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Home > Frequently Asked Questions > Measurements > Optimizing Your Measurement > What is the best sampling optic for my measurement?

What is the best sampling optic for my measurement?

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 only sees part 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.

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, giving 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 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.

A variable-angle reflection sampling system is a much easier way to measure specular reflection from surfaces as a function of angle, as the angle of incidence can be varied continuously from 10° to 50°. With convenient fiber input and output connectors, it eliminates the need for realignment when the angle is varied. Note that this system is designed for specular reflectance only, and thus must be referenced against a reflectance standard each time the angle of incidence is changed.

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 extra 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.

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