Photon-emitting devices, photon generation based on various principles.
- Halogen (VIS-NIR, stable, smooth, cw, medium, inefficient)
- Deuterium (UV-VIS, stable, smooth, cw, medium, inefficient)
- Xenon (UV-VIS, pulsed, long, inefficient)
- LED (UV-NIR narrow range, pulsed & cw, long, efficient)
- Laser (only for excitation)
- Spectral range and balance
- Stability (short-term = fluctuation, long-term = drift)
- Emission area and angle
- Warmup time
- Time behavior (continuous wave or flash)
No simple answer to what source is best, demand to be analyzed in detail.
First selection criteria: what spectral range is covered, combination of sources possible.
D2 and Xe sources with different window materials available, for various UV cut-offs
The emission of halogen can be influenced by the so-called color temperature, i.e. the temperature of the filament. The warmer source the more output and the mean wavelength shifts towards the UV, the cutoff as well, but lifetime degrades.
LEDs cover only a small spectral range (FWHM 10…30 nm), various have to be covered for broader rage spectroscopy, but ideal for narrow-band applications
Stability: D2 and Hal short-term: 10-5, long-term: drift: < 1%/d; Xe pulsed sources: 2% peak to peak variation, drift and variation can be compensated by reference channel, LED sensitive to current and temperature control
Emission area: D2 sources are limited by pinholes, 0.5 and 1mm are typical; the pinholes wider over time. The halogen emission area is given by the filament, which from a rectangular area. A Xe arc has length of a few mm, depending on design.
Efficiency: few percent with most sources, generate mainly heat (Hal, D2, XE), warming of instrument; LED much higher
Warm-up: D2, Hal, minimum 30 minutes to 1h, Xe (in-)stable right away, LED instantaneous
Operating mode: Xe and LED can be flashed, energy concentrated in time, important for triggered measurements. Xe pulse can be few µs, flash rates up to 100 Hz.
D2 and Hal continuously emitting, shutter required for dark current measurement.
Lifetime / Maintenance: Halogen and Deuterium have to be exchanged after 2000h, log lifetime Hal = 10000h), Xe 109 flashes (depends on energy setting), ideal for low maintenance, no shutter required and long exchange intervals: 0.3 x 108 flashes/a at 1 Hz rate: lifetime ca. 30 years.