DOI Gloss and Haze
Why use a Rhopoint IQ?The Rhopoint IQ is unique as it supplies surface gloss and texture information in one quick, simple and easy to understand measurement. Gloss, Distinctness of Image, Haze and RSpec measurements are taken in a single reading at 20° or 60° . Whilst glossmeters are widely used throughout industry to assess the reflective qualities of products, gloss information alone is limited as it tells us how much light is reflected, but nothing about the imaging forming qualities of a surface. The DOI, Haze and RSPEC values measured by the Rhopoint IQ allow the user to quantify and control the surface textures that reduce the perceived quality of manufactured products.
A striking example of this can be seen when measuring the (PCI) powder coating smoothness visual assessment panels. These are a set of specially prepared powder coated panels with varying amounts of orange peel. To demonstrate the effect of this orange peel, each panel has been photographed with a specially prepared test chart reflected in it. The images clearly showing how distinctness of image improves as orange peel reduces. When measured with a conventional glossmeter at 60° (the standard gloss measurement angle), the glossmeter does not measure any significant difference between all the panels. Despite large differences in appearance, the gloss readings for the panels are almost identical. This is because the glossmeter can only measure the amount of reflected light and is not sensitive to distortions which affect appearance quality. The Rhopoint IQ perfectly describes reflective appearance by giving DOI, Haze and Gloss information in a single reading. The DOI readings from the Rhopoint IQ perfectly show the reduction in orange peel from panels 1 to 10, whilst gloss values can be seen to equate to those measured using a conventional instrument. Peak specular reflectance is a measure of the peak gloss value of a surface; this value is obtained very close to the specular angle. Rspec is the gloss value measured at +/- 0.099121° from the specular angle (20 degrees in this case), because it is the gloss measured only at the specular angle it is the peak gloss reading. From the curve on the right of the screen it is the peak reading at the center of the graph. If RSpec is the peak gloss value at the specular angle then why is this value lower then the 20 degree gloss value in this example? Gloss ASTM D523 is measured +/- 0.9 degrees from the specular angle. The glossmeter simply measures the amount of light reflected through this angle (without looking at varation). We do the same thing by measuring the area under the curve at the same angular tolerance. In this case the RSPEC is lower than the gloss because sample has orange peel on the surface. This sample is high gloss, but the ripples on the surface are spreading the light causing a low (peaking at 43.8 GU) curve. If the same coating was measured on a panel without orange peel, the amount of light reflected would be the same (it would have the same gloss value), BUT the light would be reflected in a sharp curve. If the DOI of the panel was 100.00 (no orange peel at all) all of the light would be completely reflected at the specular angle and RSPEC would EQUAL the GLOSS value. Distinctness of image is, as the name implies a function of the sharpness of a reflected image in a coating surface. Two surfaces finished with similar coatings may exhibit identical gloss values but visually the quality of one coating can be seen to be very poor. Upon closer inspection the visually substandard poor coating has a highly textured dimpled appearance known as "orange peel". When a reflected object is viewed in such a coating the image becomes fuzzy and distorted. The automotive industry was the main driver to increased quality gloss coatings. Orange peel drastically reduces the perceived quality of automotive finishes. Orange peel, waviness, texturing, pin holing and similar effects can be problematic in many high gloss coating applications including automotive, powder coating and any other industries that requires a smooth homogenous finish. All these effects can be measured with the Rhopoint IQ. DOI has been measured instrumentally and subjectively in the automotive industry for many years. Instruments that measured this value in the past were bulky expensive and some had poor repeatability. DOI measurement was not common outside of automotive because of the cost of equipment was high and the demand for high quality gloss finish was not as crucial. The Rhopoint IQ measures the DOI of a surface by quantifying the way a reflected measurement beam is spread and distorted around the specular angle. An example of test panel with low and high DOI values. Orange peel, texture, flow out and other key parameters can be assessed in coating applications where high gloss quality is becoming increasingly important. The DOI value of a surface is number between zero and one hundred; a surface that exhibits a perfect undistorted image returns a value of 100, as the values decrease and as the image becomes less discernable. Measuring DOI with the Rhopoint IQ The Rhopoint IQ can measures DOI at 20 and 60 degrees, The 20 degree angle is the prescribed angle for DOI measurement in ASTM D5767. Due to the larger measurement spot size at 60 degrees, tests have shown that this angle is more sensitive to longer wave length ripples than 20 degree. Haze describes the milky halo or bloom adjacent to the reflected image. Reflection haze on a vehicle body causes a "milky" finish and a drop in reflection contrast. Surface haze can be problematic in most coating applications including automotive manufacture, powder coatings and other high gloss coatings. It can be attributed to a number of causes including incompatible materials in a formulation, difficulties in application and problems encountered during drying/curing/stoving. Reflected light sources in surfaces with medium and high haze values. Coatings without haze can be seen to have a deep reflection and have high reflected contrast. Those with a lower level of reflection haze exhibit a slight "milky" finish which can be seen at the surface. When viewing the reflection of a strong light source in a surface with high haze the image "blooms" and has a bright halo around it. Haze is an important measure for highly polished metals and is often associated with polishing marks and machining direction. Gloss-haze meters are traditionally used to measure this parameter and use a standard glossmeter design with additional light stops and diodes to measure the haze component 2⁰ either side of the specular. Gloss-haze instruments have not found mainstream acceptance in the coating industry because to take repeatable measurements the sample surface needs to be completely flat and un-textured. The relative high cost of these instruments has also been found to be prohibitive. Measuring Haze with the Rhopoint IQ The Rhopoint IQ automatically compensates for sample surface flatness and texturing allowing haze measurements to be taken on most product surfaces. Log Haze Values are commonly quoted for paints and coatings as this scale has increased resolution at the haze levels commonly seen on this type of surface. There are several factors that influence the gloss and appearance quality of a coating; The chemical composition of the coating/topcoat/varnish vehicle, the refractive index of this component largely determines how light is reflected from the surface. The size and frequency of structures on the surface, these can be deliberate in the case of matting additives which scatter light, or unintentional and unwanted such as additive bloom, orange peel, reticulation etc. Uneven substrates may cause texturing to the coating surface. For transparent coatings the reflective nature of the substrate is also important. Metallic and Effect Pigments, the size, distribution and alignment of these larger size particles have an effect on the reflective characteristics of a coating. By measuring Gloss, Haze and DOI, the Rhopoint IQ quantifies the visual impact of a product and the nature and frequency of textures on its surface. Most painted and coated objects have some degree of surface texture which impacts their visual appearance. These textures can be described by their size and the visual effect they have on the paint surface or the quality of a reflected image. The smallest textures (<0.01mm) result in gloss reduction, the largest can result in visible ripples on the coated surface (> 1mm ). The Rhopoint IQ is sensitive to surface structures up to 10mm in length. A diagram showing the effect of structures on surface appearance. The smallest structures cause gloss reduction, larger texturing causes haze, the largest structures such as waviness and orange peel reduce the DOI of a surface. A crucial factor in assessing the appearance of any manufactured product is the "glossiness" of its finish. Gloss finishes are often used make products visually appealing. Highly reflective, extremely smooth finishes are used to enhance the aesthetics of cars, luxury yachts and high end consumer electronics. Matt finishes are used for contrast furniture and architectural features. The Rhopoint IQ measures standard gloss within existing standards, and uses new technology to examine in more detail the distribution of light around the specular angle. The instrument provides goniophotometric profiles, gloss, haze and distinctness of image values which can be used to assess the visual qualities of a surface. As well as assessing the visual impact of a surface, the Rhopoint IQ can also be used to identify problems with coating formulation and application. Specular reflectance from a "perfect mirror" surface- all light is reflected in the specular direction |
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