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.