Modular Spectroscopy Offers Sampling Solutions for Every Application
Modular spectroscopy offers tremendous flexibility to adapt to a wide variety of sample types with the change of a few components, and to move between those sampling optics quickly and easily for measurements in the lab or field.
When properly measured, spectral reflectance can yield much of the same information as the eye, but it does so more quantitatively and objectively. A spectral reflectance measurement can compare two similarly colored objects, or different textures. It can also offer information about the material from which a sample is made, since light that is not reflected from a sample is absorbed due to its chemical composition, otherwise it is scattered or transmitted.
Reflection may be specular or diffuse. Very smooth surfaces like mirrors or glass exhibit specular reflection, in which the angle of reflection is equal to the angle of incidence. In reality, most surfaces are rough when viewed at a microscopic level, and thus exhibit diffuse reflection, reflecting light through a wide range of angles. The color we perceive from objects is largely diffuse reflection. Many surfaces we perceive as being somewhat shiny or having gloss exhibit a mix of specular and diffuse reflection. When measuring reflection from an object, it is important to consider which type of reflection is dominant, and whether only some or all of the reflected light needs to be collected. With food applications, most reflection measurements will be diffuse reflection.
Reflectance measurements can measure the CIE color coordinates of a sample, or examine the difference between objects for sorting or quality control. The samples may be automotive parts, paint, coffee beans, dyed human hair or lizard skin; we have a sampling accessory suitable for every measurement. Understanding how each optic works to probe the sample is the first step in configuring a system, so let’s begin there.
A reflection probe is great for making quick measurements and for applications where a small spot size needs to be sampled. It can measure either specular or diffuse reflectance, and is compatible with a preconfigured UV-Vis or Vis-NIR spectrometer and any light source (provided the probe fiber matches the wavelength range of the light source). The downside is that it illuminates and detects from the same direction, so it sees only a portion of the reflected light. Measurements made with a reflection probe are relative measurements.
A reflection stage with reflection probe is convenient for granular samples, or when transmission also needs to be measured. The illuminated stage even has active cooling to reduce the risk of overheating samples placed directly on the sample stage, which can be important with biological and organic samples, or those with low melting points.
A reflection probe collects light at the same angle as it illuminates, and can be used for either specular or diffuse reflection measurements. It’s made of 6 illumination fibers around a single read fiber, which results in a 25° full angle field of view. It seems intuitive to connect the 6 fiber leg to the spectrometer, but it is actually more efficient to use the single fiber leg for detection. That’s because each illumination fiber projects a cone of light from the source. All of them overlap at the sample in the center, exactly where the read fiber is “looking.”
To measure specular reflectance, position the probe at 90° to the sample. For diffuse reflectance, work at an angle. Remember that the rays from the illumination fibers need some space to overlap and create reflected light, so the reflection probe needs to be pulled back slightly from the surface of the sample. Be careful not to scratch or dent the surface of the sample with the probe tip. Using a reflection probe holder keeps the working distance consistent from one sample to the next, and when taking a reference measurement. A standard probe holder for works well for flat samples, while a curved surface probe holder with an O-ring contact adapts more easily to irregularly contoured surfaces.
Example food applications:
Determining the Spot Size of a Reflection Probe
When working at 90°, the diameter of the sample area being measured will be equal to ~½ of the distance, d, between the end of the probe and the sample. At 45° angle of incidence, it becomes an oval that is 0.44d by 0.63d.
To see exactly where the reflection probe is reading light from the sample, attach the read fiber to the light source temporarily. The spot illuminated on the sample is exactly where that read fiber is “looking.” If using the RPH-1 probe holder, turn the holder over and place a piece of paper on the contact surface to see the spot size illuminated through the paper.
An integrating sphere is a good idea if the reflectivity of the sample seems to change at different viewing angles. This happens with rough surfaces like brushed metal, fish scales, and seeds. An integrating sphere has a 180° view of the reflected light, yielding more accurate (and absolute) reflectance measurements. An integrating sphere can even be used for convex curved surfaces, or to measure the color of objects that are small enough to fit into the sphere. Ocean Optics integrating spheres view a 5 mm to 8 mm spot size of the sample.
An integrating sphere’s magic comes from the perfectly diffusing interior surface. Light enters through a circular input port and is scattered repeatedly by the sphere’s inner wall until the light inside the sphere is uniform, regardless of any spatial, angular, or polarization variations in the input. A fiber placed at 90° to the input port then samples a tiny fraction of the light within the sphere, sending it to the spectrometer. A baffle in front of the fiber port helps block any light rays making their first reflection from the sample port.
Free-Space Sampling with Collimating Lenses
Collimating lenses can be used at the ends of individual fibers to truly customize the angle of incidence and angle of collection when making reflectance measurements. Specular or diffuse reflectance can be measured this way inexpensively, but much more alignment is needed up front, as is additional fixturing. The collimating lenses need to be adjusted carefully to avoid beam divergence and get good signal, making this a more time-consuming method. We also find that color measurements taken using collimating lenses and fibers are not as accurate as those made using an integrating sphere.
Dynamic Sampling for Granular Samples
Dynamic sampling is a technique where rotational or linear motion is used to measure an “average” signal. This removes localized variance from the results, improving measurements of inhomogeneous samples such as grains, feed and soil.
Also, dynamic sampling is a great way to improve the consistency of any measurement — in color applications, for example, or in UV-Vis reflectance setups. It is a simple yet powerful way to improve the repeatability and accuracy of your spectroscopic measurements. Our dynamic sampling stage rotates the sample beneath the measurement probe and allows the probe to scan multiple areas over the sample, over multiple acquisitions.
Example food application: Dynamic Sampling for NIR Food Measurement
Working with Powders and Dense Liquids
An angled reflection probe works well for measuring granular samples or dense liquids. It is designed with a 45° angled window offset from the end of the reflection probe, allowing the light from the fiber to expand before illuminating the sample material. The window provides a consistent sampling surface, allowing the probe to be immersed directly into the powder or liquid to yield repeatable reflectance spectra.
A regular reflection probe also can be used in combination with a petri dish or microscope slide to look at powdered samples in a pinch. Illuminating the petri dish or slide from below creates a flat sample surface at a consistent working distance from the reflection probe. A flat sample surface will give more consistent results with similar baselines as compared to reflecting off the uneven upper surface of the sample.
Ocean Optics offers many sampling accessories for reflectance measurements, allowing the flexibility to change between sample types dynamically in the lab or the field. Our application engineers can assist you in choosing the right sampling accessory options for your measurement, matching the wavelength range and amount of light needed to the spectrometer, light source and connecting fibers. Call us to get started today!
VIDEO: Basic Reflection Spectroscopy Setup
Make Your Own Reflectance Measurements
|Torus||Torus works well for applications where low stray light, high throughput and thermal stability are necessary. It is responsive from 360-825 nm.|
|HL-2000 Tungsten Halogen Light Source||Flexible Vis-NIR light source (360-2400 nm); also available in high-power and long-life versions|
|QR600-7-UV-VIS Reflection Probe||General-purpose reflection probe, 600 µm diameter; also available in an angled version for use with powders and dense liquids|
|WS-1 Reflectance Standard||Diffuse reflectance standard, PTFE diffusing material|