Understanding Spectral Irradiance Curves

This article discusses the significance of source size and delineates the intense spectral lines from the arc lamps.

Source Size

In studies by Oriel Instruments spectral curves can be deceptive especially in the selection of a lamp. A quick glance shows an increase in the irradiance with increasing lamp power. It is not necessarily better to have more irradiance. Re-imaging of the source is required for several applications. The small radiating areas, arcs or filaments of lower power lamps are as bright as or brighter than the larger arcs or filaments of the higher power lamps, providing as much flux density on target as with a large.

Small lamps are sufficient if the target is small. It is to be noted that the smaller arcs of certain xenon lamps deliver superior performance in several application when compared to mercury and QTH lamps. It is easier to operate smaller lamps, which need and generate less total power. In certain cases, a liquid filter needs to be used with high power lamps to protect optical components from high power.

For a small target, the power produced on target by the re-imaged smaller lamp is significantly larger than the re-imaged kW lamp. Besides causing damage to optics, a constantly running kW source will heat the lab instruments. Hence, careful analysis of the optical system needs to be done to select lowest power lamp suitable for the specific requirement. The kW lamp is recommended to irradiate a very large area.

The Spectral Lines from Arc Lamps

The intense UV lines of the mercury lamp make it suitable for several UV sensitive processes and for excitation of luminescence, especially for exciting with 313, 365, 404 or 436nm radiation. The smoother xenon lamp spectrum is recommended to scan the wavelength of the excitation source. With the xenon lamp, users have fewer worries about dynamic and linear range of a detection system.

The rapid variation in the output of the mercury lamp with respect to wavelength also necessitates wavelength reproducibility in any application, where users scan the source and ratio or subtract separate resulting scans. Oriel Instruments has published the wavelengths for the lines from its mercury spectral calibration lamps. Some of these lines show structural broadening, while others are very narrow. The wavelength values of these lines from the low pressure lamps, and the previously given alphabetical designation of some of them are still utilized as labels for all mercury lines.

Oriel Instruments’ irradiance data reveals broader lines, with some lines deviating from these generic values. These deviations are caused by Doppler broadening and self reversal, which relies on the passage of radiation through colder mercury. Hence, the exact spectral profile relies somewhat on the lamp type and envelope temperature. Figure 1 depicts the 254nm line that governs the output of the 6035 Spectral Calibration Lamp is considerably absorbed in the 350 W Hg lamp, model 6286. An interference filter needs to be selected for a 250nm line and some of the other lines.

Figure 1. “254”nm line from a 350 W Hg arc lamp shown with calibration line. The line width (FWHM) of the calibration line, as recorded by MS257™ with a 1200 l/mm grating and 50µm slit and InstaSpec™ II photodiode array, was 0.58 nm, instrument limited.

Figures 2 and 3 delineate the high resolution scans showing the line broadening. The actual line shape is less significant for most calculation purposes, when compared to the total irradiance in the band of wavelengths near the line. This will be the same for a low resolution scan as for a high resolution scan. Although there may be difference in the line shape and height, the areas under the scans are almost the same.

The g and h lines from the 350 W Hg lamp shown with a calibration line recorded on the same equipment. These data were recorded with MS257™ and InstaSpec™ II so the wavelength calibration is exact.

Figure 2. The g and h lines from the 350 W Hg lamp shown with a calibration line recorded on the same equipment. These data were recorded with MS257™ and InstaSpec™ II so the wavelength calibration is exact.

The green 546.1 nm line and yellow doublet.

Figure 3. The green 546.1 nm line and yellow doublet.

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 leading instruments, such as light sources covering a broad range from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators that allow you to simulate hours of solar radiation in 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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