We propose a novel structured light method, namely trapezoidal phase-shifting method, for 3-D shape measurement. This method uses three patterns coded with phase-shifted, trapezoidal-shaped gray levels. The 3-D information of the object is extracted by direct calculation of an intensity ratio. Theoretical analysis showed that this new method was significantly less sensitive to the defocusing effect of the captured images when compared to the traditional intensity-ratio based methods. This important advantage makes large-depth 3-D shape measurement possible. If compared to the sinusoidal phase-shifting method, the resolution is similar, but the processing speed is at least 4.5 times faster. The feasibility of this method was demonstrated in a previously developed real-time 3-D shape measurement system. The reconstructed 3-D results showed similar quality as those obtained by the sinusoidal phase-shifting method. However, since the processing speed was much faster, we were able to not only acquire the images in real time, but also reconstruct the 3-D shapes in real time (40 fps at a resolution of 532 x 500 pixels). This real-time capability allows us to measure dynamically changing objects, such as human faces. The potential applications of this new method include industrial inspection, reverse engineering, robotic vision, computer graphics, medical diagnosis, etc.