Fig. 1 Spectral irradiance of 6334NS 250 W QTH Lamp, and IR ElementsIdeal sources for IR spectroscopy
4 models available; different sizes and irradiance values
Many sources emit infrared radiation to some degree, but there are only a few convenient economical sources with sufficient radiant output and spectral uniformity to be practical in IR instrumentation. We offer four IR elements, usable over a broad spectral range, 1 to 25 µm.
Large radiating area
Uniform emissivity
Similar to Nernst Element
High irradiance
Most economical IR source
Small, efficient radiating area
3.8 x 5 mm SiC element
Fig. 1 Spectral irradiance of 6334NS 250 W QTH Lamp, and IR Elements
Quartz tungsten halogen lamps also provide strong infrared output. Emission in the NIR is from the hot filament. The quartz lamp bulb cuts off direct radiation from the filament, above 3.5 µm, but the hot envelope acts itself as a broadband infrared emitter with output in the far IR. Use caution when your application is sensitive to longer wavelengths.
You need to consider several factors when choosing and comparing IR sources:
1. Operating temperature
Each source has a range of operating temperatures which allow for a reasonable output and lifetime.
2. Color temperature
The shape of the emissivity curve for a particular element affects its spectral radiance or irradiance. Color temperature is derived from the convolution of the operating temperature and emissivity curve.
3. Radiating Area
The large area of the IR emitter has advantages for irradiance of large targets. When the target is small, such as a monochromator slit, a smaller source may be the best choice.
Spectral Radiance and Irradiance both increase with increased operating temperature of the source element. However, element lifetime decreases very quickly at high temperatures. Choose an element with a reasonable lifetime at the desired operating temperature.
Fig. 2 Color temperature vs wattage of 6363 IR element.
Fig. 3 Surface temperature vs. operating wattage of6575 Ceramic Element.
The color temperature of a source is usually defined by the temperature of a blackbody, which would produce similar spectral radiance or irradiance. This works fine if the emissivity of the source is spectrally flat, but is more problematic when it has a shape like that of our 6575Ceramic Element. We define the color temperature of our sources by taking a flat section of their particular emissivity curve and fitting the black body equations to that section of their irradiance curve. The color temperature evaluated in the above manner should not be used for any Photometric calculations - i.e. it should not substitute for appearance related to Coordinated Color Temperature, which is subject to a much more rigorous algorithm.
The radiating area of an infrared element is the area that reaches the color temperature specified for the element. It is typically in the center of the element. We specify radiating area and total area for our sources.
Fig. 4 Emissivity of 6575 and 6363 IR elements
A perfect thermal radiator is fully described by the Black Body equations. Real thermal radiators can only approximate black body performance. The emissivity curve represents the departure from the black body standard. Commercial Black Bodies, get very close to unity emissivity by employing special cavity designs. However, they are large and expensive. Infrared elements and QTH lamps exhibit significant emissivity departures from unity. They are, however, much easier and economical to use. Use source size, emissivity, and color temperature curves to estimate irradiance from a particular element.