrumental Measurement of Distinctness-of-Image Gloss of Coating Surfaces.

H51: Gloss, Perception and Measurement

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