Optical design and measurements
Fig. 1a and b illustrates the complete arrangement of the laser micrometer setup principle. The laser is Coherent laser 0222-200-00EF with 4 mW of power, operates at 635 nm. The laser light falls at an angle of 45° to the facets of a rotating polygon mirror. The polygon mirror is eight facets of Lincoln laser USA with microcontroller PWB 1-2-3060-610-00 P1BB. The microcontroller provides an RPM in the range from 1000 to 10000. The F-theta lens is of Edmond Optics Model no.63-311 with an effective focal length of 165 mm. The collimating lens size is 27 mm with a focal length of 60 mm. The detector system is an 818-BB detector connected to a digital storage oscilloscope (DSO) through BNC. Without any object in the illuminating path, the pulse width is proportional to the lens size. In our case lens size of 27 mm, gives the pulse width of 64.10 µs at 8000 RPM Fig. 2a. When an object is placed, the light in the central portion of the pulse is interrupted causing a dip in the pulse width Fig. 2b. The different pulse sizes are plotted for a number of solid objects from 3 mm to 13 mm Fig. 3. The curves show linearity. The analysis predicts that an RPM value of up to 2000 can be chosen for object size measurements. The slope of the curve gives sensitivity in µs/mm. The reciprocal of this when multiplied with the width of the pulse dip of an unknown object gives the object size in mm.