All industries are concerned with the appearance of their products. The overall appearance of any object is a combination of its chromatic or color attributes and its geometric attributes (such as gloss or haze). When buyers evaluate a potential purchase, they look at appearance and expect uniformity within any grouping of the same product. If a difference is noted among several items of the same product on display, this difference is associated with poor quality. The visual or instrumental assessment of appearance is a critical step in product manufacturing and release. This application note considers the geometric attributes of samples.
Geometric attributes are associated with the scattering of light off the surface of an opaque object, or within a transparent object. For instance, a flat cotton weave fabric is very different geometrically from a corduroy. A glossy photo print looks quite different than a matte one. There are many geometric attributes and this application note considers the following:
• Gloss, the property of a surface responsible for shiny or lustrous appearance.
• Haze, the scattering of light within the surface of a transparent sample that is responsible for a cloudy appearance.
• Opalescence, the scattering of light in two colors (dichroism)
Gloss measurements quantify the amount of light reflected at the specular angle from an object‘s surface. The specular direction is the angle equal to but opposite the angle of incidence. This specular light is responsible for the highlights visible on shiny materials. Gloss measurements quantify the amount of shine coming from a surface. A glossmeter has a configuration similar to that shown below.
(see attached file below)
Glossmeters are configured so that the light generated is incident on the material at a particular angle relative to the normal or perpendicular to the surface being 0 degrees. The detector is then placed at that same angle on the other side of normal so that only the light reflected at the specular angle is collected. Generally, a green filter corresponding to the CIE Luminosity Function is placed in front of the detector to enable the instrument to simulate human visual reflectivity. ASTM Designation D523-89, Standard Test Method for Specular Gloss, specifies how gloss is measured. Glossmeters provide light at one angle or a combination of three angles. The most common angles are 20°, 60°, and 85°. ASTM Designation D523 specifies that a 60° angle may be used for most materials. The test method recommends the use of a 20° angle when the 60° gloss value is greater than 70 (which would be a highly-reflective material). The use of an 85° angle is recommended when the 60° gloss value is less than 10 (which would be material with a matte surface). TAPPI gloss for paper is specified by TAPPI T480 and is measured at a 75° angle.
Haze is the cloudiness of a product that is caused by scattering of light through a transparent material. Haze is an important appearance attribute which can be quantified and then used to assess the quality of objects, particularly those that are near clear. Light scattering resulting in haze can be caused by suspended particles, contaminants, or a roughened surface. Haze is instrumentally determined by reflectance or transmission to match the way an object is viewed. A transparent or is measured by transmitting light through it (transmission haze). Reflection haze seen as a milky finish on an opaque, mirror-like surface, and measured in reflectance.
Transmission haze is defined as the scattering of light through a nearly clear specimen when viewed in transmission. The back scattering of light is not included in the measurement. Only light scattered by more than 2.5° from the incident light is considered to contribute to haze. When measuring haze, the percentage of light diffusely scattered compared to the total light transmitted is reported. ASTM Method D1003 describes the type of instrument to be used for measuring transmission haze. This haze meter uses a pivoting sphere and a single collimated beam of light. The light enters one side of the sphere at an entrance port and is directed to an exit port on the opposite side of the sphere. When the sphere is in the first position, the light leaves the exit port and is absorbed by a light trap placed at that port as shown in Figure 2. When the sphere is pivoted, the beam of light is directed toward the sphere wall and diffused. The detector is filtered to Illuminant C and the y function of the 2° standard observer and the %Haze is reported to the nearest 0.1 %. HunterLab sphere spectrophotometers have diffuse light sources that originate from the inside of the sphere rather than a collimated beam light source as described in ASTM D1003. Also note that the spheres of both of these instrument types are in a fixed position and cannot be pivoted. The method used to get similar haze results is to use a white tile at the reflectance port to simulate the pivoted position when light is directed at the sphere wall. There are drawbacks to using this method. The first is that the white standard that is placed at the port to complete the sphere may have a slightly different reflectance than the actual sphere wall and the second is that the reflectivity may vary slightly with each new placement of the standard. To increase accuracy, one can use a standard with reflectivity similar to the material which coats the sphere.
ASTM E430, defines the measurement of reflection haze as the spread of the specular component of the reflected light from a glossy surface. The specular component is the light that is reflected from an object at an angle equal to but opposite to the angle of the incident light. Most glossmeters measure at the specular angle plus or minus several degrees and therefore cannot report the amount which the specular component spreads. The light that is spread 0.3° from the specular is responsible for distinctness-of-image gloss. The light that is spread 2° is responsible for a quality known as bloom or narrow-angle reflection haze. The light spread 5° is referred to as wide-angle reflection haze.
Opalescence is a type of dichroism seen in highly dispersed systems with little opacity. The material appears yellowish-red in transmitted light and blue in the scatter light perpendicular to the transmitted light. The phenomenon is named after the appearance of opals. Common examples of opalescence include blue skies in the daytime and the yellowish-red skies at sunset. One can make a quick experiment by adding a few droplets of milk to a glass of water: the milk looks bluish. If one looks through the milk at the light source, it looks yellowish-red. Opalescence is actually related to the degree of scattering of near-transparent liquids. It is not a common colorimetric application in most industries; although one application encountered is the opalescent measurement of bleaching solutions used in dental whitening.
In the European Pharmacopoeia (EP) [www. pheur.org] Section 2.2.1, clarity and degree of opalescence of liquids defines visual clarity and describes a standard for opalescence (haze) relative to water. A primary opalescent liquid suspension of 25 mL hydrazine sulfate solution and 25 mL of hexamethylenetetramine defines four levels of opalescent standards mixed with distilled water and a fifth being 100 % distilled water (no opalescence) (Table 2).
(see attached file below)
For visual evaluation, the liquid standards and a sample are placed in identical flat-bottomed test tubes (15-25 mm in diameter) to a depth of 40 mm. The evaluator looks down the tubes placed sideby-side against a black background under even daylight lighting. The sample is considered clear if its opalescence is similar to that of distilled water or no more than that of Opalescence Standard I. If the dichroic color difference in the opalescent sample is of interest, it is possible to measure the color difference based on two measurements, one straight through the sample (regular transmission) and the other of the diffuse, or scattered, transmission. The measurements would need to be made separately and manually.
It is usually the degree of scattering with the appearance of opalescence that is of interest. Opalescence can be determined instrumentally for liquid samples by using a correlation between the EP Opalescence Liquid Standards and a Transmission Haze measurement.
The correlation method can be summarized as follows:
1. Select Haze as a read method/procedure for measurement using your software. Note: It will only be available for a bench top sphere instrument such as a ColorQuest XE, UltraScan XE, UltraScan PRO, or UltraScan VIS.
2. Standardize the instrument in TTRAN mode using a 50 mm transmission cell filled with distilled water as a blank when setting the top of scale.
3. As an operational qualification (OQ) step, read back the cell of distilled water as a standard or sample. The reading should be very close to:
L* = 100.0 ± 0.05,
a* = 0.0 ± 0.05,
b* = 1.0 ± 0.05, and
Haze % = 0.0
4. If any of the values are more than ± 0.05 unit away from these values, stop and determine what the problem is.
5. Measure the four EP Liquid Opalescence Standards in the 50 mm transmission cell and save the measurements.
6. The Haze % of the standards should increase with the standard number.
7. Using Microsoft Excel or another spreadsheet/graphing program, determine the optimum correlation between the EP Standard Number (0 for distilled water, then 1, 2, 3, and 4) and the Haze % values for your measurements.
8. Implement the formula below in one of your software’s custom formula fields in the Color Data Table/Master Color Data view so that EP Opalescence will be automatically calculated and displayed (see below)
9. Refill the 50 mm transmission cell with a liquid sample and measure it with your instrument.
EP Opalescence = m* (Haze %) + b, Where, m and b are the correlation coefficients determined
While EP visual Opalescence is reported in a rating system of single digits (corresponding to the standard the sample is most like), instrumental EP Opalescence may be reported to additional decimal places if desired.
The overall appearance of any object must take in to consideration geometric attributes as well as chromatic or color attributes.
ASTM D1003, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics.
ASTM D4039 Standard Test Method for Reflection Haze of High-Gloss Surfaces.
ASTM D523 Test Method for Specular Gloss.
ASTM E430 Test Methods for Measurement of Gloss of High-Gloss Surfaces by Abridged Goniophotometer.
European Pharmacopoeia (EP) Section 2.2.1, Clarity and degree of opalescence of liquids, www.pheur.org
Hunter, Richard S., and Harold, Richard W., The Measurement of Appearance, 2nd Ed., John Wiley and Sons, Inc. New York, NY USA 1987.
(See attached for the complete article)