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Monday, August 11, 2014

What is the difference between gloss and haze?

There are three basic forms of reflection: specular, diffuse and mixed.

Specular Reflection

Specular reflection is when all of the light incident at a certain angle is reflected at the equal but opposite angle. In other words, the light coming in at 75° is reflected at the opposite 75°.

Diffuse Reflection

Diffuse reflection is when all of the light incident at a certain angle is evenly reflected at all possible angles (diffusely).

Mixed Reflection

Mixed reflection is a combination of specular and diffuse reflection.  The incident light has some measurable specular reflection, however there is still some diffuse component.

The following figure illustrates the relationship between specular reflectance and diffuse reflectance from low, medium and high gloss materials.  A light beam striking a low gloss sample will be diffused by the paper and reflect in all directions.  A sample with a high gloss surface will diffuse some of the incident light energy, however, much of the light will be reflected in mirror-like fashion which produces a "blip" on the curve at the specular angle, as shown.  A glossmeter will view the specular portion of the reflected light while excluding as much as possible of the diffuse reflectance.

High quality surfaces are expected to have a clear and brilliant appearance. Microstructures, e.g. poor dispersion, can cause a milky appearance.  The effect is described as milkiness or haze.  A high gloss surface with microscopic texture has diffused light with low intensity adjacent to the main direction of reflection.  The majority of the incident light is reflected in the specular direction (gloss) which makes the surface appear highly glossy with image forming qualities, but with a milky haziness on top of it. 

When gloss is measured, a certain aperture is used to capture the specular reflection. Haze is the reflection close the specular angle, but excluding the specular reflection (see below).

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