Glassfiber diameter measurement

Glass wool is a common product as insulation material and produced worldwide.
Manufactures are still trying to increase its quality.
The major parameter of its thermal insulation is the fiber diameter. By decreasing the diameter of the fiber the insulation quality increases. The major problem of measuring the fibre diameter fast and accurate within the range of 4 to 20 microns.
Measuring a microscoop needs an objective with a large focus depth.

Vision inspection setup

To measure a fibre diameter between 4 and 20 micron a microscopic setup is used. The sample (image left)are a few glass fibers pressed between two glass plates. Glass_fiber_low_focus_depth Glass_fiber_test_small The image created with a digital camera and a LED back light. The field of view of this image is 0.8x0.6 mm to show the focus problems. In this image contains a lot blurred fibers and 1 sharp fiber, a consequence of a low focus depth.
Measurement of these blurred fibers is not accurate because the diameter is focus dependend. Measuring only the in-focus fiber is accurate.
To solve this focus problem J.A.I.M.S. developed a large focus depth microscope objective. As a consequence of increasing the focus depth the Modulation Transfer Function (MTF) decreases.
For automatic fiber diameter measuring it's acceptable. The measurement is done over the whole fiber in the image. The results of the large focus depth objective is shown in the second image. The size of this image is 1.2x1.0 mm scaling is 1 micron/pixel.

Diameter_vs_focus_depth Standard dev vs height normal The back light is also improved and more homogenious.
To measure the optic quality of the objecive, the diameter of one fiber is measured as a function of the displacement on the optical axis (height). Three types of modifications have been tested and one measurement has been done without modification (normal). The results are shown in the diagrams and show a huge increase of the focal depth for Mod-2 and Mod-3.
Also visible is an absolute offset of 2 microns. This is a consequence of the measuring algorithm and a correction can be solved by software.
Needless to say a calibration will be necessary. Also the standard deviation (= SQRT(variance)) is given. The average value is smaller then 0,2 micron over the full focus depth of about 500micron. And is slightly higher then the minimum of the normal objective (Norm Obj).

Software

To detect the fibers indivually and rejecting the fiber crossing (resuls in a bad measurement) the bundle of fibers are virtually cut in pieces (measurement areas).

In every measurement area the fiber is located followed by a very accurate edge detection algorithm perpendicular to the fiber. Fiber_measurement Frequency plot fiber diameter
This algorithm detects both sides of the fiber. Using these edge points the width of the fiber is calculated.
The edges used for calculating the diameter are shown in red (left image). The graph (right) shows the frequency plot of the diameters.
The scale in this situation is 1 micron per pixel.

For more detailed information about Glass Fiber Diameter Measurement download the pdf (Only in Dutch available, sorry): Glasfiber meten NL.pdf