Why measure gloss? |
Why measure gloss?Gloss is an aspect of the visual perception of objects that is as important as colour when considering the psychological impact of products on a consumer. It has been defined as 'The attribute of surfaces that causes them to have shiny or lustrous, metallic appearance.' The gloss of a surface can be greatly influenced by a number of factors, for example the smoothness achieved during polishing, the amount and type of coating applied or the quality of the substrate. Manufacturers design their products to have maximum appeal- highly reflective car body panels, gloss magazine covers or satin black designer furniture. It is important therefore that gloss levels are achieved consistently on every product or across different batches of products. Gloss can also be a measure of quality of a surface, for instance a drop in the gloss of a coated surface may indicate problems with its cure- leading to other failures such as poor adhesion or lack of protection for the coated surface. It is for these reasons that many manufacturing industries monitor the gloss of their products, from cars, printing and furniture to food, pharmaceuticals and consumer electronics. How is gloss measured?Gloss is measured by shining a known amount of light at a surface and quantifying the reflectance. The angle of the light and the method by which the reflectance is measured are determined by surface and also aspect of the surface appearance to be measured. Different aspects of the gloss of a surface are described by Hunter. Hunter's 6 ways to describe gloss.
What gloss standard should I be using to measure gloss?Many industries have adopted the 20/60/85º geometries as specified in ISO2813/ ASTM D523, however consult the table below for more information on specific industries and their industrial standards. General Gloss measurement ASTM D523 1999 (USA) Test method for specular gloss The principal ASTM specular gloss standard. Very similar to ISO 2813 ASTM D3928 1998 (USA) Test method for evaluation of gloss or sheen uniformity ASTM D4039 1999 (USA) Test method for reflection haze of high-gloss surfaces ASTM D4449 1999 (USA) Test method for visual evaluation of gloss differences between surfaces of similar appearance ASTM D5767 1999 (USA) Test methods for instrumental measurement of distinctness of image gloss of coating surfaces ASTM E430 1997 (USA) Test methods for measurement of gloss of high-gloss surfaces by goniophotometry MFT 30-064 (South Africa) Local version of ASTM D523 JIS Z8741 1997 (JAPAN) Method of measurement for Specular glossiness Paint IS0 2813 1994 (International) Paints and varnishes - determination of specular gloss of non-metallic paint films at 20°, 60° and 85° The principal ISO specular gloss standard. Very similar to ASTM D523 The following are technically similar to ISO 2813: BS 3900: Part D5 1995 (UK) Methods of test for paints - optical tests on paint films - measurement of specular gloss of non-metallic paint films at 20°, 60° and 85° DIN 67530 1982 (Germany) Reflectometer as a means for assessing the specular gloss of smooth painted and plastic surfaces NFT 30-064 1999 (France) Paints - measurement of specular gloss at 20, 60 and 85°. AS 1580 MTD 602.2 1996 (Australia) Paints and related materials, methods of test – introduction and list of methods. JIS Z8741 1997 (Japan) Specular glossiness – Method of measurement.
SS 18 41 84 1982 (Sweden) Paints and varnishes - measurement of specular gloss of non-metallic paint films at 20, 60 & 85° Plastics BS 2782: Pt 5, Method 520A 1992 Methods of testing plastics - optical and colour properties, weathering - determination of specular gloss Similar to ISO 2813 ASTM D2457 1990 Test Method for Specular Gloss of Plastic Films and Solid Plastics Specifies the primary standard as a perfect mirror with a defined gloss value of 1000. 20°, 60° and 45°; the 45° method is as ASTM C346 for ceramics. Metals BS6161: Part 12 1987 Methods of test for anodic oxidation coatings on aluminium and its alloys - measurement of specular reflectance and specular gloss at angles of 20°, 45°, 60° or 85° Ref. Std BS 3900: Part D5 (1980); technically equivalent to ISO 7668 replaces BS 1615:1972. At 45°, dimensions of source image and receptor aperture are as for 60°. Squares with sides equal to the shorter sides of the rectangles are also recommended. Alternatively, total reflection in a 45° prism is used as a reference; source image and receptor aperture are then circular, both with angular diameter 3.44° ± 0.23° (1.5 mm ± 0.1 mm at 25.4 mm focal length) IS0 7668 1986 Anodized aluminium and aluminium alloys - measurement of specular reflectance and specular gloss at angles of 20°, 45°, 60° or 85°. IS0 5190 Anodizing of aluminium and its alloys - evaluation of uniformity of appearance of architectural anodic finishes - determination of diffuse reflectance and specular gloss ECCA T2 (European Coil Coating Association) Specular gloss at 60°. Paper DIN 54502 1992 Testing of paper and board; reflectometer as means for gloss Assessment of paper and board ASTM D1223 1998 Test method for specular gloss of paper and paperboard at 75°. Has unusual converging beam geometry. Specifies the primary standard as black glass of refractive index 1.540, not 1.567, at the sodium D-line having a defined gloss value of 100. ASTM D1834 1995 Test method for 20° specular gloss of waxed paper Another unusual converging beam geometry, different to the previous one. TAPPI T480 OM-90 1990 (USA) Specular gloss of paper and paperboard at 75° Same text as ASTM D 1223 TAPPI 653 1990 Specular gloss of waxed paper and paperboard at 20° Probably the same text as ASTM D 1834 JIS - Z8142 1993 (Japan) Testing method for 75° specular gloss Furniture BS 3962: Part 1 1980 Methods of test for finishes for wooden furniture - assessment of low angle glare by measurement of specular gloss at 85° Similar to ISO 2813: 1978 Floor Polish ASTM D1455 1987 Test method for 60° specular gloss of emulsion floor polish Ref. std ASTM D 523 Ceramics ASTM C346 1987 Test method for 45° specular gloss of ceramic materials Ref. std ASTM D 523 ASTM C584 1981 Test method for 60° specular gloss of glazed ceramic whitewares and related products Ref. std ASTM D 523 {Sheen} Fabrics BS 3424: Method 31: Part 28 1993 Testing coated fabrics - determination of specular gloss What is a gloss unit ?The measurement scale, Gloss Units (GU), of a glossmeter is a scaling based on a highly polished reference black glass standard with a defined refractive index having a specular reflectance of 100GU at the specified angle. This standard is used to establish an upper point calibration of 100 with the lower end point established at 0 on a perfectly matt surface. This scaling is suitable for most non-metallic coatings and materials (paints and plastics) as they generally fall within this range. For other materials, highly reflective in appearance (mirrors, plated / raw metal components), higher values can be achieved reaching 2000 Gloss Units when measured at 20° What difference in gloss units is visable to the human eye?If two different coatings are measured, what number of gloss units would be detectable by the human eye, how many units would be perceived as significantly different? When measuring at 60 Degrees these detectable differences depend on the gloss level of the sample, for instance 3.0 GU difference measured on a very matt surface (perhaps 5GU), would be seen by the human eye but on a higher gloss coating (perhaps 60 GU) the difference would be very difficult to notice. The only way that you can determine tolerances for your products would be experimentally, perhaps preparing printed samples at different gloss levels that you can show to end users of your coatings or internal "experts" The other option is to change to a 20/60/85 degree instrument, the 85 degree glossmeter is more sensitive to differences in gloss below 10 GU @ 60º and the 20 Degrees has higher resolution on high gloss coatings (above 70 GU @ 60º). The advantage of using the three angles is that there is more equality to the gloss differences, in our experience a gloss difference of 5 GU, when measured with the correct geometry is just visible to a trained observer. |
