A new depth measurement method is described in this paper. This method uses the parallel planes of light each of which flickers in the time domain according to the binary code uniquely assigned to it. When the code length is n bits, we input n pictures projected on a certain object in which we locate and "identify" each slit image. Then the depth to the points on the slit images can be calculated by triangulation. The experimental results show that this method is faster and stronger for noise than the conventional methods. We aim to investigate the importance of knowledge about the task domain being used in signal level processing. Picture processing systems are usually task dependent, so the knowledge about the domain can be applied even in signal level processing. The knowledge is, we believe, more powerful to be used at earlier stage in picture processing than to be used at the latter one. A result about the transformation from knowledge to signal-to-noise ratio is shown as an example.