Sampling Optics Define the Measurement
All matter interacts with light, and detecting that interaction is how we sense our universe.
Sampling optics defines what type of interaction we are viewing. The two design criteria are the Geometry – how the light source and detector are arranged with respect to the sample, and the Field of View of the detector.
Geometry describes the angle between the light source and the detector. In our products, these angles are defined by optical fibers that couple light sources and spectrometers (detectors) to fixtures to hold the fibers in the proper angles. The physical phenomena that being measured are:
Reflection Light energy bounces off of a surface due to the change in refractive index. The reflection equal but opposite in angle between the light ray and the surface is called specular (mirror like) reflection. Light that scatters and comes off the surface at any other angle is termed diffuse reflection. Typically the light that is detected as diffuse reflection has travelled within the material and interacted with the material such that light has been absorbed and scattered and the emitted light is changed spectrally.
Scatter Scattered light occurs when light encounters a particle. In Raleigh scattering, the interaction of the light with the particle changes its direction. In Raman scattering, the light interacts with vibrations of the molecular bonds and the frequency of the light is shifted.
Absorbance Light energy can be captured or absorbed by a molecule. The energy can result in the elevation of an electron to a higher energy state. That energy can be re-released as light in fluorescence (luminescence). The energy may also be capture as increased kinetic energy of the molecule (heat) and in fact can result in a mechanical wave or acoustic energy.
Transmittance Light that is not reflected, scattered or absorbed re-emerges from a sample or has been transmitted through the sample.
Field of View impacts the data being collected by a particular geometry. The amount of light that is being collected and the range of angles are both impacted. Often, conventional definitions or industry standards will dictate both geometry and field of view requirements. Our basic tools for modifying the field of view are fibers, lenses and diffusers.
Fibers – The acceptance angle or numerical angle (na) of a fiber depends on the difference in refractive index between the core and cladding material. We offer fibers with 3 different na values, chosen for specific purposes.
Silica core/silica clad na = 0.22 is our work-horse optical fiber material that is used in almost all of our patch-cords and fiber optic probes. It offers the best transmission efficiency over the widest wavelength range of any of the fiber materials available. The acceptance angle of the silica/silica fiber is ~ 25 degrees full angle in air (in liquids it will be less depending on the refractive index). This angle or na matches the optics of our spectrometers. Some radiometry applications require defined FOV < 25 degrees. These are easily accomplished by using a Gershun Tube adapter which has aperture discs for various angles down to 1 degree,
Hard Clad Silica – 0.39 na This fiber has a silica core and a hard polymer cladding. The acceptance angle in air is about 46 degrees full angle. While the higher na allows the fiber to gather more light from a diffuse source, this extra light is lost when coupling to our standard spectrometers. Our Ventana Raman spectrometer was designed to capture this extra light from the 0.39 na fiber and that is the fiber used in the Ventana Raman probe. Higher na fibers are also useful for illumination purposes, to gather more light from LEDS or incandescent bulbs and flooding an area on the other end of the fiber.
Plastic Optical Fiber – 0.5 na This fiber has a 60 degree field of view and transmission is limited to the visible range. However it is very inexpensive and very rugged. It can be used for disposable sensors and illumination.
Collimating Lenses – Classic reflection, transmission and absorbance measurements call for collimated beams of light. This is accomplished with our screw on fiber optic collimators. Our standard 74 series lens and the Spectrecology SE – vacuum compatible lens use a standard external thread (3/8-24) that fits all of our fixtures and light sources. Our lens also features a removable fiber connector for use as a free-beam direct attachment to spectrometers and light sources, and a replaceable lens held with a retaining ring. You can specify AR coatings and lens material (quartz, sapphire) and type (achromatic doublet or single)
The collimation depends of the fiber diameter (d) and the focal length of the lens (f). the divergence angle (a) is:
Equation (1) tangent (a) = d/f
|3D Model – Zipped STEP file||112689, 112720.zip|
|Materials||Stainless Steel, Fused Silica|
|Max. Bake Temperature||250ºC|
|Max. Operating Temperature||250ºC|
|Max. Vacuum Level||1×10-10 Torr|
|Length / Height||1.10 Inch|
|Operating Wavelength||190~2500 nm|
Cosine Correctors – Cosine correctors are planar diffusers placed in front of a fiber or a detector. This provides a 180 degree field of view, and the energy that reaches the detector is the component of the light that is normal to the plane of the surface of the diffuser. These are used primarily in radiometry, to define a measurement plane. They can also be used to randomize light entering a fiber, to eliminate phase, polarization or directional components that are unwanted.
Integrating spheres – Spheres coated with highly reflective diffuse materials are used both for sampling 180 degree FOV or 360 degrees for light sources held within the sphere. Our spheres are equipped with fiber optic ports to bring the collected light to the spectrometer, or to deliver illumination light.
|Optical diffuser:||Opaline glass||Spectralon||PTFE||Spectralon|
|Wavelength range:||350-1000 nm||200-2500 nm||200-2500 nm||200-2500 nm|
|Dimensions (OD):||6.35 mm||6.35 mm||6.35 mm||12.7 mm|
|Field of View:||180°||180°||180°||180°|
|Replacement diffuser:||Yes||Yes||Yes||Yes, with restrictions*|
* In spectroradiometrically calibrated setups, the CC-3-DA must remain attached to the spectrometer to maintain calibration. Removal of the CC-3-DA to replace the diffuser would require recalibration of the setup.
The FOIS-1 integrating sphere collects light from emission sources such as LEDs and lasers and is used to measure light fields with a 360° field of view. The FOIS-1 sphere coating comprises a thin layer of Spectralon, a highly Lambertian diffusing material with response from 250-2500 nm.
Versatile – measures relative and absolute irradiance
Flexible – connects to LED power supply-controller accessory for LED measurements
Configurable – can be spectroradiometrically calibrated as part of a complete system
|Dimensions:||56.8 mm x 62.4 mm x 38.1 mm (LWH)|
|Spectral range (most effective):||250-2500 nm|
|Sample port diameter:||9.5 mm|
|Top cap mounts:||(2) 8-32 threaded holes (hardware not included)(1) ¼-20 threaded hole in center (screw/adapter included)|
|Side mounts:||(1) SMA 905 connector for coupling optical fiber to the spectrometer(1) 8-32 threaded hole for post mounts|
LED measurement using a sphere