Disenchantment in Britain

The light source is a key component of the IBBCEAS approach and influences the performance of the whole system. As the name of technique suggests, the light source used is both broadband and incoherent. The spectral output of the light source should cover the absorption spectra of the species to be investigated. Desirable characteristics are a high radiance in the spectral region of interest and stable intensity over the duration of measurement. Although most light sources do not meet these requirements, both Xenon arc lamps and high power light emitting diodes (LEDs) are stable, high radiance devices that are widely used in IBBCEAS systems.

Xenon arc lamps have a wide thermal emission spectrum covering the ultraviolet, visible, and near-infrared wavelength regions (approximately 180–2000 nm). A typical xenon arc lamp has a very high radiance that allows efficient imaging of light into the optical cavity. The brightest emission of the arc lamp usually comes from a spot within 1 mm of the lamp electrodes. Modifications of the arc lamp performance are possible under some circumstances, including a special short arc lamp running in so-called hot spot mode. The hot spot source differs from the conventional diffuse mode arc lamp in that the spot of the light is much smaller (~ 150 μm), with extremely high radiance. A hot spot with spectral radiance of 18 W cm^-2 sr^-1 nm⁻¹ at 400 nm, which is about one to two orders of magnitude higher than in the diffuse mode, is formed close to the cathode surface (Fig. 2.2 ) (Welz et al., 2006b). Lamp intensity

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variations affect the measured extinction coefficients and are another point to be considered. The long term dimming and short-term variation of a 75 W Xe arc lamp are shown in Fig. 2.3. The lamp output is highly stable, with short-term fluctuations of 0.2% and a further dimming of 0.27% per hour. This stability is likely to be worse for lamps nearing the end of their life.

Figure 2.2 Hot Spot of the Arc Lamp. Comparison between the brightness of the normal Xe arc lamp (left) and that of the “hot-spot” mode lamp (right) showing the very bright spot near the cathode (Welz et al., 2006a)

However, the arc lamp has disadvantages as well. a) The xenon gas has some emission lines which vary depending on the temperature and pressure inside the bulb and may produce spectral artefacts. b) Transmission of the broadband emission (especially in the IR range) into the spectrometer greatly increases stray light levels. Stringent filtering is required to attenuate this background signal to sufficiently low levels. c)

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The life time of xenon arc lamp bulbs varies from 4-12 weeks, and effort and expense is needed to replace the bulb. d) Only a small portion (usually 5%) of the isotropic emission of the xenon arc lamp is collected (Kern et al., 2006).

Owing to these drawbacks of the arc lamp, high power LEDs have been adopted as convenient and cost effective alternative light sources for IBBCEAS (Ball et al., 2004). Compared to Xe arc lamps, LEDs are compact, efficient, and often have higher peak spectral radiances than arc lamps. In the near-UV range, where high power LEDs only extend down to 365 nm, the Xe arc lamp remains the better light source for measuring spectra below 360 nm.

0 1000 2000 3000 4000 5000 6000 7000 0.996

0.998 1.000 1.002 1.004 1.006

Relative Intensity

Time [s]

Figure 2.3 Temporal dependence of the intensity of the arc lamp output at 355 nm. The long term trend is given by a slope of 0.27% per hour and the short term fluctuations from the trend line are approximately 0.2%. The measurement plot of pixel at 355 nm was based on a 1 s integration time over 6820 s.

46 2.3.2 Spectral filtering

As an incoherent source, a Xe arc lamp produces spectral output from around 180 nm to 2000 nm. The highly reflective mirrors of the optical cavity function as a highly efficient band rejection filter. On the other hand, light outside of the cavity mirror range is transmitted very efficiently through the cavity. Coupling all transmitted light into the spectrograph would cause serious stray light problems and possibly saturate the CCD detector. In addition, the background light would vary with absorption outside the cavity spectral region. In the current work focusing on near-UV spectroscopy this problem is more acute because the UV output of the arc lamp is weaker than in the visible range. In addition, there are fewer standard optical elements and filters in the UV. Hence, obtaining a small spectral region in the near-UV (320 nm to 380 nm) to match that of the cavity proved to be challenging.

Nevertheless, a combination of several bandpass filters, dielectric mirrors, and short-pass filters allowed reasonable control of the spectral output.

Fig. 2.4 shows the transmission of three filters: a band pass filter centred at 357 nm (Semrock), a short pass Schott UG11 filter, and an IR absorbing filter (KG1).

Combining these three filters achieved a spectral window located within the high reflectivity range of the cavity mirror and reduced stray light to an acceptable level.

47 0.0

0.2 0.4 0.6 0.8 1.0

200 300 400 500 600 700 800 900 1000 1100 1200 10^-4

10^-3 10^-2 10^-1 10⁰ Relative Transmission Relative Transmission

Wavelength [nm]

Figure 2.4 TOP: Filter transmission curves of Semrock 357 (blue), UG11 (red) and KG1 (green). BOTTOM: Calculated filter transmission curve of all three filters (purple) as well as the transmission curve of the optical cavity (cyan).

48 2.3.3 Detector

With low light levels reaching the CCD, it is essential that the detector be highly sensitive over the whole spectral range monitored. The quantum efficiency (QE), which indicates the conversion efficiency of photon to electrons, determines the sensitivity of the detector. A higher relative QE of the CCD at the wavelength of interest will give a larger signal and a more sensitive spectrometer. Conventional CCD detectors have at most 30% QE in the near-UV range. However, back-thinned CCD detectors are more efficient in the UV range. Combining this detector with an anti-reflection coating, the detector selected for our system (Andor DV420-BU) has an average 80% QE from 300 to 400 nm (90% at 400 nm and 60% at 300 nm). Different integration times of 200, 400, 600, 800 and 1000 ms were measured and indicated that the detector displayed good linearity (Fig. 2.5).

0 200 400 600 800 1000

0 1x10⁴ 2x10⁴ 3x10⁴ 4x10⁴ 5x10⁴

Intensity [counts]

Exposure time[ms]

335nm 360nm 375nm

Figure 2.5 Linearity of the detector response to different integration times for several wavelengths.

49 2.4 Instrument performance