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Home > Applications Blog > Technical Tips > An In-Depth Study of the DH-mini Light Source

An In-Depth Study of the DH-mini Light Source

DH-miniThe DH-mini light source is the latest in compact, affordable deuterium-tungsten halogen light sources from Ocean Optics. These dual light sources combine the continuous spectrum of a deuterium UV source and a tungsten halogen Vis-NIR source in a single optical path, with output from 200-2500 nm.

The DH-mini has many of the same performance advantages as our signature DH-2000 series dual light sources – especially powerful output and great stability — but in a much smaller and less expensive instrument footprint. In this technical note, we compared the spectral performance of the DH-mini with an earlier Ocean Optics dual light source model and a competitor’s deuterium-tungsten halogen source. The results offer guidance in choosing the most appropriate dual light source for your application.

Introduction

The DH-mini comprises a deuterium bulb and a tungsten halogen bulb, providing illumination from the UV-Visible to NIR (200-2500 nm). Each bulb can be operated individually, and the intensity of the halogen bulb can be adjusted for maximum flexibility, allowing the instrument to be manually balanced as appropriate. This reduces stray light and eliminates any spectral features from the second bulb when not in use.
The DH-mini has been carefully designed to produce high intensity while maintaining stable output. That’s important because a powerful light source produces more light and makes measurements easier, especially for absorption and transmission applications using samples with high optical density.

Also, the stability of a light source is paramount to taking accurate spectral measurements. For example, for applications where accuracy and repeatability are crucial, the instability of the light source can be an important source of error. If the light source can be kept stable over time, you can be confident that any observed effects are characteristics of your sample.

This tech tip will study key performance parameters of the DH-mini, investigating its long-term stability and comparing its intensity with other deuterium-tungsten halogen sources. It’s important to note that all light sources require a warm-up period while they reach equilibrium; after this warm-up they can be expected to have a much more stable output. This stable output is shown in the results.

Experimental Setup

The signal from a DH-mini was measured over time using the QE Pro spectrometer. A 450 nm diameter UV-Vis fiber was used to collect the light source output. Measurements were taken at a sample rate of 15 seconds over 12 hours, with a 70 ms spectrometer integration time. The measurements began when the light source was first turned on in order to characterize the warm-up time.

The QE Pro was chosen as the most appropriate spectrometer for measuring the light source output as it covers the relevant UV-Vis wavelength range (~200-1100 nm) and has a thermoelectrically cooled detector to help maintain stable readings for the duration of the experiment. (To compare the output of the sources at wavelengths >1100 nm requires a NIRQuest spectrometer, which was not used in this experiment.) Also, it’s important to note the variation in the signal that occurs when the fiber in the setup is moved; for this experiment, the fiber was fastened in place to ensure constant transmission.

Results: Intensity

A comparison of the intensity of the DH-mini to its predecessor, the DT-MINI-2-GS, and to a market-equivalent competitor light source is presented in Figure 1. These spectra were measured with both lamps in use at 40 minutes after start-up to allow ample time for the light source to warm up.  

Figure 1 shows a clear difference in intensity of the output from the dual light sources, with the DH-mini reaching over 110,000 counts from the deuterium bulb in the UV, and over 150,000 counts at longer wavelengths from the halogen bulb. By comparison, the DT-MINI-2-GS only reaches about 55,000 counts in the UV, and just over 80,000 counts for the halogen bulb. The competitor light source is even less intense, reaching only 27,000 counts from the deuterium lamp, and just over 60,000 for the halogen lamp. The increased intensity observed in the DH-mini can be vital both for fiber probe-based measurements where light throughput is a challenge and for transmission and absorption applications involving substances with high optical density.

Figure 1: The signal measured by the QE Pro across the entire spectrum for the DH-mini and two other deuterium-tungsten halogen sources shows clear differences in intensity.

Figure 1: The signal measured by the QE Pro across the entire spectrum for the DH-mini and two other deuterium-tungsten halogen sources shows clear differences in intensity.

Generally, it’s difficult to produce a very high number of counts from a deuterium lamp. However, the DH-mini meets this challenge and is able to generate a strong signal from the deuterium bulb that is comparable to its halogen output, and is even higher than the halogen output of its predecessor.

Also, because each bulb in the DH-mini can operate individually, users can adjust the output to control for spectral artifacts such as the D-alpha peak at 656 nm, which can affect results. The ability to adjust each source independently also allows users to produce a more balanced output across the spectrum and to maximize the most useful parts of the spectrum for a particular application.

Results: Stability and Warm-up Time

Figure 2 shows the change in DH-mini intensity measured over 720 minutes (12 hours) at selected wavelengths. This data was taken with both bulbs in use, with a sample rate of 15 seconds. A short-term variation can be seen due to the spectrometer noise, and a long-term drift or change in stability of the output can be seen from the slow change in intensity over time.

Figure 2: Measurement of DH-mini intensity shows the signal drift over time, by wavelength. Each line was normalized for presentation purposes. The y-axis values are arbitrary and do not represent the intensity of the signal at these wavelengths, only the amount the intensity changes over time. The same approach was used for Figures 3 and 4.

Figure 2: Measurement of DH-mini intensity shows the signal drift over time, by wavelength. Each line was normalized for presentation purposes. The y-axis values are arbitrary and do not represent the intensity of the signal at these wavelengths, only the amount the intensity changes over time. The same approach was used for Figures 3 and 4.

During the allocated lamp warm-up time, prior to 30 minutes after switch on, non-uniform variation can be seen in the light source output across all wavelengths. After 30 minutes the light’s output largely stabilizes and the drift becomes more regular. Notably, the stability appears to be wavelength-dependent, especially during the warm-up period, highlighting the importance of careful and frequent referencing-taking when making measurements.

It is inherent to higher intensity light sources that signal varies more significantly. However, with the careful development of the DH-mini, Ocean Optics has been able to achieve high intensity output while maintaining good stability. It is important to be aware of this effect when using the DH-mini, as adjusting the halogen bulb to just below the maximum intensity will help to optimize stability. For example, in our experiment, we operated the halogen bulb at maximum intensity and at two lower intensities. At maximum intensity, light source stability was reduced. A more moderate intensity in the halogen is likely to be sufficient for most applications, and ensures a more stable output.

The experiment also was run with the bulbs individually. We observed that most of the instability in output comes from the halogen bulb. Conversely, the deuterium bulb displays a very stable output with a predictable drift that can be characterized (Figures 3 and 4). This difference is due to the way that the light is produced in each case. A tungsten halogen lamp is simply a tungsten filament bulb in a halogen gas. This design is subject to variation in the electrical current and the temperature, as well as any irregularities in the filament itself.

A deuterium arc lamp is a gas discharge lamp that operates by the excitation and subsequent de-excitation of an ionized gas due to an electrical discharge passing through it. This produces a much more stable output as it is not subject to fluctuating electrical current.

It is important to be aware of these properties when taking measurements so their effects can be minimized. Results can be improved by frequent referencing during the experiment and by using the bulbs at appropriate intensity levels.

Figure 3: A temporal plot containing five wavelengths in the UV and Vis is measured to compare the long-term stability of the signal from the deuterium bulb in the DH-mini.

Figure 3: A temporal plot containing five wavelengths in the UV and Vis is measured to compare the long-term stability of the signal from the deuterium bulb in the DH-mini.

Figure 4: A temporal plot containing six wavelengths in the Vis and near-IR ranges is measured to compare the long-term stability of the signal from the halogen bulb in the DH-mini.

Figure 4: A temporal plot containing six wavelengths in the Vis and near-IR ranges is measured to compare the long-term stability of the signal from the halogen bulb in the DH-mini.

Summary

The DH-mini deuterium-tungsten halogen light source produces a considerably more powerful output than both its Ocean Optics predecessor and the market-equivalent competitor source studied here. Significantly, the DH-mini maintains its stable output despite the challenge that high intensity output brings to stability. Also, the warm-up time specified for the light source has been demonstrated, emphasizing the importance of allowing ample time before taking measurements.

We have seen that the deuterium bulb has a more uniform intensity trend over time than the halogen bulb, which has some fluctuation due to the way its light is produced. Additionally, with the halogen bulb intensity lowered slightly from the maximum we observe increased stability as well as a more balanced output across the spectrum. Adjusting the halogen bulb may be useful for applications where attenuation of light is not a limiting factor.

All these considerations are important to account for when taking measurements and interpreting results to maximize the instrument’s capabilities. The DH-mini’s high output and great flexibility make it a versatile option for a range of applications, and its compact size and great reliability are attractive features for users in research, academia and industry. By configuring the DH-mini with a broad-range spectrometer and appropriate accessories, users can create a multifunction spectroscopy system for UV, Vis or NIR measurements or for measurements where wide-range response is necessary.