Fiber Painting

Fiber Measurements
The paint thickness on the fibers should be between 10 and 20 microns. It takes practice to achieve a consistently good layer, as the measurements from several tests show.


 * Fiber Painting Measurements
 * Paint Thickness Histogram

Fiber Pictures

 * Painted Fibers

Procedure for Painting
Wear gloves and hold the fiber in one hand while spraying on the paint with the other. It is best to paint while sitting down at the painting table rather than holding the fiber and brush at waist level. If your eyes are lined up with the paintbrush you will be able to see where the paint stream is going. Spray a little, then pause and look at the fiber. If it appears speckled with clear points (when painting a waveguide), or green points (when painting a scintillating fiber), it needs more paint. If the paint layer is covering the intended area, it still may be too thin. If you can see through it too much, it needs more paint. The paint does not have to be completely opaque, but you should have to rotate the fiber or hold it up to the light to see anything other than white.

The distance of the brush from the fiber during painting is important. Holding the brush too close to the fiber causes the paint to bead up along the edges and be too thick there. Four inches from the fiber is a good distance for spraying the paint.

To paint a long fiber, paint one short section at a time, starting at one end and working your way to the other.

Getting a Steady Stream of Paint
The biggest challenge in spray painting the fibers is maintaining a good stream of paint from the brush. It is important to clean the spray paintbrush, the jar and the tube for the paint after using them. The spray paintbrush sometimes does not spray well because of clogging somewhere. The paint can even clog the tube. To avoid a blocked tube, the tube should be rinsed after use with water at an adequate pressure: as much as possible without splashing too much.

If the paintbrush is blowing air and no paint, the seal where the tube passes through the lid of the jar might be loose. If so, you should use a new lid.

If the paintbrush is not working when you are trying to paint, you may want to clean it partially by spraying water. If you do not want to do a thorough cleaning, do not separate the tube from the paintbrush. Inserting the tube back into the inlet sleeve without washing the tube and the sleeve will cause the tube to stick to the sleeve when the paint dries. Pulling hard on the lid to detach the tube from the sleeve will then loosen the seal, and the paint will no longer spray. A better plan for incomplete cleaning is to leave the tube in the sleeve, unscrew the lid from the jar, and screw it onto another jar full of water or dip the brush and tube in a bucket and operate the brush underwater.

To avoid damaging the seal mentioned above, it is best not to pull on the jar lid to remove the tube from the inlet sleeve. To this end, there is a bulge in the tube that you can grip if you cannot hold on closer to the tube exit.

Cleaning the Paintbrush with Isopropyl Alcohol - After Using Water
We are using acrylic paint, which can dissolve in isopropyl alcohol. After cleaning the paintbrush with water, it should be further cleaned by diluting the alcohol in water and spraying the solution. This cleaning should be done near the hose that draws air out of the lab to avoid breathing the alcohol vapors too much. After spraying the water and alcohol, remember to spray some water without alcohol to clear the brush of alcohol.

Looking at the Fiber
What you see depends on how you hold the fiber. The fibers have (approximately) a square cross-section. (What look like the sides of the square are in fact curved.) If your line of sight is perpendicular to a face of the fiber, you may see only the paint. But holding the fiber up to the light or rotating it can make the underlying color visible. If you rotate the fiber, you can see the color on the shaded underside. Gaining familiarity with the appearance of the fiber as it is rotated may help the worker to recognize a good paint coat.

Measuring the Paint Coat
Using a micrometer to measure the paint thickness can damage the paint on a fiber. The sharp edges of the spindle and the anvil can easily scrape off paint as the the fiber is placed between them and as the screw is tightened. Putting masking tape over these sharp edges may reduce damage to the paint. To avoid scraping the paint after recording a measurement, let the fiber fall rather than holding on to it as you are loosening the screw.