Effect Of Lighting On Probe Diameter Measurements

The type of lighting and intensity has a significant effect on the diameter of a probe image which will be "seen" by an image capture system. In the Probilt image capture system, the probe shows up as a bright object in a dark background. In simple terms, light which reflects back towards the center of the image capture system will be seen by the CCD camera as image. Light which reflects to the outside, will not be seen and that area of the object will remain dark like the background. Different incident angles of light and different probe shapes will produce different (and reasonably predictable) results.

Figure 1 shows a probe tip with a rounded (hemisphere) profile lighted by only direct incident light. This is equivalent to the "Bright Field" lighting in the Probilt system and is directed at the probe through the lens of the optical system. In general, the rays of light (A, B and C) are traveling in a straight line toward the probe. Because of scattering, lens aberrations and the fact that the light is not a point source, this is not strictly true but is a good approximation. Note that in this case, only the center ray of light (C) is reflected back toward the center or source. Thus the only area which will show up as a bright object is a small dot in the center of the probe tip.

Figure 2 shows the same type of lighting in combination with a "flat tip" probe. Note that the probe is not really flat around the edge. This curvature will vary with the diameter of the probe, the type of sanding used, the age (amount of use) of the tip, the material, and other factors. In any case there will always be some curvature on the edge. The only question is, "how much?". Note that the flat portion of the tip will reflect the light back to the source and it will show up as a bright object. Part of the edge may show up depending on the curvature. In trying to determine the diameter of this type probe tip, a decision must be made as to what point on the curved edge is really the outside of the probe tip. In mechanical engineering circles, this is usually considered as the point where a line tangent to the curved surface would be 45 degrees from the centerline of the circle. Of course, this is virtually impossible to determine by optical or physical measurements. In the Probilt system, the lighted portion of the probe will determine the diameter. Using only the direct (Bright Field) lighting, most of the curved edge will remain dark and will not show up as image.

Figure 3 shows a configuration much closer to the actual Probilt lighting system. A direct (Bright Field) light is combined with an angled incidence (Dark Field) light. The intensities of these two lights may be individually adjusted to light as much of the curved edge as desired. In attempting to correlate this diameter measurement with manual methods, it must first be determined if the manual method is repeatable and reproducible. One problem is to determine where the actual edge of the probe occurs. If we assume the edge is defined to be a point where a line tangent to the curve of the edge would form a 45 degree angle with the centerline of the circle, probably the best we can do is approximate it by setting up the light sources to include about half of the curved part. An optical capture system will include only the lighted portion of the object so the diameter is primarily determined by lighting. If a repeatable and reproducible manual method can be devised, the Probilt can be made to correlate with it by adjusting the two light sources to obtain the proper image size. The Dark Field light will have the largest effect on this. Proper adjustment of the Dark Field light can enable the system to capture a reasonable diameter on flat or spherical tip probes. A combination of Bright and Dark Field lighting is usually the best choice since the probe tip is typically a mixture of flat and curved surfaces.


The effect of focus on the problem is easily seen. If the image is slightly out of focus, the diameter will appear to be slightly larger. This is a second order effect and is not nearly as important as lighting. One surprising effect of a slightly out of focus image is that an image capture system may actually be more repeatable than one with a sharply focused image. This is because very small changes in the position of the probe tip can make significant changes in the reflected image size when the image is sharply focused. Because real life probes are usually not simple flat or spherical shapes, the problem is more complex than shown in these examples. Probe tips may not be round and the measurement technique must take this into account. The algorithm for handling the processing of the captured image is significant and will be the subject of another paper.

Procedure developed by Rod Schwartz with help from ITC's Application Engineering Department.