Interchangeable Slits and Internal Shutter Enhance NIRQuest and QE Pro Spectrometers
Interchangeable slits and an optional internal shutter are available on high performance QE Pro and NIRQuest spectrometers. Adjusting slit size allows users to adapt throughput and resolution; internal shuttering manages dark measurements when external shuttering is not feasible. As a result, users can help to mitigate some of the trade-offs associated with fixed-slit optical bench designs and applications where shuttering is integral to effectively managing light throughput in the spectrometer.
Dark Measurements and Shutter Control
A “dark” (or baseline) measurement is needed in every spectroscopic experiment. Even without incident light any photodetector produces a signal that depends on the detector’s temperature, history and age; the integration time of the measurement; and the specifics of the electronic circuits employed to turn the detector’s photocurrent into a number in the computer. In some types of detectors (most notably, InGaAs detectors in the NIR spectral range), this background signal varies from pixel to pixel and a complete background spectrum needs to be acquired.
A shutter that is integrated into a spectrometer simplifies such a background measurement significantly. It is not just convenient to use an electronically controlled shutter rather than blocking the beam path by hand (for example, in a cuvette holder); in some applications, such as remote measurements, full automation is needed, making an internal shutter essential to record meaningful data.
The shutter in the spectrometer allows users to measure the background signal of the detector (often called “dark” signal). In a different approach to record a background signal, the light source output is blocked by an external shutter. At first sight this approach seems equivalent, but upon closer consideration this measurement records a slightly different signal: In addition to the dark current from the detector, this experiment also captures any environmental (ambient) light making its way into the spectrometer.
Which approach is better – closing the spectrometer input or closing the illumination output? The answer depends on your application. For example, finding the right answer can be tricky with a reflection measurement. In one approach, the measurement consists of four measurements: first, a reference sample (usually a white standard, a type of diffuse reflector), recorded once with the light source turned on and once with the light source blocked by a shutter. A second set of measurements is taken from the actual reflection sample under test; again, one spectrum is needed with the light source on and one spectrum with the light source blocked by a shutter. During these four different measurements the environmental lighting conditions are not allowed to change. Once all of these measurements are completed, the reflectance spectrum can be calculated from the ratio of the two differences between light source on and light source blocked.
However, it is rare that one encounters such circumstances: The environmental lighting conditions could change in between those four measurements, or the setup, for example in the case of a handheld device, could be oriented differently during reference and sample measurement, picking up different amounts of ambient light.
Often reflection probes work with direct illumination from the light bulb, as in the case of the Vivo light source or the DR diffuse reflectance probe, which does not allow users to block the illumination light path. Turning the light source on and off electrically is no alternative either as it introduces large variations and instability in the light output. In all of these common cases, the best approach is to eliminate as much environmental light as possible through the design of the reflection setup (or by performing the measurements in a dark area) and to use a shutter in the spectrometer instead. One might even want to argue that such a superior experimental design is generally preferable to relying on subtracting any interference out of the signal after the measurement.
In a different use case the integration time on the detector needs to be adjusted dynamically to measure light emissions over a wide range of intensities — for example, to determine the output power and color of a light source, such as an LED in a test setup for different drive currents. In such a device an automated system would adjust the integration time of the detector to match the LED intensity. However, as the detector’s background signal increases with integration time, a separate “dark” measurement is required for every new setting. A shutter in the spectrometer will allow this type of control. There’s a good example of this use case for a QE Pro emissive color measurement application at Internal Shutter Improves Accuracy in Color Measurement.
The background (or “dark”) signal from the detector is an indispensable element for any spectroscopic measurement. A shutter in the spectrometer provides convenience and flexibility in the experimental setup and allows users to perform high-quality measurements that would otherwise be difficult, complicated or even impossible to execute.
Interchangeable Slit Design for Optimizing Measurements
Precision laser-cut slit and aperture assemblies in the QE Pro and NIRQuest provide users with a degree of measurement flexibility not typically available with most spectrometers. Spectroscopy is a technique in which the design criteria exist as a set of trade-offs. The optimal spectrometer depends upon the application. For some users, one of the most frustrating trade-offs involves the choice of entrance aperture (slit). A larger slit increases throughput, but at the expense of optical resolution. A smaller slit yields higher optical resolution, but decreases throughput. Often, changing the slit requires spectrometer rework that has to be performed at the manufacturer’s facility.
With replaceable slits, users can change the spectrometer’s performance directly in the field (see The Benefits of Modularity in Slit Sizes in the Flame Spectrometer). Changing slits can be accomplished within minutes, with minimal tinkering. For example, QE Pro users who need great sensitivity for low light applications such as fluorescence in one experiment can change slits to avoid saturation in absorbance applications (for an example of such an application, download Modular Spectroscopy Tools for Measuring Intrinsic Protein Fluorescence). Or a Flame configured to resolve closely spaced emission peaks for one application can be adjusted to perform low light level measurements for another application.
In most cases, as our testing has demonstrated, there’s no need to recalibrate the spectrometer when changing the slit. There is one exception: You cannot change from a standard slit to a slit with a filter because it changes the optical focus and wavelength calibration of the spectrometer. In this case you would need to send the spectrometer to Ocean Optics for installation and calibration. A filter must be ordered for each slit (if your application requires a filter) and the spectrometer needs to be calibrated and focused with the filter installed. This only applies to filters installed inside the slit assembly.
Another caveat is to avoid using a wide slit with a narrow fiber. Unlike the slits, the fibers are not positioned accurately. Depending on the fiber, it can be off-center slightly and attaching the same fiber multiple times can change the position in front of the slit. This is not a problem, as the slit defines the entrance aperture and is the object being imaged, not the fiber. However, if the slit is wider than the fiber, the slit will not be filled — now the fiber is the object being imaged with a position that can change with attachment. This will subsequently change the image position – i.e., shift the spectrum.
The interchangeable slit design, which is also available in the Flame, Torus, Jaz and Maya LSL spectrometers, is a great option for research settings, where experiments can change from day to day. Even for single-use applications the optimum slit size for a good signal might not be certain ahead of time. Resolution can be reasonably well calculated, but intensity not so much. For example, consider a solar reflection measurement in the field, with a cosine corrector attached to a spectrometer, measuring the light backscattered from vegetation. From the spectrum one can deduce the health of the plants, perhaps for agricultural monitoring.
The only parameters available to optimize the signal are the integration time and the slit width. If there are constraints on the integration time — for example, to achieve a desired acquisition frequency – flexibility in the light throughput is needed during the initial setup. Sometimes it’s just simpler to optimize the configuration on site. Even if the same slit will stay in the spectrometer for the rest of its life, during the initial setup this flexibility is very useful.