What is a Globar Light Source?
A Globar light source is a type of thermal infrared emitter that produces broadband radiation by heating a rod of silicon carbide (SiC) to high temperatures. It is commonly used in infrared (IR) spectroscopy, especially Fourier-transform infrared (FTIR) spectroscopy, as a stable and high-intensity source of mid-infrared radiation. Other thermal emitters, such as the Nernst glower, are also used for similar purposes.
Globar sources are favored in applications that require continuous, broadband IR output from approximately 2 µm to 25 µm (5000 to 400 cm⁻¹), covering the mid-infrared spectral range where many molecular vibrations occur.
Operating Principle
A Globar functions by electrical resistance heating. When an electric current passes through the SiC element, it heats up to temperatures typically around 1000–1650 °C (depending on the power input and cooling conditions). As a result, the Globar emits thermal radiation that follows Planck’s law — the spectral output resembles blackbody radiation, with intensity and peak wavelength depending on the temperature.
Unlike incandescent filaments (e.g., tungsten), which require a gas-filled bulb and cannot emit beyond 3 µm due to the envelope material, silicon carbide can be operated at high temperatures in air, making it ideal as a cost-efficient IR light source.
Spectral Characteristics
The emission spectrum of a Globar is continuous and roughly blackbody-like, peaking in the mid-infrared range. The intensity drops off significantly toward the near-infrared and far-infrared ends of the spectrum.
Filters or monochromators may be used in combination with Globar sources to select specific wavelengths for measurement.
Applications
Globar light sources are widely used in:
- FTIR spectroscopy for chemical and materials analysis
- Gas and vapor sensing based on mid-IR absorption
- Optical system calibration in the infrared range
- Infrared microscopy and imaging for biological or materials research
They are typically integrated into benchtop or portable instruments that require a stable IR source for high-resolution spectral analysis.
Advantages and Limitations
Advantages:
- Broad spectral output across the mid-IR
- Long operational life and stable emission
- Relatively low cost and straightforward operation
Limitations:
- Low brightness compared to laser or synchrotron sources
- Slow modulation response due to thermal inertia
- High power consumption and significant heat generation
For applications requiring high spatial coherence or fast modulation, alternative sources like quantum cascade lasers (QCLs) or synchrotron radiation may be preferred.
Comparison with Tungsten filament lamps
Tungsten filament lamps emit primarily in the near-IR and visible range. While tungsten filaments can reach high temperatures, they require a sealed bulb filled with inert gas (e.g., halogen gas) to prevent evaporation and ensure longevity. However, the bulb is typically made of glass or fused silica, which is opaque to wavelengths beyond approximately 3 µm. As a result, tungsten-halogen lamps do not emit in the mid-IR.
