UPK-50-S UV Excitation Fluorescence Measurement Set-up

Picture 1: Sensor with Controller and Readout Electronic

Gigahertz-Optik GmbH manufactures a wide range of components and instruments for light measurement applications. Services in the form of application solutions and calibrations for its products are provided. These application services may also include product improvements and new product development. Our product application notes are meant to provide general information concerning the use and handling of our products and their application. In this format detailed technical specifications are not included.

Application Description:

Fluorescence describes the emission of light during the transition from a physical system into a state of lower energy. The emission occurs spontaneously and the emitted light is typically of lower energy or longer in wavelength than the excitation source. Physical systems with fluorescent characteristics are atoms, molecules and ions. Fluorescence as a form of luminescence stops immediately when stimulation is removed and is therefore different than phosphorescence which exhibits a decay effect. Usually the light stimulus is of shorter wavelength than the emitted light e.g. ultraviolet and visible. The application range of fluorescence is wide and includes fluorescence spectroscopy, lighting, decoration, marking, displays, CRTs, biochemistry, analytical chemistry, medicine, mineralogy, geology, forensics and organic liquids. For materials made to fluoresce through penetration or coatings by inks the level and effect of the fluorescence can be measured with the use of a excitation light source and a light measurement instrument.

Picture 2: Volume Fluorescence Principle

Measurement Method:

Device under test (DUT) stimulation using a diffuse type irradiation source was chosen to avoid measurement inaccuracies caused by differences in surface structure. Three different type LEDs with peak emission at 365nm, 315nm and 255nm enable stimulation in the UV-A, UV-B and UV-C spectral range. To get quantifiable measurement results the irradiance E in W/m² effected by the stimulation source on the DUT is calibrated. The measurement quantity for the fluorescence is luminance L in cd/m². Luminance is a photometric quantity usually termed brightness. The ratio of the irradiance and luminance is the fluorescence efficiency.

Picture 3: Stimulation Spectrum of UV-B 315nm and UV-A 365nm sources

Measurement Set-up:

Three LEDs at three different wavelengths are used for DUT irradiation. The light source is built around an integrating sphere which offers diffuse and homogenous sample irradiation. The model UPK-50-S integrating sphere made out of OP.DI.MA. material was chosen for this UV application. This diffuse reflecting plastic coating is highly reflective in the ultraviolet spectrum, is stable under long term ultraviolet irradiation and has low self-fluorescence. The compact size integrating sphere at 50mm / 2inch diameter offers a 10mm / 0.4inch diameter light output port.

Each LED light input port is baffled with a ‘collar baffle’ which allows the use of several LEDs assembled behind the baffle around the light output port. Due to multiple diffuse reflections along the inner sphere surface the radiation is uniformly distributed before exiting the sphere. The light output irradiates the sample and stimulates the fluorescence.

Picture 4: Principle Draft of Measurement Set-up

The light output is measured by a SiC photodiode used as reference for the UV irradiation. The UV reference detector is solar blind and does not detect the visible light component of the fluorescence. This UV detector is calibrated and adjusted in irradiance at the reference plane at the light output port.

A luminance detector located opposite and in axial orientation to the light output port measures the fluorescence luminance of the sample surface. The luminance detector is built with components from Gigahertz-Optik’s MD-37 series. The UPK-50-S integrating sphere was modified with an additional measurement port. Easy modification to UPK type integrating spheres is possible because of the plastic coating. The measurement spot on the sample is approximately 8mm in diameter purposefully made smaller than the port to avoid any extraneous light being detected from the sphere surface. Apertures narrow down the measurement spot diameter. Photometric luminance sensitivity L is calibrated in cd/m². For measurement, the instrument is
placed with its light output port on the surface of the device under test. The DUT size must be larger than the measurement spot.

Picture 5: Measurement Port of Integrating Sphere

The electronics to power the LEDs and read-out the irradiance and luminance detectors are all standard components manufactured by Gigahertz-Optik.

The LPS-25-RM power supply operates the LED in constant current mode with high current stability and low noise. Its 16bit resolution digital analog converter (DAC) enables high resolution current adjustment within the maximum permissible range. For higher irradiance levels several LEDs can be operated connected in series. The LPS-25-RM supplies offers up to 48V for serial systems set-ups. Three power supplies allow individual operation of the UV-A, UVB and UV-C LEDs. The power supplies are remote controllable and set via their RS485 interface.

The X1-RM Optometer offers signal inputs for up to four photodiodes and can therefore readout and display the irradiance and luminance detector simultaneously. For each stimulation wavelength in the UV-A, UV-B and UV-C spectrum an individual calibration factor is stored in the optometer and applied according to the selected wavelength. In manual operation the X1-RM also supports the remote setup and control of the LPS-25-RM power supplies via RS485 interface. Remote control operation of the X1-RM and 3 x LPS-25-RM is possible via the X1-RM USB interface. Simple to use software is supplied offering both a single shot and data logger measurement mode with data transfer into Excel.

X1-RM and LPS-25-RM are rack-mounted into the BTH-19 bench top housing with BTH-19-Z03 main power supply.    

Picture 6: Sensor Head on Device Under Test

Calibration:

The luminance detector sensitivity is calibrated with a luminance standard source.
The UV reference detector sensitivity is calibrated by measuring the radiant power for each LED source at the reference plane of the integrating sphere output with a 7mm precision aperture in front of a large area photodiode calibrated in spectral radiant power sensitivity. The irradiance is then calculated using the aperture area. The LEDs current is set for 20mA. Both reference source and detector offer traceability to the PTB. NIST / PTB mutual recognition agreement statement available on demand.

Specifications:
Irradiated Area : 10mm diameter
UV-A365nm Irradiance : ~ 5.2W/m²
UV-B310nm Irradiance : ~ 0,36W/m²
UV-C255nm Irradiance : ~ 0.06W/m²
Luminance Sensitivity: ~ 2 cd/m² (0.04 cd/m² resolution)

Summary:

This application note describe a measurement device to qualify fluorescent samples. Gigahertz-Optik’s modular design concept and wide range of components and instruments for light measurement applications is also described. The measurement set-up introduced can be supplied in similar
configurations or modified for other applications. Please contact the
Gigahertz-Optik sales department or your local representative.
     

Picture 7: Electronic Device with X1-RM and 3xLPS-25-RM

Picture 8: Display of Single Shot Measurement

Picture 9: Display of Data Logger Measurement Mode