The measurement head / probe has to accomplish the measurement principle as effective as possible, adapt to fibers for light transfer and last but not least withstand the sample.
The measurement principle, i.e. which physical effect is used to interact with the sample, defines the design principle and thus the probe type. I.e. there are heads or probes for:
For details please see Sample Interaction
Emission measuring probes can be quite simple, for example, just a light guide with a decently sized fiber in front of a light source will do the job. A slightly more professional way is a (cosine) diffusor disk at the entrance of the light guide, the distance of the fiber end in combination with the effective NA of the fiber defines the spot size, over which light is detected. It might actually be the NA of the attached spectrometer which limits it if the spectrometer has an acceptance angle smaller than the NA of the fiber.
A cosine diffuser is a (diffusive) disk where the efficiency with which light is scattered (“transmitted”) depends in a cosine way from the angle of incidence.
For refined measurements or if light from all directions should be covered in a similar fashion, an integration sphere should be used. A sphere surfaces should be the ideal reflector so that after tons of bounces the light eventually hits the detector area undampened. Since this is not the case spheres, are very inefficient, the larger the worse. The measurement port size should be as small as possible, for such a port does not contribute to the internal reflection, and if it does (because the sample to be measured is too close), it influences the wavelength neutrality of transporting the light. Back to top▲
A transmission setup is straightforward – two light guides with a collimating optics each create a parallel light path through the media desired. This concept can be used in the same straightforward way to measure solids (e.g. glass filters or foils). For liquids, one can rig in a similar fashion a tube – two opposing observation windows on the tube provide access to the liquid contained within. Immersion probes allow measuring the transmission within a liquid in a very convenient way.
For short path length, the Attenuated Total Reflection (ATR) principle is exploited: even light under total reflection contained within a solid transparent material such as quartz or sapphire seeps into the lower refracting environment (the liquid sample) and thus might get absorbed. The interaction length is in the range of the wavelength.
Transmission measurements are of high efficiency, for the light follows a defined path. 40% is a typical value. Back to top▲
Reflectivity measurements are more demanding: with direct reflectivity measurements, the direction of the reflected beam depends on the surface orientation, following the Law of Reflection (αIn = αOut). Small variations (e.g. flutter and/or uneveness of a glass pane) change directions and influences the pick-up efficiency and thus the measured light intensity. Spheres can be used to counter the variation of the reflection direction, but determine the specular reflection accurately only if there is no diffuse contribution. Specular reflection measurements are of high efficiency.
The challenge for diffuse reflection is to find setups which allow collecting as much light as possible, for a diffuse surface distributes light in all directions, not following the Law of Reflection.
For inelastic interaction (Fluorescence, Raman), the excitation is typically narrowband (LED or Laser). Additional spectral filtering is applied to suppress the strong excitation also a specific geometrical arrangement: fluorescence is typically observed under 90°. Also, the emitted light is often quite narrowband, that eases the selection of materials and limits chromatic errors.
Technical Realization / Design:
In modern process technology, most spectrometer systems are fiber-optic coupled. Therefore, measurement heads/sample interaction devices are designed around optical fibers.
The fiber diameter is typically limited by the input of the spectrometer, not necessarily by the probe design. Back to top▲
A bifurcated fiber-optic bundle can do a pretty good job to detect reflected signal under normal incidence. E.g. interference fringes from white light interference can be easily detected. To reduce the sensitivity to distance changes – not to increase efficiency – a collimating lens may be added.
Probes which can be directly immersed into liquids (or powders and such). Typically rod-like designs of diameters from 3 to 20 mm, with the optical head at the penetrating end. For transmission, two main designs are used: either a gap is realized by e.g. quartz blocks which contain the collimating lens equipped light guides (single pass) or by one collimating lens equipped Y light guide and an opposing reflection mirror at a defined distance, i.e. the light passes this distance through the media twice (“transflection” probe). The latter allows very compact designs but are less sturdy, the minimum mechanical gap for any liquid to penetrate is only half of that of a single-pass probe with a related disadvantage.
An ATR probe have a head design which provides 3 or 5 bounces where total reflection takes place, thus multiplying the short interaction length accordingly. Few micron absorption length can be realized, to measure highly absorbing materials such as inks. This length scales with the wavelength, a more complex analysis is required. The overall efficiency is similar to standard transmission setups, although significantly dropping in the UV.
Designs with a compact diffuse reflection setup are also integrated into rod-like structures, able to measure reflection on a few mm spot size.
Immersion probes for fluorescence measurement incorporate the complex setup within relatively small space, but follow otherwise the same design principle (e.g. fibers with collimating optics).
Raman probes follow similar design principles but are more sophisticated in respect to focusing the light to the desired measurement spot and to filtering of unwanted light.
For more details please visit https://www.hellma.com.
Hollow sphere, internal surface coated with high-reflecting material (Spectralon, gold), with an opening for sample, and small ports for observation and illumination. Light guides (fibers) can be directly connected to the illumination and observation ports. Instead light delivered by a fiber, a lamp can be integrated into the sphere. The sample is directly illuminated from the opposite side of the sample port, rays from the sample are reflected diffusely back into the sample port under all directions and are transferred eventually to the observation port by a multitude of reflections within the sphere (works the other way round too). Back to top▲
Link to professional sites https://www.labsphere.com/support/system-product-brochures/
Diffuse Reflection Heads
To avoid the high losses of spheres, designs with a couple of fibers facing the sample under various directions are possible. The coarser the sample surface, so more pickup directions are important. Wide-spread are arrangements with 7 fibers, for these form a nice fiber bundle. 6 or 7 for pickup, and 1 or a source for illumination. The angle between incoming and outgoing is set to 30° or 45°.
Resistance covers aspects such as chemical resistance but also resistance against shock, temperature, pressure, mistreatment. Such challenges are mainly faced by immersion probes, which are immersed into a liquid or gas cells interacting with gases, while all other measurement principles have the advantage that no contact to the medium is required.
The aggressiveness of sample materials is countered by the choice of the material for the probe or head in contact with the liquid, gas or solid. Besides chemical interaction also abrasive aspects have to be encountered as well as clogging of the probe.
Assembling techniques especially for immersed probes are important, such as molecular bonding or braising are favorable, avoiding any cementing. Sapphire is the window material of choice to withstand mechanical ablation forces of particles.
Probes can be designed and manufactured so they can withstand up to 350°C and pressures up to 700 bars, e.g. for use in extruders. Back to top▲