We have designed and constructed the calibration system of line standards such as tape and rule for the secondary
calibration laboratories. The system consists of the main body with linear stage and linear encoder, the optical
microscope with digital camera, and the computer.
The base of the system is a aluminum profile with 2.9 m length, 0.09 m height and 0.18 m width. The linear stage and
the linear encoder are fixed on the aluminum profile. The micro-stage driven by micrometer is fixed on the carriage of
the long linear stage, and the optical microscope with digital camera and the tablet PC are on the this stage. The linear
encoder counts the moving distance of the linear stage with resolution of 1 μm and its counting value is transferred to the
tablet PC. The image of the scale mark of the tape is captured by the CCD camera of optical microscope and transferred
to the PC through USB interface. The computer automatically determines the center of the scale mark by image
processing technique and at the same time reads the moving distance of the linear stage. As a result, the computer can
calculate the interval between the scale marks of the tape. In order to achieve the high accuracy, the linear encoder
should be calibrated using the laser interferometer or the rigid steel rule. This calibration data of the linear encoder is
stored at the computer and the computer corrects the reading value of the linear encoder.
In order to determine the center of the scale mark, we use three different algorithms. First, the image profile over
specified threshold level is fitted in even order polynomial and the axis of the polynomial is used as the center of the line.
Second, the left side and right side areas at the center of the image profile are calculated so that two areas are same.
Third, the left and right edges of the image profile are determined at every intensity level of the image and the center of
the graduation is calculated as an average of the centers of the left and right edges at all intensity levels.
The system can measure the line standards up to 2.5 m. The expanded uncertainty for the tape calibration is U = [(0.04)2+ (0.015•L)2]1/2 mm, where L is measured length of the tape or rule in meters. At this system, the long distance
measuring instruments such as ultrasonic distance meter or laser displacement sensor can be also calibrated.