Fringe pattern can be projected fast by digital projector using DLP technology. The projection speed is higher when patterns with lower bit-depth are adopted. The phase error of sinusoidal fringe pattern with different bit-depth is studied with three-step phase-shifting algorithm. The uniform quantization algorithm (UQA) and quantization algorithm with error diffusion (EDA) are used for pattern quantization. The conclusions are as following. 1) With UQA, the maximum of phase error will less than 1% of 2π when bit-depth is higher than 4 bits. If the projector is defocused, the error will be decreased. 2) With EDA, the maximum of phase error is larger than that with UQA. But the error will be decreased significantly when the projector is defocused. The phase error of pattern with EDA is smaller than that of pattern with UQA when the projector is nearly focused and the period of pattern is long (for example longer than 20 pixels). If the period of pattern is short, the performance of UQA is always better. 3) The error difference of UQA and EDA will be very small when the bit-depth is higher than 4 bits.
A method for high-speed three-dimensional measurement with low-speed camera is proposed. The spatial frequency encoded fringes are projected with high frame rate and deformed fringes are captured with low frame rate. Several fringes are integrated in one captured image. The directions and/or frequencies of these fringes are different. The spatial frequency spectrum of these fringes is separate in spatial frequency domain. So, the phases of different fringes can be obtained by filtering the image with different filters. Then several 3D shapes of different time are obtained from one captured image. The experiments are carried out to verify proposed method and measurement results are demonstrated. The method improves the speed of 3D shape measurement and reduces the cost of measurement system as well.