Controlling Light in Useful and Surprising Ways
Everyone enjoys having control, though it takes experience and knowledge to know what to do with it. Every day, we try to maintain control in our personal lives, our professional lives, and countless other aspects.
Working in the lab is no different. More specifically, working with optics and spectroscopy brings with it a special need for heightened experiment control. In this article, we look at the versatile uses of optical attenuators to control light in unique ways and help you achieve results you may not have imagined otherwise.
From Attenuators to Sample Stages
Ocean Optics accessories have helped our customers achieve amazing discoveries and push the boundaries of spectroscopy for decades. We offer a variety of accessories for controlling, filtering and attenuating the light in spectroscopy setups. One of the most powerful yet least appreciated accessories is the FVA-UV Fiber Optic Variable Attenuator, which has many more configuration possibilities than what’s immediately obvious. Learn how the FVA-UV delivers unexpected value in different experimental scenarios:
1. When you have too much light to the detectorLet’s start with the most obvious application: to attenuate light. Here’s how it works: Fibers screw into either side of the FVA-UV, which has collimating lenses that project light across a metal disk into which a slit has been cut. The width of this slit varies as a function of the manually adjusted radial position (see the rotating wheel at the top of the FVA-UV in Figure 1). Rotating the wheel allows you to vary the attenuation from the closed position to fully open.
Figure 1. The FVA-UV fiber optic variable attenuator can be used to attenuate light uniformly at wavelengths from the UV-Vis through Shortwave NIR.
Often, your lab will have a single spectrometer configuration and no other option. If the rest of the optical components or samples lead to detector saturation at the lowest (shortest duration) spectrometer integration times, then there is no way to make a good measurement. The attenuator saves the day here, and allows you to set up your system to the maximum-light scenario and then dial the attenuator to the point of smooth spectral performance. Then you can lock the wheel set-screw into place and be sure you will not saturate the detector throughout the experiment (Figure 2).
Using an attenuator does not improve the resolution of the signal in the way a smaller entrance slit inside the spectrometer will do. For example, a fluorescence-configured spectrometer with 200 µm slit will still achieve the same resolution when attenuated to work with high light-level systems. Perhaps more important is that the attenuator does not affect the resolution of the light interaction with the sample, since the same number of photons are interacting with the sample but are merely scaled-back before reaching the detector. The importance of this is further discussed in #2.
Figure 2. If you have too much light in your setup, use the attenuator on the spectrometer side to mitigate the possibility of detector saturation.
2. When you have too much light to the sampleIn the prior configuration we put the FVA-UV between the spectrometer and sample holder (Figure 2), not between the light source and the sample holder (Figure 3), and this is important to ensure sufficient photons interacting with the sample. Light interacting with any sample will have some portion of photons compromised, so by limiting the light before the sample we are left with fewer “good” photons reaching the detector and much more noise.
However, some samples are highly photosensitive and simple interrogation with light can destroy the samples themselves. Whether a UV-curing compound or ancient artifact, there are some samples that simply cannot be exposed to too much light. In this scenario, the attenuator helps to protect samples while ensuring the desired optical feed to the detector.
Figure 3. If your samples are highly photosensitive, place the attenuator closer to the sample holder.
3. When your light source has no shutterSome models of light sources on the market, whether from Ocean Optics or other suppliers, do not have a shutter built into the design. Or you may be using the sun or a non-scientific, shutter-free source for your application (Figure 4). In these scenarios the attenuator has two useful click-in positions for 100% and 0% transmission, which work perfectly as a de facto shutter. Now you can take dark references with confidence without the problematic methods of turning off the bulb or, heaven forbid, disconnecting any fibers. (Turning the light source off/on frequently can affect lamp stability; disconnecting fibers can create alignment issues that affect measurements.)
Figure 4. If your light source has no internal shutter and you don’t want to disturb the setup, use the attenuator to shutter the light instead.
4. To adjust light from multiple sources uniformlyPerhaps one light source isn’t covering the wavelength range you need, so you have two or more sources feeding into your sample/spectrometer setup. This could be a set of discrete LEDs illuminating at the desired wavelengths, or broadband sources such as tungsten-halogen for the VIS-NIR range and deuterium for the UV range (Figure 5). Despite the multi-light source setup, all sources can feed from a multifiber assembly (bifurcated, trifurcated and so on) into a single FVA-UV attenuator, which then can uniformly adjust the total signal that reaches the spectrometer as though it is coming from a single, unified source. This allows precise tuning of the combined light feeding into your system, and allows for a unified dark measurement without disconnecting the fiber or cutting power to the bulbs.
Figure 5. With multiple light sources feeding into your setup, use the attenuator to adjust the light uniformly.
5. To create an optical equalizer for use in parallel on bandpass channelsIn the scenario in #4 we adjusted the combined light feeding in from several sources, which controls all sources at once. However, what if we placed attenuators immediately after each source, and then combined them into a common fiber leg (Figure 6)? Now we’ve created an optical equalizer that can tune the signal from each light source. Whether you’re using segmented LEDs or variable filters on several broadband sources, this arrangement is a clever way to individually adjust the UV, Visible or IR regions in much the same way you’d adjust the bass, mid and treble knobs on your stereo’s equalizer.
Figure 6. With a slight variation on the approach described in Figure 5, you can create an optical equalizer for your setup.
6. To achieve extreme attenuation of high-power light sources
Of course, the attenuator will cut some percentage of the passing light on its own, and when strung along in a series this effect can be tightly tuned to the lowest levels. For extremely bright sources or applications where saturation seems unavoidable, using two or more attenuators in a series (Figure 7) can drop your signal to 5% on the first pass, and then 5% of 5%, or 0.25%, on the second pass.
Figure 7. Using two attenuators can be useful for controlling light in setups having very high-intensity light sources.
7. To gate multiple optical signals feeding to a single detector
So far, we’ve covered various options for dealing with light sources, although some experimental setups may be limited to a single spectrometer with multiple analyte signals feeding into it (Figure 8). For example, perhaps there are several fluorescence cuvettes all routing into a single Ocean HDX spectrometer. An attenuator placed after each cuvette holder will allow for precise adjustment of each fluorescent signal. This could be useful in isolating each signal for analysis or for that specific portion of the experiment, or to scale multiple feeds to the same level for an aesthetically pleasing and presentable screenshot plot.
Figure 8. With multiple attenuators, you can create a setup for gating optical signals feeding to the spectrometer.
8. To roughly calculate fluorescence quantum yield at various levelsIn an experimental fluorescence setup where you can measure both the absorption of the source channel and the emission of the fluorophore, you can place an attenuator between the light source and sample holder to adjust both absorbance and fluorescence values up and down (Figure 9). This gives you the ability to calculate an average fluorescence quantum yield (or photons emitted divided by photons absorbed) across a range of excitation intensities, and may even be an avenue to discover some unique property or behavior of your analyte.
Figure 9. In fluorescence setups, use the attenuator to help calculate quantum yield.
9. To automate the light source duty cycle
Nos. 1-8 on our list have focused solely on the manually controlled FVA-UV attenuator. Ocean Optics also offers the FHSA-TTL, a combination filter and cuvette holder that provides comparable manual operation as the FVA-UV, but with electronic software control of the overall shutter as well (Figure 10). With this product coupled to any light source, you can now automate the duty cycle of that source through TTL communication. This may be important for time resolved applications, longevity simulations, or to prevent extended exposure of a sensitive sample.
10. To automate QC processes for fluorescent and other materialsIn addition to manual attenuation and automated shutter control, the FHSA-TTL offers a cuvette and filter holder to house the sample or help clean up the optical signal through filtering. Long term-photobleaching studies may be arranged with this setup to both hold the fluorescent material and automatically control when light is being exposed. Also, several of these accessories could automatically control/rotate various light sources to look at the isolated effects of each over time.
Figure 10. Where automated shutter control is required, the FHSA-TTL combination filter-cuvette holder offers software control shutter functions.
Our attenuator list has described many of the unique optical arrangements achievable with a simple accessory that fits in your hand. What exciting experiments and applications have you worked on that use our attenuators or other accessories in clever ways? Email us with your accessory tech tip and you could be eligible for a special reward.
And be sure not to close the attenuator of your mind too tightly to new ideas, because as the late, great Jerry Garcia reminds us, “Too much of a good thing is just about right.”
- Frequently Asked Questions: How does the Fiber Optic Variable Attenuator attenuate light?
- Frequently Asked Questions: Why use a Fiber Optic Variable Attenuator?
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