Measuring the Opacity of Plastic Tubing with VIS-NIR Transmission Spectroscopy
Plastic tubing is available in a variety of opacities ranging from clear to translucent. The transparency of plastic tubing is varied for a number of reasons including providing contrast for visual monitoring of fluid flow, decreasing exposure to ambient light and making the tubing more distinct for machine vision technology. In this application note, VIS-NIR transmission spectroscopy is used to assess the amount of frosting applied to plastic tubing to determine if the tubing meets the required opacity level.
Background
Plastic tubing is used everywhere – from the beverage dispenser at your favorite restaurant to the gas and liquid delivery lines in life-saving medical devices. Whenever fluid transport is required, some type of plastic tubing is most likely involved. With a diverse range of uses, it is not surprising that there are so many types of plastic tubing on the market. In addition to varying plastic formulations for transporting materials as different as inert biological fluids and corrosive slurries, tubing sizes and shapes are available to meet every fluid delivery need.
In many applications employing plastic tubing, visual contact with the flowing material is required to confirm flow and check for bubbles. Visual monitoring is facilitated by enhancing the contrast between the fluid and plastic tubing. Coatings, frosting and other surface modifications are used to vary the interaction of light with the tubing making it easier to observe fluid flow. For example, less transparent, more translucent tubing is used to provide contrast and reduce photodegradation caused by exposure to ambient light. The ability to visually distinguish plastic tubing is also becoming more critical as robots and vision technology are used to automate work.
Modular spectroscopy components can be used to assemble a range of setups to measure the interaction of light with plastic tubing. In the case of frosted tubing, where light transmission must be kept within a narrow range to provide the desired tubing characteristics, VIS-NIR transmission measurements provide a straightforward method to assess frosting level. In this application note, the frost levels for several samples of plastic tubing were assessed using VIS-NIR transmission measurements. Samples with both passing and failing frost levels were measured.
Measurement Conditions
Five plastic tubes with varying levels of frosting were used for the analysis. A picture of the samples is shown in Figure 1 with more details on the frosting level provided in Table 1.
Figure 1: Frosted Plastic Tubes Analyzed with VIS-NIR Transmission Measurements
Table 1: Details on the Frost Level of the Plastic Tube Samples
Transmission measurements were made using a USB2000+VIS-NIR-ES enhanced sensitivity VIS-NIR spectrometer covering the range from 350-1000 nm, an HL-2000-FHSA tungsten halogen light source, a STAGE-RTL-T reflection and transmission stage and two QP400-2-VIS-NIR 400 µm optical fibers. The transmission configuration of the STAGE-RTL-T sampling accessory is shown in Figure 2.
The tubes were placed between the two collimating lenses of the transmission setup with the tubes moved in the setup until the most reproducible orientation was found. To ensure accurate results, a more repeatable measurement method and sample holder to accommodate the round tubing is recommended. The sample holder should always measure the tube in the same position.
Figure 2: Transmission Setup Using the STAGE-RTL-T Stage
Results
The transmission spectra measured for the frosted plastic tubing samples are shown in Figure 3. The transmission intensity measured for these samples correlates with the frost levels reported for the plastic tubing in Table 1. The plastic tubing sample with the lowest transmission (Sample 0) is rejected due to low transmission caused by frosting levels higher than the passing tubing sample.
The plastic tubing samples with the highest transmission (Samples 3, 4 and 5) are rejected for high transmission due to insufficient frosting on the plastic tubing. Note that even though the transmission intensity for Samples 1, 2 and 3 is very similar, the transmission spectra are sufficiently different to reject Sample 3 as having insufficient coating. While it is difficult to see this in Figure 1, visual observation of the samples showed that the frosting level for all three plastic tubing samples were very close.
This illustrates the power of the transmission measurements to discriminate plastic tubing with very similar frosting levels that are difficult to discriminate visually.
Figure 3: Transmission Spectra for Frosted Plastic Tubing
Conclusions
The power of VIS-NIR transmission measurements to discriminate similar samples is demonstrated by the different transmission intensities measured for plastic tubing samples with similar frost levels. The ability to discriminate samples separated by less than 1% transmission make this technique a good candidate for use in QA or QC methods to ensure the plastic tubing has the desired light interaction properties and characteristics.
The use of transmission spectroscopy described here is just one of many applications where this type of measurement can be used. Coatings applied to other transmissive samples including optical filters, polymer films and lenses can be characterized over a broad spectral range using modular spectrometers that measure spectral data across the 150-2500 nm wavelength range.
The availability of spectral data over the UV-NIR range enables extensive coating characterization well beyond transmission at a single data point. Furthermore, with the modular spectroscopy approach, the instrumentation is robust and flexible enough for use well beyond the laboratory setting, from a process line to portable instrumentation for field measurements.
Find Related Application Notes (from the Ocean Optics Application Blog
- Measurement of Filters used in Night Vision Applications
- Transmission Measurements of Polymer Thin Films
- Reflection and Transmission of Chlorine Content Color Wheel
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