haze

Reflection Haze

Reflection Haze is an optical phenomenon usually associated with high gloss surfaces, it is a common surface problem that can affect appearance quality. The reflection from an ideal high gloss surface should be clear and radiant, however, due to scattering at imperfections in the surface caused by microscopic structures or textures (≈ 0.01 mm wavelength) the reflection can appear milky or hazy reducing the quality of its overall visual appearance.

Causes of this could be due to a number of factors –

Poor dispersion

Method of applying the coating

Variations in drying, curing or baking

Types of materials used in the formulation

Polishing or abrasion

A high gloss surface with haze exhibits a milky finish with low reflective contrast- reflected highlights and lowlights are less pronounced.

On surfaces with haze, halos are visible around the reflections of strong light sources.

Measurement

Measurement of reflection haze is primarily defined under three International test standards:

ASTM E430 comprises three test methods:

Test method A specifies a 30° angle for specular gloss measurement, 28° or 32° for narrow-angle reflection haze measurement and 25° or 35° for wide-angle reflection haze measurement.

Test method B specifies a 20° angle for specular gloss measurement and 18.1° and 21.9° for narrow-angle reflection haze measurement.

Test method C specifies a 30° angle for specular gloss measurement, 28° or 32° for narrow-angle reflection haze measurement and 15° wide-angle reflection haze measurement.

Test method specifies gloss measurements to be made at 20° and 60°, the haze index is then calculated as the difference between the 60° and 20° measurements.

Source:

Test method specifies a 20° angle for specular gloss measurement and 18.1° and 21.9° for narrow-angle reflection haze measurement.

All test methods specify that measurements should be made with visible light according to CIE spectral luminous efficiency function V(λ) in the CIE 1931 standard observer and CIE standard illuminant C.

As most commercially available glossmeters have gloss measurement angles of 20°, 60° and 85° haze measurement is incorporated at either 20° (ISO 13803 / ASTM E430 method B) or at 20° and 60° ( ASTM D4039). There are however some manufacturers that offer glossmeters with measurement angles of 30° and haze measurement in accordance with ASTM E430 Method A and C but are fewer in number, therefore for the purposes of detailing haze measurement theory only the first three methods will be included.

Both test methods measure specular gloss and haze together at 20° that means light is transmitted and received at an equal but opposite angle of 20°.

Applications

Generally measurement of reflection haze is confined to high gloss paints and coatings and highly polished metals. Although there has been some degree of success using this measurement method for films it has proven unreliable due to variability caused by changes in the film thickness (internal refraction variations) and the background colour on which the film sample is placed. Generally haze measurement of films is performed using a transmission type hazemeter as described hereafter.

Transmission Haze

Light and transparent materials

When light strikes the surface of a transparent material the following interactions occur –

• Light is reflected from the front surface of the material

• Some light is refracted within the material (depending on thickness) and reflected from the second surface

• Light passes through the material at an angle which is determined by the refractive index of the material and the angle of illumination.

The light that passes through the transparent material can be affected by irregularities within it; these can include poorly dispersed particles, contaminants (i.e. dust particles) and/or air spaces. This causes the light to scatter in different directions from the normal the degree of which being related to the size and number of irregularities present. Small irregularities cause the light to scatter, or diffuse, in all directions whilst large ones cause the light to be scattered forward in a narrow cone shape. These two types of scattering behaviour are known as Wide Angle Scattering, which causes haze due to the loss of transmissive contrast, and Narrow Angle Scattering a measure of clarity or the "see through quality" of the material based on a reduction of sharpness.

These factors are therefore important for defining the transmitting properties of a transparent material-

Transmission – The amount of light that passes through the material without being scattered

Haze – The amount of light that is subject to Wide Angle Scattering (At an angle greater than 2.5° from normal (ASTM D1003))

Clarity – The amount of light that is subject to Narrow Area Scattering (At an angle less than 2.5° from normal)

Measurement

Measurement of these factors is defined in two International test standards-

ASTM D1003 comprises two test methods:

Procedure A – using a Hazemeter

Procedure B – using a Spectrophotometer

Source:

Part 1 – Using a single beam Hazemeter

Part 2 – Using a dual beam Hazemeter

The test methods specify the use of a Hazemeter as shown below -

A collimated beam of light from a light source (ASTM D1003 - Illuminant C, BS EN ISO 13468 Parts 1 and 2 - Illuminant D65 ) passes through a sample mounted on the entrance port of an integrating sphere.

The light, which is uniformly distributed by a matte white highly reflective coating on the sphere walls, is measured by a photodetector positioned at 90° from the entrance port. A baffle mounted between the photodetector and the entrance port prevents direct exposure from the port.

The exit port immediately opposite the entrance port contains a light trap to absorb all light from the light source when no sample is present. A shutter in this exit port coated with the same coating as the sphere walls allows the port to be opened and closed as required.

Total transmittance is measured with the exit port closed.

Transmittance haze is measured with the exit port open.

Commercially available Hazemeters of this type perform both measurements automatically, the only operator interaction being the placement of the sample material on the measurement (entrance) port of the device.