Analysis of Radiometric Data- Spectral Irradiance Measurements

Spectroradiometric systems measure spectral irradiance in absolute values as a function of wavelength. Such systems can be used to measure the spectral output of an unknown source, or to calibrate measurement equipment.

The radiometric data presented in this analysis was measured in the Oriel Standards Laboratory. Oriel Instruments’ calibration lamps were used for the spectral measurements, including a deuterium lamp for wavelengths less than ~300 nm, while a NIST-traceable calibrated Quartz Tungsten Halogen (QTH) lamp was used to collect the irradiance data between 250 – 2500 nm. The lamps were mounted in their optimal orientation for maximum flux prior to measurements.

 

Oriel QTH lamps with dense flat filaments have highest irradiance along the axis normal to the filament plane through the filament center.

Figure 1. Oriel QTH lamps with dense flat filaments have highest irradiance along the axis normal to the filament plane through the filament center.

Oriel Instruments utilized calibrated, multichannel detectors to validate these measurements as well as interpolation to deduce the calibrated lamp irradiance, except at discrete NIST-calibration points. The detectors were mounted to integrating spheres on the MS125 spectrograph, thereby averaging out the polarization of the incoming radiation. It’s important to note that stress birefringence in the arc lamps and in the filament structure of the lower power QTH lamps can lead to measureable polarization of the light output which may detract from the measurements.

Radiometric Measurement

Figure 2 depicts the experimental setup for the radiometric measurement with the lamps positioned such that the center of the radiating filament or arc was aligned in the horizontal plane.  The measurements were taken with lamps early in their life and operated in open air. The flux density at 0.5 m was even over at least a 25 mm x 25 mm area for these Oriel lamps. These lamps resemble point sources up to the electrode shadowing limit.

Setup for a radiometric measurement

Figure 2. Setup for a radiometric measurement

The irradiance followed an inverse square law, providing that the distance d was greater than 20 – 30 times the radiating element size. The shortest distance was set at 300 mm. Figure 3 illustrates the linear display of the graph, with the height of the peaks clearly shown, but the values at lower levels are missing.

Linear display of the graph

Figure 3. Linear display of the graph

The logarithmic compression can be erratic in the estimation of the area under a section of the spectral irradiance curve. Since the peaks are significant and narrow, the logarithmic scale makes the calculation of the amount of irradiance in any peak difficult.

Despite the half maximum no longer being halfway between the top of the peak and the bottom of the graph, it can be easily determined by measuring the distance from 1 to 2, or 10 to 20, etc. on the logarithmic axis scale. Moving down this distance from the peak determines the half maximum, as shown in Figure 4.

Calculating the FWHM from a log graph

Figure 4. Calculating the FWHM from a log graph

Oriel has cross checked its data with full radiant power meters and calibrated filters. Thermal conditions vary for lamps run in lamp housings compared to open air, and there is a slight change in the spectral distribution with aging lamps, especially mercury lamps. Oriel observed +/- 15% deviation in output from lamp to lamp even within the same batch of lamps.

Oriel observed significant deviation in the UV output (< 280 nm). Envelope materials, in both standard and ozone-free versions, are constantly changing, and envelope thicknesses are not subject to tight mechanical tolerances.

Conclusion

Oriel Instruments trusts that this data set is the most complete and reliable for lamps of this type and is an outstanding source for first estimates. However, it is recommended not to design a tightly-toleranced system based on this data alone without performing additional characterization of the lamp in its actual operating conditions.

About Oriel Instruments

 

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents industry-leading instruments, such as continuous light sources covering a broad range from UV to IR, and from low to high power.

Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectroscopy solutions, making it easy for users across many industries to build instrumentation for specific applications. Oriel is also a leader in the area of Photovoltaics, with its offering of solar simulators allowing users to simulate hours of solar radiation within a matter of minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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