Determination And Validation Of The Spectral Power Distribution (SPD) OF Artificial Weathering (Lightfastness) Instruments Matthew McGreer, Atlas Material Testing Technology LLC, Chicago, IL, USA; George Coonley, KHS Technical Lighting Systems GmbH, Chicago, IL, USA will adversely affect the quality and quantity of light that impinges Abstract upon test specimens. Automatically controlled irradiance (Ci) laboratory Resulting variations in light output will also cause weathering (lightfastness) instruments were first introduced in unreliability in test results, given its importance to the 1970. The technology has since become universally used. Today, lightfastness phenomenon. The control irradiance feature which is virtually all lightfastness standards specify irradiance control, designed to automatically hold the output at one wavelength range and all major manufacturers of lightfastness instruments currently constant throughout a test, is intended to maintain the irradiance, offer instruments that feature a version of irradiance control. especially that in the ultraviolet range, thereby mitigating the Until now, controlled irradiance devices have been limited to negative impact of variable light on test results. This feature, maintaining and providing irradiance data at, or about, a single which was introduced by Atlas Material Testing Technology LLC wavelength or wavelength range. With the introduction of Atlas in 1970 represented a significant step at the time, and has since Material Testing Technology’s proprietary on-board, real-time served the weathering community well. All major manufacturers full spectrum monitoring (FSM) system, the complete spectral of weathering instruments currently offer instruments that feature power distribution (SPD) of the light source can now be a version of irradiance control. displayed. However, controlled irradiance technology, which, in principle, is very similar regardless of supplier, is not without History And Evolution Of Light In Weathering some notable weaknesses. It provides information only at the And Lightfastness Tests single wavelength (or wavelength range) for which it is As the name obviously implies, light is the most important configured. Secondly, the systems have little or no flexibility. The aspect of natural and simulated lightfastness tests. Numerous user, for the most part, is restricted to controlling and monitoring a excellent studies of it’s importance to the can be found in industry test at the wavelength which had been pre-selected and which may literature. only be changed to another single wavelength (range) by tedious The first, fairly crude, laboratory weathering tests employed hardware reconfigurations often requiring complex recalibration. carbon arc lamps as the solar simulator. Instruments using xenon arc light sources, which are inherently superior solar simulators, Full Spectrum Monitoring (FSM) eventually succeeded such devices. Figure 1 shows a comparison The introduction of the Full Spectrum Monitoring (FSM) of solar radiation and filtered xenon spectra. system is the first major innovation for light control in laboratory weathering instruments in thirty years. It is meant to address the Q/Q Xenon with Colipa #1 and #2 normalized at 420 nm vs. Sunlight weaknesses associated with standard controlled irradiance, as well 4.500 Q/Q Xenon with Colipa Filter#1 as to provide researchers the critical spectral data now being Q/Q Xenon with Colipa Filter#2 4.000 Miami Peak Sun 26 S demanded by progressive weathering methodologies and 3.500 approaches. 3.000 In contrast to with current technology which controls and/or 2.500 monitor fixed, single, discrete portions of the xenon spectrum, the 2.000 Full Spectrum Monitoring (FSM) system permits control of any Irradiance in W/m^2*nm in Irradiance 1.500 single user-selected wavelength, or any user-selected wavelength 1.000 range, while monitoring and collecting data at all other 0.500 wavelengths in the 250-800nm range. The many implications of 0.000 this capability are discussed later. 6 0 4 8 4 50 6 98 14 3 62 78 94 10 26 42 58 7 90 06 22 3 70 86 18 34 50 82 98 1 46 62 78 2 2 282 2 3 3 346 3 3 3 4 4 4 4 4 4 5 5 5 554 5 5 602 6 6 6 666 6 6 7 730 7 7 7 794 Wavelength in nm The system, which is fully integrated in an Atlas Weather- Ometer®, is meant to be used on a full time basis. As such, it is Figure 1. Comparison of spectral power distribution of natural sunlight and necessarily designed to be robust, to withstand the inherently filtered xenon. hostile environment in, and around weathering instruments. As with other light sources, the output of a xenon arc will vary with electrical input power, the stability of its enclosure and surrounding optical filters. The filters will generally tend to degrade or solarize with use. The combination of these variables Specimen rack Xenon Input optics lamp sealed cosine receiver Auxiliary filter lantern Figure 2. Graphical display of the lightfastness chamber interior. CCD array spectroradiometer is mounted below the test chamber. Output - FSM Output – OL - 754 Detailed specifications are proprietary, but in general the FSM system is comprised of a CCD array spectroradiometer with Figure 3. Comparison of conventional spectroradiometer output with FSM one nanometer resolution over a range of 250 – 800 nm, including system order-sorting filters to ensure appropriate stray light rejection. Figure 2 shows the mounting of the input optics in the weathering chamber. The input optics system incorporates a sealed robust FSM Capabilities cosine receiver and patented quartz diffuser. An on-board industrial computer manages the system with customized software User Selected Control Points for system calibration, data management and user interface. Current instruments are limited to controlling irradiance at a predetermined wavelength, or wavelength range that is likely not Comparison of FSM and Conventional to coincide with a test material’s critical wavelength(s). Critical Spectroradiometer: wavelengths are those at which a material demonstrates ASTM G-138-03, Calibration of a Spectroradiometer Using heightened sensitivity to light. In addition, a lamp undergoing a Standard Source of Irradiance [1] has become the normative temporal changes not captured by a single-wavelength-monitoring standard for the calibration of spectroradiometers. A traditional system will compound the problem. This has implications for spectroradiometer requires typically several pieces of hardware to repeatability and reproducibility. For example, replication of comply with the ASTM standard, including a NIST-traceable current tests that monitor and integrate irradiance data at non- irradiance standard, a calibrated digital voltmeter, a calibrated critical wavelengths will indicate that the tests are identical. current shunt, wavelength calibration source, very stable power However, assuming there are changes to the lamp (there always supply, etc. After calibration is complete, spectral irradiance are – small and gradual for the most part) at the critical measurements in a Weather-Ometer® are done as in situ in the wavelengths not “seen” by current instrumentation, true radiant weathering chamber. Both steps require a skilled operator. dosage replication, where it matters most, is not accomplished. Comparisons have been made between a standard The flexibility of FSM system allows unprecedented utilization of spectroradiometer and the system used in the FSM configuration. activation spectra data, to control and monitor tests at material- The standard spectroradiometer (in this case, an Optronic specific critical wavelengths to ensure more repeatable and Laboratories OL754) was calibrated by the ASTM procedure and reproducible tests. set-up to make measurements in situ as described in the previous It should be made clear that the selection of a control paragraph. The FSM was calibrated by its unique and proprietary wavelength coincident with a material’s critical wavelength does technique. The FSM was used to make measurements immediately not limit testing to a single type of material. For while it is only following the OL 754’s measurement of different filter possible to control at a single wavelength or range, since the full combinations, in the same Weather-Ometer and under identical spectrum is available, data may be monitored and accrued at any conditions. other wavelength(s). For each xenon filter combination, the spectral power distribution plots measured and produced by each system are Photometric Monitoring And Control virtually indistinguishable. In each case, numerical data at selected In the photo imaging industry for example, lightfastness tests wavelengths and wavelength ranges also show excellent are required to maintain specified lux values, which for current agreement for repeated, alternating measurements. See Figure 3 as instrumentation meant costly re-engineering of the controlled a comparison between the two measurements. Numerical outputs irradiance system. By contrast, systems with FSM could, through of the two systems are shown at the bottom of the figure. its software, automatically produce the lux value by convolution of its spectral irradiance measurements and the known photometric, “standard observer” weighting curve. Performance-based Specifications more repeatable and reproducible tests, or to demonstrate The trend towards performance-based weathering test compliance with performance-based test methods. specifications over the recent years has meant the elimination of any technical