Orange Peel & DOI FAQs

What is orange peel?

When a person hears the term “orange peel” they instantly think of the outer skin of an orange and the textured appearance it has; to a coatings specialist however these words can mean a major headache in terms of how they are going to control the surface appearance of their coatings.

For many years in the coatings industry, orange peel has been used as a term to describe the visual sensation of texture on a painted surface. This texture is a combination of different structure sizes that can be caused by a number of different variable in the surface preparation and painting process.

In some industries such as the decorative paint industry, orange peel is desirable as it can create an attractive patterned surface for walls. However in the automotive industry, specific actions are taken to reduce the orange peel effect to a minimum in order to give a sharp, high contrast “quality” visual sensation, so that when the car in the showroom is viewed by a potential customer they get that “wow” feeling which eventually leads them to purchasing.

Factors that influence the levels of orange peel during manufacture can be classified into specific process areas. For example, during substrate material preparation, the use of incorrect abrasive materials can create fine structures on the material surface which can cause problems later down the line during paint application. Variations in the coating itself, i.e. coating thickness, viscosity and flow characteristics, particle size distribution and raw material quality can create larger structure sizes in the finished surface; also the orientation in which the coating is applied will cause more or less orange peel to be created, vertical application will always be greater than horizontal.

So from a coatings specialist point of view, the control of all these factors is critical in maintaining consistency of surface appearance quality of the finished product.

The Rhopoint IQ can detect changes in the orange peel of high gloss finishes. This instrument is used to measure orange peel in the paint industry.

How is orange peel measured?

Orange peel has typically been measured using a set of standardised test panels with varying degrees of orange peel as a visual comparison.

This method, not only being time consuming and highly subjective is also not in any way accurate as there is no real data available to identify potential problem areas in the process. Instrumental methods of providing quantitative information have evolved enabling a greater understanding of Orange Peel and its causes resulting in the creation of a new metric, DOI (Distinctness Of Image) able to express the degree of orange peel present numerically. By measuring the clarity of reflected images the smoothness of a surface can be determined. The higher the degree of orange peel present, the less defined the reflections. 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.

What is DOI? (Distinctness of Image)

DOI or Distinctness Of Image is, as the name implies a function of the sharpness of a reflected image from a surface.

Painted surfaces finished with similar coatings may produce identical gloss values when measured using a glossmeter however when visually assessed the quality of one surface against the other may be seen to be better than the other. Typically upon closer inspection the visually different surface will contain a degree of Orange Peel causing the reflection to become fuzzy and distorted.

The images above demonstrate this measured versus visual difference, all surface measure identical gloss values however visually they appear different due to varying levels of Orange Peel present.

The DOI value of a surface is a number between zero and one hundred; a surface that reflects a perfect undistorted image returns a value of 100, as the value decreases the image quality deteriorates.

How is DOI (Distinctness of Image) measured?

DOI can be measured using a similar principle to gloss measurement by projecting light onto a surface at a particular angle. As orange peel is more noticeable on highly reflective surfaces a smaller measurement angle at 20° is preferred. The reflected light is collected at an angle equal but opposite to the normal specular angle using a wider angular band of measurement. This enables the amount of light deflected away from the specular angle to be determined.

The Rhopoint IQ is designed using the 20°/60°/85° geometry of a standard gloss meter but incorporates 512 element, linear, photo-diode array (LDA) at the 20° angle instead of a single detector enabling measurement of the distribution of reflected light. The spacing of the pixels of the LDA is such that it measures at the 20° ± 7.25° in steps of 0.02832°. The orientation of the source slit/aperture is set perpendicular to the plane of the incident and reflected beams to comply with the standard ISO 2813 – Determination of specular gloss, while the LDA is set in the plane of reflection.

The light source used in the instrument is a high power white LED filtered to correspond closely to the required spectral response, i.e. the photopic response function V(λ). The instrument calculates gloss values using the response from elements of the LDA which correspond to the angular tolerances in ISO 2813.