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Procedures Hinge on Nonlaser Light Sources’ Optimal Spatial Intensity Distribution

BioPhotonics
Jun 2016
Commercially available laser beam profiling systems are commonly employed to not only test the quality of lasers but also to evaluate the spatial intensity distribution of other light sources important to the medical, forensic and dental professions.

KEVIN KIRKHAM, OPHIR-SPIRICON LLC

Unsterilized medical products, dental restorations with incompletely cured restorative materials and adhesives that fail in their application are a few of the potential outcomes when light-based processes are poorly understood. Laser beam profiling (LBP) products are used to evaluate these and other critical light sources. For example, forensic light sources are tested for their intensity distribution patterns at various wavelengths, and low-level light therapy products are evaluated for the correct irradiance patterns for assured efficacy. As Sir William Thomson (Lord Kelvin) said, “If you cannot measure it, you cannot improve it.”

 Poor analysis of nonlaser light from a dental curing lamp could result in dental restorations with incompletely cured restorative materials.
Figure 1.
Poor analysis of nonlaser light from a dental curing lamp could result in dental restorations with incompletely cured restorative materials.

LBP products are typically composed of a 2D sensor array; optical imaging and attenuation optics; and software that displays, analyzes and records the visual representations of the distribution of light for measurement, archival and retrieval. The heart of a LBP system is a scientific grade CCD or CMOS camera. The camera is selected and tested for linearity over the usable range of the system, uniform response and accuracy. There are no blemishes or inoperative pixels in the sensor arrays that detect the scene and convert it to an electronic signal. The camera is selected for its wide dynamic range and typically has had the window and IR cut-off filter removed so that there are no interfering optics to distort the image or limit the spectral response. Beam profiling cameras typically provide images with a signal-to-noise ratio of 58 dB or better and are digitized to 12 to 16 bits or more.

LBP software — the brains of the product — provides user selectable images that intuitively describe the intensity distribution of the light source under test. False color, 2D and isometric 3D displays inform the user of variations in the light intensity pattern emitted from the texted devices.

Measuring nonlaser light sources

Given that LBP systems are designed and manufactured to provide high fidelity images, it is easy to measure and document the performance of other light sources where the intensity distribution is critical to the application. Light-emitting devices, ranging from LED and luminaires to dental curing lamps and sterilization systems, may benefit. The list of applications is extensive, and these systems are capable of providing very useful information for the alignment and analysis of any light source with an intensity distribution pattern that is critical to the efficacy of its application.

Measurement systems can be quickly and easily constructed to assist in the manufacture, alignment and documentation of such products. Commercial off-the-shelf software provided with these products permits the user to make high-resolution spatial measurements as a function of spot size and intensity distribution of light. With known spatial and intensity calibration, beam profiling systems can be employed to provide an accurate, traceable understanding of the distribution of light intensities from many commercial devices.

Dental curing lamps

A useful example of a LBP system application on nonlaser light is the analysis of dental curing lamps (Figure 1). These monochromatic or multiwavelength, handheld systems are used in dentists’ offices around the world to cure polymer resins for restorative dental procedures. UV and visible light-cure compounds can provide lifelike, color-matching restorations. These realistic restorations are made possible by the LED-powered curing of these composites. Fully and uniformly cured and hardened restorations can now be quickly tailored thanks to new, light-polymerized resin compositions and high-intensity UV and blue light sources that feature evenly distributed or homogeneous light.

 A diffusive target and camera comprise a system for measuring a highly divergent light source.
Figure 2.
A diffusive target and camera comprise a system for measuring a highly divergent light source. Filter wheels can be used to place optical bandpass filters in the light path to selectively measure — by wavelength — the output of each source that comprises the light-empowered product. Courtesy of Dr. Fred Rueggeberg.

The distribution of light produced by these illumination sources can be quickly measured with the use of a modified LBP system. Nonlaser optical metrology applications of the LBP products utilize a diffusive target and imaging lens. The target provides a flat, uniform surface that can impinge the light source being tested. The target must be secured in the light path and is typically placed at the same plane or distance from the source where the light or illumination system will be employed.

Target material must be transmissive and diffusive, but it must not introduce any pattern or structure of its own. Ground glass and flashed opal are common choices for targets. The grain or grit of the material must be smaller than what can be resolved by the imaging system. Targets should also be selected so as not to be damaged or discolored by the light from the device under test.

When light from the curing lamp is impinged on the target, for example, an imaging lens will relay the target image to the camera detector array (Figure 2). The camera collects images of the light intensity distribution of the lamp under test and provides them to the system software for display and analysis (Figure 3).

Beam profile of dental curing lamp.
Figure 3.
Beam profile of dental curing lamp. Courtesy of Ophir-Spiricon LLC.

In another example, when the output of two LED-based, battery-operated curing lights is measured with a modified Spiricon LBP system, a false color scheme can represent varying levels of irradiance falling onto the target (Figure 4). These measurements provide a subset of pixels on which to distribute the total power. The power measurement is divided by the total number of digital counts to obtain a count per intensity scale. From this, each pixel is assigned a power over area value.

Radiant exitance, spatial nonuniformities and filters

The LPB system can be calibrated for total intensity, or radiant exitance, with the use of an integrating sphere and silicon photo diode sensor. The total integrated power can be read by Ophir’s BeamGage software application, which provides a 2D map of the intensity distribution of the light source under test. Once calibrated for total intensity and effective pixel spacing, each pixel has a power density value associated with it.

The output of two LED-based, battery-operated curing lights measured with a modified Spiricon laser beam profiling system.

Figure 4.
The output of two LED-based, battery-operated curing lights measured with a modified Spiricon laser beam profiling system. One unit has a total integrated optical power of 310 mW and average power density of 3.26 W/cm2 (a). The other device has total power of 714 mW and an average irradiance or power density of 1.19 W/cm2 (b). Courtesy of Ophir-Spiricon LLC.

Spatial nonuniformities, such as those due to the placement of the internal light sources, can be plainly seen and quantitatively measured. Most noncoherent light sources can be effectively measured with commercial LBP systems. By associating a known, externally measured total power, or radiant flux, of the device under test with the beam profile, 2D maps of the distribution of the light source can then be made.

Filters that limit spectral transmission can be used to measure the distribution of the various sources that comprise the light output from the product. Such measurement systems can be employed to ascertain the goodness of the light source per the application by manufacturers and users of the product. The addition of a spectrometer confirms the wavelength or wavelengths of interest. It also assures the spectral output of the device under test falls in the 350- to 1100-nm spectral range of the CCD camera and optics.

While goniometric radiometers, which measure divergent angles, are still the standard for measuring the output of widely diverging light sources, camera-based radiometer systems, such as the LBP, continue to increase in market share due to ease of configuration and use. As LBP-based light measurement systems are enhanced with new target materials and shapes, the use of telecentric lenses, and automated filter selection, new measurement applications will be quickly realized.

Meet the author

Kevin Kirkham is a senior manager of product development for Ophir-Spiricon LLC in North Logan, Utah; email: kevin.kirkham@us.ophiropt.com


Best Practices for Measuring Nonlaser Light Sources

Below are several tips for using commercial laser beam profiling (LBP) on nonlaser light sources:

• Each wavelength should be measured separately, as the response of silicon CCD cameras is not spectrally uniform. The total integrated power of each spectral contributor should also be measured individually and associated with the spatial profile of that wavelength.

• Care must be taken to ensure that the camera exposure and gain controls are not modified once the calibration of the total radiant flux that is impinging onto the target has been made. These controls change the camera sensitivity and therefore modify the calibration of the light source.

• If filters are used as attenuation devices, the filters must be consistent with the profile and power measurements and not modified once calibrations are made.

• Place something of a known dimension in the image plane so that the measurement software can be spatially calibrated. Most laser beam analysis systems allow users to modify the dimension of the pixel to accurately represent the optical minification of the imaging system. The measurement may be compromised when the source-to-target distance is changed.

Three-dimensional image of the irradiance distribution of a matrix of monochromatic LEDs.
Figure 5. 
Three-dimensional image of the irradiance distribution of a matrix of monochromatic LEDs. Courtesy of Ophir-Spiricon LLC.

• Not all of the light exiting the source is impinged on the target. As the target-to-source distance is increased, the amount of light from a divergent source impinging on the target is decreased. This distance must remain constant when comparing light sources.

These techniques may also be used to measure the output of low-level light therapy devices. With the matrices of LEDs used in a clinical photobiomodulation product, the total contribution of multiple wavelengths can be combined by measuring each separately (Figure 5). The images can be combined and the individual power measurements added to obtain a useful map of the intensity distribution of the device.



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