Shape and colour measurement of colourful objects by fringe projection

Abstract

In this paper, we present a novel method to measure shape and colour information of a colourful object by projecting separate red, green and blue colour fringe patterns onto the object surface. With regard to the object surface's colour, the modulation at each pixel position in the three colour channels has different values. For example, when projecting blue fringe patterns onto a red point, the corresponding pixel has too low a fringe modulation to accurately calculate the phase (shape) information; but with red fringe patterns a high fringe modulation is obtained. Therefore, phase information of the red point can be calculated by projecting red fringe patterns. For each object point, by comparing the modulation values from the three colour channels, it is possible to choose the channel having maximum modulation, and hence phase information can be reliably obtained by the phase-shifting algorithm. The fringe order information is obtained by using the optimum three-frequency selection method, so there is a maximum reliability in determining the fringe order and the 3-D shape of an object with step or large slopes on the surface. Since three colour channels are used, colour information of the object surface can be extracted with high dynamic range from the same fringe patterns. Chromatic aberration between colour channels is unavoidable and can be eliminated by a software-based method. Using the recently developed colour fringe projection system, separate colour fringe patterns are projected onto a mug having different colour patterns, a colourful box and plate, and a colour checker card to test the proposed method. The results show the range of colours that can be measured and that shape and colour information of colourful objects can be reliably obtained.