The optical sensor is a measurement sensor that is widely used to measure the 3-D shape profile of object surface as a non-contact type. However, because the operating range of the sensor should be wide to measure the shape of objects that have lots of variation of a slope or bend, the accuracy of height measurement gets weak. If the surface of objects has a lot of variation of slope or bends, the gain of the optical sensor is varied according to the slope. Because height information obtained with the constant gain is incorrect information, it should be compensated without a large error. In order to improve performance measurement for this reason, if the method of scanning the object surface along the x and y directions using a gap control between the optical laser displacement sensor and object surface is applied, the nonlinearity could decrease. Moreover, compared with the method of maintaining the constant height of the sensor, the method of the gap control could reduce the measurement range of the sensor, so that the accuracy of height measurement for the sensor increases. In addition, the accurate information of shape
measurement can be obtained, because the error caused by the slope or bend on the object surface can be reduced. Therefore, 3-D shape profile measurement using the method of constant gap control between the optical laser sensor and object surface shows better performance rather than the method of maintaining the fixed height of the sensor from the bottom.