PATCHED 7th Sphere PortScan 1.1
Kian Trip
Thorlabs' Modular Integrating Spheres each provide a durable, highly reflective surface to optimally diffuse light. The sphere is constructed inside a cuboctahedral (fourteen-sided) housing that can be configured for a wide variety of applications. Each of the eight triangular faces can support a Ø5 mm port with SM1 internal threads, enabling easy mounting of our mounted photodiodes or fiber-coupled components such as LEDs or spectrometers (SM1 adapters may be needed). Either three or four of the square faces can be configured with inserts selected by the user as described in the Modularity section below. The two faces that are not modular provide a universal 8-32 / M4 x 0.7 tapped hole, allowing mounting to the 25 mm rail system stand (included with stock items) or a Ø1" post. These integrating spheres are available from stock as one of two pre-set configurations or ordered as a custom configuration using the tool below.
The spheres are manufactured from PTFE-based bulk material that has high reflectance in the 250 - 2500 nm wavelength range (see Specs tab) and is resistant to heat, humidity, and high levels of radiation. This reflective surface is designed to have specific roughness and diffusive reflection properties and should not be cleaned using solvents, as this could damage the inner surface. We only recommend using compressed air for cleaning the inner surfaces of the integrating spheres.
Integrating spheres enable high sensitivity measurements of optical signals in a variety of setups. An integrating sphere causes the incoming radiation to undergo multiple reflections over the sphere surface, which diffuses and depolarizes the light beam. This makes it the ideal instrument for many applications such as laser power, flux, reflectance and radiance measurements, as well as creating a uniform light source for camera calibration.
Modularity
These integrating spheres come with either three or four modular faces. Each modular face accepts an interchangeable port insert, available below with a variety of port sizes, SM threading options, tapped holes for cage system compatibility, and a cuvette holder. Spheres available from stock are pre-configured for common applications and include a preset selection of port inserts and two fixed Ø5 mm ports. Custom-configured spheres allow the user to choose the inserts and up to eight fixed Ø5 mm ports.
Calibration
Thorlabs can calibrate integrating spheres with user-selected sensors upon request and provide the pertinent NIST- and/or PTB-traceable certificates of calibration. Contact Tech Support for more information.
A material of choice for coating the light-diffusing cavities of integrating spheres is polytetrafluoroethylene (PTFE). This material, which is white in appearance, is favored for reasons including its high, flat reflectance over a wide range of wavelengths (see the Specs tab for details) and chemical inertness.
However, it should be noted that integrating spheres coated with both PTFE and barium sulfate, which is an alternative coating with lower reflectance, emit low levels of ultraviolet (UV) and blue fluorescence when irradiated by UV light. [1-3]
Hydrocarbons in the PTFE Fluoresce
It is not the PTFE that fluoresces. The sources of the UV and blue fluorescence are hydrocarbons in the PTFE. Low levels of hydrocarbon impurities are present in the raw coating material, and pollution sources deposit additional hydrocarbon contaminants in the PTFE material of the integrating sphere during its use and storage. [1]
Fluorescence Wavelength Bands and Strength
Researchers at the National Institute of Standards and Technology (NIST) have investigated the fluorescence excited by illuminating PTFE-coated integrating spheres. The total fluorescence output by the integrating sphere was measured with respect to fluorescence wavelength and excitation wavelength. The maximum fluorescence was approximately four orders of magnitude lower than the intensity of the exciting radiation.
The UV and blue fluorescence from PTFE is primarily excited by incident wavelengths in a 200 nm to 300 nm absorption band. The fluorescence is emitted in the 250 nm to 400 nm wavelength range, as shown by Figure 1. These data indicate that increasing the excitation wavelength decreases the fluorescence emitted at lower wavelengths and changes the shape of the fluorescence spectrum.
As the levels of hydrocarbon contaminants in the PFTE increase, fluorescence increases. A related effect is a decrease of the light output by the integrating sphere over the absorption band wavelengths, due to more light from this spectral region being absorbed. [1, 3]
Minimizing Fluorescence Effects
Minimizing and stabilizing the fluorescence levels requires isolating the integrating sphere from all sources of hydrocarbons, including gasoline- and diesel-burning engine exhaust and organic solvents, such as naphthalene and toluene. It should be noted that, while hydrocarbon contamination can be minimized and reduced, it cannot be eliminated. [1]
Since the history of each integrating sphere's exposure to hydrocarbon contaminants is unique, it is not possible to predict the response of a particular sphere to incident radiation. When an application is negatively impacted by the fluorescence, calibration of the integrating sphere is recommended. A calibration procedure described in [4] requires a light source with a well-known spectrum that extends across the wavelength region of interest, such as a deuterium lamp or synchrotron radiation, a monochromator, a detector, and the integrating sphere.
Absolute transmittance and absolute diffuse reflectance spectra of optical samples can be found using integrating spheres. These spectra are found by performing spectral measurements of both the sample of interest and a reference.
Conditions Leading to Sample Substitution Errors
An integrating sphere's optical performance depends on the reflectance at each point on its entire inner surface. Often, a section of the sphere's inner wall is replaced by the sample when its transmittance and diffuse reflectance spectra are measured (Figure 2). However, modifying a section of the inner wall alters the performance of the integrating sphere.
Sample substitution errors are a concern when the measurement procedure involves physically changing one sample installed within the sphere for another. For example, when measuring diffuse reflectance (Figure 2, bottom), a first measurement might be made with the standard reference sample mounted inside the sphere. Next, this sample would be removed and replaced by the sample of interest, and a second measurement would be acquired. Both data sets would then be used to calculate the corrected absolute diffuse reflectance spectrum of the sample.
This procedure would result in a distorted absolute sample spectrum. Since the sample of interest and the standard reference have different absorption and scattering properties, exchanging them alters the reflectivity of the integrating sphere over the samples' surface areas. Due to the average reflectivity of the integrating sphere being different for the two measurements, they are not perfectly compatible.
Solution Option: Install Sample and Reference Together
One experimental technique that avoids sample substitution errors acquires measurement data when both sample and reference are installed inside the integrating sphere at the same time. This approach requires an integrating sphere large enough to accomodate the two, as additional ports.
The light source is located external to the integrating sphere, and measurements of the sample and standard reference are acquired sequentially. The specular reflection from the sample, or the transmitted beam, is often routed out of the sphere, so that only the diffuse light is detected. Since the inner surface of the sphere is identical for both measurements, sample substitution errors are not a concern.
Alternate Solution Option: Make Measurements from Sample and Reference Ports
If it is not possible to install both sample and standard references in the integrating sphere at the same time, it is necessary to exchange the installed sample. If this must be done, sample substitution errors can be removed by following the procedure detailed in [1].
References
[1] Luka Vidovic and Boris Majaron, "Elimination of single-beam substitution error in diffuse reflectance measurements using an integrating sphere," J. Biomed.Opt. 19, 027006 (2014).
The 4P3 sphere is pre-configured for light measurement, designed so that the sphere is continuous opposite the Ø1" port that would be used for a light source. This reduces the probability of primary reflections occurring on a seam around a port plug.
The 4P4 sphere is pre-configured for reflection measurements, as shown in the middle diagram above. An SM1-threaded, Ø1" port insert allows samples in SM1-threaded mounts (for example, an SM1L05 lens tube) to be positioned opposite a light source. Two Ø14 mm ports on the opposing face are designed with an 8 geometry so that light input to one Ø14 mm port will reflect off the sample in the Ø1" port, allowing the direct reflection to be detected from the second Ø14 mm port. Indirect reflectance can be detected using one of the two fixed Ø5 mm ports.
These easily interchangeable port inserts are designed for use with Thorlabs Ø100 mm Modular Integrating Spheres. Two included M3 x 0.5 screws secure each insert to the sphere, allowing the insert of any modular face to be swapped out using a 2 mm (5/64") hex ball driver. The sphere-facing sides of the 4P10, 4P11, and 4P13 inserts have the same radius of curvature and are made of the same highly reflective bulk material as the rest of the sphere, minimizing losses from changes in geometry. The 4P12 insert does not have any bulk material in its design as the aperture is closely matched with the hole in the integrating sphere for the modular face.
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