We see gloss when the positions of the light source, the object, and the observer are such that the object's surface is viewed from the direction of the specular reflection (regular reflection) of light coming from the source. If you view it from a direction away from the specular reflection, the surface will appear less glossy. In fact, the stronger the glossiness of the reflecting surface, the darker it will appear from that off-angle.

When an object's surface is illuminated from a certain direction, how much of the incident light is reflected in each direction—the light distribution characteristics of reflected light—is deeply related to the perception of gloss.

In figure (c), specular reflection (regular reflection) is exactly what occurs with a mirror surface: for an incident light ray with an incident angle θ_i, the reflected light travels in a direction symmetric to the surface normal, with a reflection angle θ_r equal to θ_i.

In figure (a), perfect (Lambertian) diffuse reflection produces the greatest reflected luminous flux in the surface normal direction (θ_r = 0°), regardless of the incident angle θ_i. In all other directions, the reflected flux decreases in proportion to the cosine of the reflection angle (cos θ_r) relative to the normal direction (See Chapter 9).

The light distribution characteristics of reflected light lie between these two extremes of specular reflection (c) and perfect diffuse reflection (a). On most real surfaces, reflection is like figure (b): the reflection is strongest in the specular direction (θ_r = θ_i) and gradually becomes weaker as you move away from it. For a single narrow incident light ray, specular reflection (c) concentrates the reflected light energy entirely into the reflection angle direction, making the surface appear extremely bright from that direction. From any other angle, almost no light enters the eye, making it appear completely dark.

Now, what about perfect diffuse reflection (a), such as white paper or chalk? When discussing diffuse reflection characteristics (light distribution), as shown in the top row of the figure, it is often illustrated through luminous intensity distribution, which shows how the luminous flux per unit solid angle changes with reflection direction. From this diagram, it's tempting to assume that directions with low intensity will appear dark, and those with high intensity will appear bright. But this is not exactly correct.

As explained in Chapters 9 and 10, the human perception of surface brightness is based not on luminous intensity, but on luminance—the light intensity per apparent unit area. Therefore, when discussing gloss, it is better to look at the luminance distribution shown in the bottom row.

For perfect diffuse reflection, the luminous intensity of the reflected light changes with cos θ_r depending on the viewing direction, but so does the apparent unit area. These two factors cancel each other out, making the luminance distribution constant regardless of viewing direction ≪See Chapter 9≫. This is why surfaces like paper or chalk, which have nearly perfect diffuse reflection characteristics, appear to have almost no gloss.

On a typical reflective surface (figure (b)), the stronger the reflection in the specular direction, the greater the luminance in that direction compared to other directions, increasing the sense of gloss. In other words, the strength of the reflection component in the specular direction directly determines the degree of gloss.

The factors that influence the distribution of reflected light include:

1. The direction of the incident light relative to the reflecting surface
2. The degree of fine surface texture (whether it is rough or smooth)
3. The relationship between the object's internal material structure and the wavelength of the incident light

Another factor influencing diffuse reflection is the scattering and absorption that occur inside the object due to the interaction between the material's molecules/atoms and the incident light. As explained in Chapter 32, light that enters the material without being reflected at the surface is repeatedly scattered by the densely packed molecules and atoms, changing direction many times. During this process, some of the light is absorbed by the material, but part of it is re-emitted back toward the incident side, combining with the light reflected at the surface.

Both scattering and absorption generally have wavelength dependence. As a result, the spectral distribution of diffuse reflection re-emitted from inside the object often differs from that of the incident light. This is why objects have color under white-light illumination.

For surfaces with strong gloss, the proportion of specular reflection is large, and the diffuse reflection emitted in the specular direction from inside the material is weak. This means that the spectral distribution in the specular direction closely matches that of the light source. While the non-glossy parts of the surface appear colored, the glossy highlights themselves have little to no apparent color.

As explained above, the light distribution characteristics of reflected light are determined by a complex combination of various factors. In simple terms, the degree of gloss is determined by the strength of specular reflection at the object's surface. The challenge lies in how to make objective and quantitative comparisons and evaluations.

There are many factors to consider when evaluating the gloss of a reflecting surface, such as the positional relationship between the light source, the reflecting surface, and the observer; the magnitude and range of the incident angle; and the spectral properties of both the light source and the reflecting surface. These conditions must be clearly defined to compare and evaluate gloss.

For example, when the light source has a large emitting area, the incident light on the reflecting surface will include many different angles of incidence. Some of the specular reflection will enter the eye, and gloss will be perceived from many different positions on the surface and observation angles. In such cases, the reflectance (luminance factor) changes depending on the angle of incidence, meaning that the intensity of the specular reflection—the perceived gloss—varies with observation angle. Furthermore, the combination of the light source's spectral distribution and the object's spectral reflectance (spectral luminance factor) will change the spectral distribution of the reflected light, altering the brightness perceived by the human eye.

The method for measuring and evaluating glossiness is defined in the Japanese Industrial Standard (JIS). JIS Z 8741-1997 "Specular glossiness -- Method of measurement" specifies that glossiness should be measured using an optical setup like the one shown in the diagram at the lower right.

With this measurement system, the sample's reflective surface is illuminated with nearly parallel light (divergence angle α₁) at the incident angle θ_i, and the photodetector captures the specular reflection, which also has a narrow divergence angle α₁′. ≪1≫

The standard specifies five possible incident angles (θ_i): 85°, 75°, 60°, 45°, and 20°. Because reflectance generally increases with larger incident angles, this allows the selection of an appropriate incident angle depending on the material, application, and other conditions.

The reference standard for specular glossiness is a smooth glass surface with a refractive index of n = 1.567 over the entire visible wavelength range. For each incident angle θ_i, the specular reflectance ρ₀ (θ_i) of this reference surface is specified.

If φ_0S is the specularly reflected luminous flux from the reference surface at the specified θ_i, and φ_S is the measured value from the sample surface under the same conditions, then the sample's specular glossiness G_S (θ_i) is defined as follows, with the glossiness of the reference surface set to G_0S (θ_i) = 100%:

At each incident angle condition, the glossiness of the reference surface is defined as G_0S (θ_i) = 100%, and the sample's specular glossiness (in %) is expressed as the relative ratio to this reference.

The light source used must be non-polarized ≪2≫, and the combination of the light source and detector must have spectral characteristics equivalent to the combination of standard illuminant D₆₅ and the standard spectral luminous efficiency function V(λ).

In practice, gloss meters come with standard gloss plates (working standard surfaces) for instrument calibration. These plates are supplied with glossiness values calibrated against the primary reference surface described above. Measurements of sample gloss are calculated using these reference values and the formula above. Because the glossiness of working standard plates can change over time due to dirt or wear, periodic recalibration is recommended.

This is the JIS-specified method for measuring specular glossiness. While the brightness perceived by the human eye from a reflective surface should ideally be evaluated in terms of luminance (i.e., luminous intensity per unit apparent area), the JIS measurement system actually says to measure the reflected luminous flux φ from the surface.

In this system, as shown in the measurement diagram, the detector observes the reflecting surface from an oblique direction through aperture S₂. If the reflection angle θ_r (= θ_i) is fixed, then both the "measurement solid angle" and the "apparent area of the reflecting surface" remain fixed. Therefore, even though the measured output is in terms of luminous flux φ, it effectively has the dimensions of luminance. Since the light source and detector characteristics are also fixed, luminous intensity I = dφ / dΩ and luminance L = dI / (dA · cos θ_r) = dφ / (dΩ · dA · cos θ_r) both become directly proportional to the measured luminous flux φ.

Thus, by measuring the luminous flux φ in the specular reflection direction through aperture S₂ and calculating the reflectance relative to the reference surface, it is possible to determine the luminance ratio to the reference surface—i.e., the specular glossiness—without having to measure luminance directly, using a simpler measurement system.

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