Optical material properties

 

The optical properties of materials include such measures as reflection, transmission, absorption, fluorescence and photoluminescence. Gigahertz-Optik GmbH offers measuring instruments and accessories, calibration standards, and calibration services for measuring optical material properties. Application examples from these product ranges are presented on this page.

Our sales team will be pleased to support you regarding your particular application requirements. Please contact us via +49 (0) 8193 93700-0 or info@gigahertz-optik.de.

App. 034

Non-destructive light transmission measurement of large glass panes

The amount of visible light transmitted by the glass used in buildings, automobiles, airplanes, trains, etc. is of utmost importance to the people occupying and using them. Therefore, the light transmission of window glazing, vehicle windscreens, etc. needs to be evaluated with respect to the photometric sensitivity of the human eye. Conventional measurements with laboratory based spectrophotometers, as with the statutory qualification of car windscreen light transmission in accordance with ECE R43 [1] for example, permit only relatively small pieces of glass to be measured. Large panes of glass and windscreens must first be cut into suitably small samples. Portable spectrophotometers, on the other hand, help to avoid this time-consuming and costly effort and even make it possible to measure the light transmission of windows in-situ.

The big challenge in designing a portable spectrophotometer is the implementation of the required measurement geometry. The classical arrangement of conventional stationary measuring systems, as given in ECE R43 for example, is based on sample illumination by a parallel light beam and an integrating sphere as detector.

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Basic ECE R43 ISO 3538 measuring arrangement for light transmission EN v02

Basic ECE R43 / ISO 3538 measuring arrangement for light transmission

The implementation of an inverted measuring geometry of ECE-R-43 can be made considerably more compact. This follows the Helmholtz reciprocity principle for the reversibility of a beam path. The measuring geometry also corresponds to the luminance measuring arrangement according to CIE 130 [2] and DIN 5036 [3].

Schematic representation of the inverted ECE R43 measuring arrangement for light transmission EN

Schematic representation of the inverted ECE R43 measuring arrangement for light transmission

When implementing the inverted ECE R43 measuring arrangement in a mobile spectrophotometer for measuring the light transmission, special attention must be paid to compliance with:

  • Size ratio of the illuminated field of the integrating sphere light source and the measuring beam of the luminance meter.
  • Integrating sphere light source with spectral monitor detector for simulation of the CIE standard illuminants A, D50 and D65 as well as for the correction of back reflections of the specimen into the integrating sphere.
  • Robust semiconductor source.
  • Pulsed operation of the light source and matched detection to enable the rejection of ambient light.
  • Luminance measuring head with quasi-parallel measuring beam.
  • Luminance probe with spectral detector to simulate the CIE photometric function.
  • Luminance probe with camera-assisted positioning of the luminance measurement beam relative to the illuminated field of the integrating sphere light source

The light source and luminance meter are two independent units which must first be placed together in order to establish a 100% level. Subsequently, the light source and luminance meter are positioned on each side of the glass sample and aligned. Thereafter, the measurement of light transmission in % is measured with CIE standard illuminants A, D50 or D65 as well as the spectral transmission.

Gigahertz-Optik GmbH offers the LCRT-2005-S, a portable spectrophotometer proven in many applications. Its compact size allows measurements on glass plates of any size. The device can be integrated into stationary measurement setups and is offered with accessories to support many varied applications. 


References

[1] UN ECE Regulation No. 43 - Uniform provisions concerning the approval of safety glazing materials and their installation on vehicles. 

[2] CIE 130-1998 Practical Methods for the Measurement of Reflectance and Transmittance

[3] DIN 5036-3 Radiometric and photometric properties of materials; methods of measurement for photometric and spectral radiometric characteristics

App. 035

Universal integrating sphere design criteria for the measurement of optical material properties

The optical properties of materials are routinely described in terms of their reflection, transmission and absorption [1]. To measure these key parameters, materials are exposed to optical radiation which is then partially reflected, transmitted or absorbed by the samples. The most common type of equipment for measuring these optical properties is a spectrophotometer. These instruments comprise a light source for sample illumination, a sample holder with measuring geometry for steering the reflected or transmitted radiation and a detector for measuring the radiation. If the light source provides broadband illumination of the sample then a spectrometer, typically array based, will form the detector stage. Alternatively, the sample may be illuminated with monochromatic radiation that is scanned through the wavelength range of interest. In this configuration, single element photodetectors such as photomultiplier tubes and photodiodes are used.

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In applications where scattering materials are measured, spectrophotometers incorporate an integrating sphere [2] within the sample measuring geometry. However, most commercial spectrophotometers are rather limited regarding their available space and access for using integrating spheres. For situations where they cannot be used there is a need for universally configurable integrating spheres. Their design must be optimally adapted to the measurement requirements of each optical parameter.

Often the best possible compromise between high optical throughput and sphere size can be achieved with a 150mm diameter (6 inch) integrating sphere. Their spherical surface area of ca. 71,000 mm² allows a total area of up to 3,500 mm² to be used for application and detector ports, while maintaining the recommended <5% port area to sphere surface ratio. For a universally designed integrating sphere, a 150mm diameter sphere therefore permits five application ports each up to 30 mm in diameter (700 mm²). With these five ports the two measuring geometries 0 / d and 8 / d (with light trap) can be realized.

0 d and 8 d measuring geometry

Picture: 0 / d and 8 / d measuring geometry

The design of the application ports is of crucial importance for the precise measurement of reflection and transmission of scattering samples. Their edges must have a very narrow lip with so-called ‘knife edges’. The radiation scattered by the sample can thereby enter the integrating sphere even at shallow angles of incidence without contact with the side edges of the application port.

Port plugs allow the closure of any unused application ports and port reducers can be used to reduce the size of an application port thereby effectively increasing the sphere’s surface area. It is important that the face of any port reducer lies in the same plane as the sphere’s surface. In addition, this face must be coated as per the coating of the hollow sphere. This is the only way to ensure undisturbed radiation distribution within the integrating sphere.

To measure the reflection and transmission, the samples are attached to the outside of an application port. The required sample holders are ideally designed so that the sample is pressed flat against the area around the port. The contact pressure should be independent of the sample thickness. In addition, it must be possible to combine the sample holders with port reducers and other accessories such as light traps. These are always required when semi-transparent material samples are to be measured with regard to their reflection. In this case the light trap provides a defined black background without any re-reflection.

To measure absorption, material samples are placed inside the integrating sphere. Liquid samples are measured in cuvettes. The sample holders must be adjustable in height and rotatable to position the samples. The sample opening must be closed by the plug of the sample holder. It is important that the sphere side of the plug lies in the plane of the hollow sphere and is coated. Only in this way is the undisturbed radiation distribution in the integrating sphere also ensured in the region of the sample port. If the mounting rod of the sample holder is hollow, additional leads for electrical connection to the samples can be guided into the integrating sphere.

The south pole of the integrating sphere provides a favorable position for the radiation detector. For reflectance and transmission measurements, the detector must be baffled from the application port that holds the sample. For absorption measurements, a baffle must be placed between the detector and the sample in the center of the integrating sphere.

Gigahertz-Optik GmbH offers the UPB-150-ARTA 150mm integrating sphere with accessories for universal reflection, transmission and absorption measurements. If the applications are limited to reflection and transmission, the model UPB-150-ART offers a lower cost alternative.


References

[1] Tutorial Basics of Light Measurement – 1.8 Reflection, Transmission and Absorption

[2] CIE 130-1998 Practical Methods for the Measurement of Reflectance and Transmittance

App. 036

Calibration of spectrophotometers for 8 / d reflection

Integrating spheres are used as the reflection measuring optics within many commercially available spectrophotometers. The 8 / d measuring geometry is the usual choice for scattering materials. Here, the sample is illuminated with a beam at 8 ° incidence angle and the total diffuse reflected radiation is measured.

A reflection measurement represents the proportion of incident radiation that is returned by a surface. Therefore, to measure the reflectance, spectrophotometers must first determine the 100% level of the incident radiation.

Spectrophotometers offering 8 / d reflection measurement geometry require diffuse reflection calibration standards. Common calibration standards are either made of pressed barium sulfate powder or synthetic materials. The synthetic material is much more robust and can be cleaned in case of contamination by reworking the reflection surface. In addition, these PTFE-based white plastics offer a wide usable spectral range from 250 nm to 2400 nm.

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The reflectance of the materials used is not constant cross their usable spectral range. Therefore, a calibration of the spectral reflectance is required. Calibration is performed by comparing the calibration standard with a transfer standard. The calibration of the transfer standard must be traceable to a national metrology institute.

The use of so-called working standards is recommended for the routine calibration of spectrophotometers. The calibration of working standards is carried out by the user of the spectrophotometer for which the transfer standard is used as the reference. With at least three working standards it can be ensured by comparative measurements that the reflection properties of the working standards have not changed over time.

The calibration standard must completely cover the measuring aperture of the integrating sphere for calibration. When attaching the standard to the measurement port, its surface must not be pressed in or polished in order to rule out later incorrect measurements due to shadowing in the recesses or gloss from the polished surfaces. When not in use, the reflection standard should be sealed in a light- and dust-tight manner.

With the BN-R98-D2 Gigahertz-Optik GmbH offers a reflection standard made of the synthetic material ODM98. Its diameter is 2 "or 50.8 mm. The housing can be closed with a quick release, light-tight cap. The calibration of the spectral reflectance in the spectral range from 250 nm to 2400 nm is carried out by the Calibration Laboratory for Optical Radiation Measurements of Gigahertz-Optik GmbH.