Narrowband vs. Wideband Irradiance Control in Accelerated Weathering Tests: What You Need to Know

When it comes to accelerated weathering tests, understanding the measurement of irradiance is key to obtaining accurate results. In weathering testing, one of the most important distinctions to understand is the difference between narrowband and wideband irradiance control.

What is Narrowband Irradiance?

Narrowband irradiance is a power density, typically expressed in Watts per square meter per nanometer (W/m²/nm), centered on a very specific wavelength, such as 340 nm or 420 nm. For example, test may specify 0.68 W/m²/nm @ 340 nm, which describes the irradiance over 1 nm and centered on 340 nm (Figure 1). It's critical to include the “per nanometer” in the measurement to avoid confusion, especially when conducting accelerated weathering tests to simulate the effects of sunlight exposure.

Figure 1: Narrowband Spectral Irradiance @340 nm

What is Wideband Irradiance?

In contrast, wideband irradiance integrates energy across a larger range of wavelengths, resulting in larger numerical values. This control point is often specified for weathering tests that evaluate how materials react to a broader spectrum. The most common wideband measurement is total ultraviolet, or "TUV" (300-400 nm). Some tests also specify a "UV+Visible" irradiance (300-800 nm). The latter is less common, and generally less appropriate for weathering testing, since the UV range has far greater impact on material degradationthan visible. For instance, a TUV irradiance value might be 60 W/m², determined by integrating the spectral irradiance from all wavelengths between 300 and 400 nm.

The Importance of Correct Units in Irradiance Control

The key to accurate irradiance control in weathering tests lies in understanding the units and the wavelength range. Narrowband and wideband measurements serve different purposes and must be clearly specified. Only saying "0.55 W/m²"or "60 W/m²" with no reference to the wavelength range is misleading because, at first glance, it may seem that these two values are significantly different. In fact, they may actually be very similar, assuming the first is the irradiance at 340 nm and the second is TUV. Misunderstanding the difference can lead to incorrect assumptions, especially when comparing narrowband values with wideband values. These values are not directly comparable without taking into account the different measurement methods and the specific wavelength ranges. Figure 2 shows graphically how wideband and narrowband values are related.

Figure 2: Irradiance for Narrowband (340 nm, 420 nm) and Wideband (TUV and UV+Visible ) for a Q-SUN with Daylight-Q Filter

At Q-Lab, we emphasize the importance of irradiance control in all of our accelerated weathering tests. We recommend that customers provide full details of their desired test parameters, including both the specific wavelength and the units. This ensures that the weathering tests are conducted accurately, with precise irradiance settings that reflect real-world conditions.

Conclusion: Clear Measurements = Accurate Weathering Tests

For accurate results in weathering tests, understanding the difference between narrowband and wideband irradiance is essential. Whether you're conducting accelerated weathering tests in the automotive, construction, or coatings industry, precise irradiance control can make a significant difference in the reliability and relevance of your data. By working with a trusted partner like Q-Lab, you can ensure that your irradiance measurements are properly calibrated for your testing needs, ensuring consistent and reliable results every time.