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There is a difference between radiometric dimensions and photometric dimensions. Photometric dimensions deal with properties of electromagnetic radiation as they relate to human sight and are the customary measurement units in light technology.
Radiation | | | Photometric | | | Variable | Symbol | Equation | Variable | Symbol | Equation | Radiant Intensity | Ιe | W / sr | Luminous Intensity | Ι | cd | Radiant Power | Φ | Watt (W) | Light Flux | Φ | cd ⋅ sr = lm | Radiant Energy | Qe | W ⋅ s | Quantity of Light | Q | lm ⋅ s | Radiance | Le | W/sr ⋅ m2 | Luminance | L | cd/m2 | Specific Radiation | Me | W/m2 | Specific Light Emittance | M | lm/m2 | Irradiance | Ee | W/m2 | Illuminance | E | lm/m2 (Lux) | Radiant Exposure | H | W/m2 ⋅ s | Exposure | H | lx ⋅ s | Radiant Efficiency | ηe | % | Luminance Efficiency | η | lm/W |
Lamp Types
Halogen and Deuterium Light Sources
Halogen lamps typically emit a continuous spectrum in the range of 400 nm to 2500 nm. Deuterium lamps emit a continuous spectrum in the range of about 200 nm to 400 nm. This gap-free spectrum means that these lamp types have become the universal standard light sources for spectroscopy. Since they are ideal complements to one another’s spectral ranges, m·u·t supplies a light source which includes both types of lamp. These lamps are easy to handle, and even with a low electrical power (12 to 45 watts), provide sufficient light intensity for most spectroscopic examinations.
LED Light Sources
LED-based light sources have the advantage of needing very little energy and having a long lifespan. They are limited, however, in that they emit a very small spectral range. Special coatings can be used, similar to those in tubular fluorescent lamps, to exploit this wavelength and, thereby, to excite different materials to wider bandwidth emissions. This is how so-called white LEDs are constructed.
Arc Lamps
Mercury and xenon arc lamps are operated with high pressure gas in a transparent tube. These lamps are notable for their especially high luminosity. The wide spectral distribution starts at around 250 nm and goes on to 2500 nm. Xenon lamps produce a spectral distribution which is very similar to sunlight, and as a result, are used for illumination in all kinds of sun simulators. Arc lamps for spectroscopy need a power supply of 75 W to 1 kW.
Line Emitters
Line emitters emit only individual spectral lines that depend on the energy levels of the electrons causing the emissions. These are characteristic to the particular gas in the lamp. These spectral lines are extremely constant and their precise wavelength is known exactly. Line radiators are consequently highly suitable for calibrating spectroscopic modules.
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