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.