| 摘要 |
PROBLEM TO BE SOLVED: To stably produce a preform by detecting a shape and/or a position of a preform from measurement values of luminance of a preform and its periphery in an axial and/or radial direction, in this production that involves controlling a preform so as to have an optional shape and/or an optional position, from a detection of a shape and/or position of a preform. SOLUTION: This production method comprises the steps of measuring some rate values of change in luminance in an object; finding the differential coefficient value of each of the measured rate values of change in luminance with respect to the position in the object, at which the rate value of change is measured (measurement position); further, performing curvilinear approximation of differential coefficient values between the maximum differential coefficient valve and the minimum differential coefficient value of all the differential coefficient values thus found, by using a cubic equation; and detecting the measurement position at which the rate of change in luminance becomes maximum and therefore, the differential coefficient value shows zero, as the shape and/or position of the preform in the process of this production, such as: the position of the front end part of the preform in the axial direction (vertical direction), in which the rate of change in luminance becomes maximum; and/or the shape of the front end part of the preform, as a whole. |
