White Light in Focus
March 30, 2009
To astrologers, white light is a powerful source of healing energy. To metrologists, white light is used in sensors for measurement and integrated with multisensor measurement machines.
Almost all multisensor measurement systems today are equipped with cameras and tactile probes. Beyond these, operators choose from a range of additional sensors. Right now, white light probes are surpassing lasers as the most frequently selected add-on to many multisensor systems.
White light probes provide high data-acquisition speeds, ultra-high resolution and nearly complete insensitivity to refracted light. In many applications, this makes them the tool of choice for collecting surface point data.
White Light vs. Laser ProbingLaser probes are monochromatic, generating a single frequency of light, while white light probes are polychromatic, generating a full spectrum of light wavelengths. Both types of probes analyze the light returning from the surface of the part. The difference is that laser probes measure the intensity, or amplitude, of the returning signal while white light probes analyze the presence of certain color spectrums, or frequency.
Because lasers measure light intensity, they can be thrown off by such things as color or texture variations; translucent, transparent or highly reflective surfaces; variable ambient lighting conditions; and measurements not normal to the surface of interest. White light sensors are impervious to all of these problems and they capture point data five to six times faster.
Use of a Laser ProbeThere are several reasons why a metrologist would choose a laser probe over a white light probe. Better edge detection is one. Because of the refractive properties of laser light, it continues to return information as it drops over an edge. White light probes do not do this.
Therefore, one would not use a white light sensor to measure a part with many holes. For the same reason, a metrologist would chose a laser probe to measure a sphere, because it is capable of providing meaningful data as it follows the surface. It is worth noting that cameras are faster than either laser probes or white light probes when it comes to the simultaneous capture of large numbers of edge data points on flat features such as holes.
Focal range is another reason to consider using a laser probe. Because of its high accuracy, white light probes have very narrow focal ranges, typically ± 3 millimeters. Lasers, on the other hand, are much more forgiving. A focal range of about 50 millimeters makes laser probes better candidates for use on most coordinate measuring machines (CMMs), portable arms and hand-held measurement systems.
Today, good general-purpose white light sensors cost $25,000, while a laser costs between $7,000 and $8,000. This can deter some manufacturers, but in most cases it becomes a moot point after the benefits are understood.
White Lights ShinesWhite light sensing is an exceptionally effective tool for capturing data from free-form sculptured surfaces. The resolution of the data is so fine, in fact, that several manufacturers offer special, stand-alone white light stripe systems that rapidly scan, capture and analyze surface finish data.
On multisensor machines, operators are currently limited to using white light spot probes for capturing point data from surfaces. This is still a powerful tool, and in some systems software allows the use of a white light probe in a manner identical to using a tactile probe for linear, patch and rotary scans. The rate of data capture is limited only by the computer’s ability to process and store the collected data.
There are myriad applications that can benefit from this speed and precision. For example, an Asian mint wanted to measure the surfaces of several coins so that operators could compare the actual part surfaces to their computer-aided design (CAD) models. Scanning them with a tactile probe would have been slow and cumbersome and would have yielded data of insufficient resolution. Laser scanning also was unacceptable because of lower data resolution and detection problems created by changes in coloration of the coins from the silver outer ring to the inner copper ring.
The only feasible solution was to use a white light sensor to scan the coins and collect the data into point clouds. Next, software used these clouds to construct surface models for comparison to the original models. No other measurement technology would have allowed the customer to collect and analyze the data at the speed and level of accuracy required.
In addition to this example, white light sensors can be used for a range of scanning applications including reverse engineering; measuring small parts and features not measurable with other types of probes; and measuring many types of flexible parts.
On the HorizonWhite light probe technology is evolving rapidly. Recent developments include the introduction of white light sensors capable of measuring inside very small holes with measurement accuracies of up to 200 nanometers.
White light sensors also are seeing use in specialized, stand-alone systems to measure roundness, flatness and, as mentioned earlier, surface finish. These are not yet ready for use on multisensor systems, but it will not be long before sensor manufacturers learn how to adapt these products for mounting on multisensor machines. When that happens, today’s advanced measurement software will incorporate the sensors into the multisensor toolbox easily-just like any other probe.
As a prime example of this, stand-alone, white light stripe scanning machines capture large amounts of surface data at unprecedented speeds. Today, these probes are not available on multisensor systems. However, measurement machine manufacturers and software developers are working to support them, and it should not be long before they are available.