Quantum Efficiency Integrating Spheres

The Labsphere QE-060-SF is a 150mm (6-inch) diameter integrating sphere coated with Spectraflect high reflectance paint which is purpose-designed for measuring the photo-luminescent quantum yield (PLQY) or quantum efficiency (QE) of phosphorescent materials, such as LED phosphors and quantum dots. The QE sphere features a rotating centre mount sample holder with interchangeable clip, jaws, cuvette and powder sample holder accessories.

Phosphorescence is that property possessed of certain materials whereby light at shorter wavelengths is absorbed and reemitted at longer wavelengths. As a common example, consider that nearly all modern lighting is based upon blue LEDs with phosphor coatings. The blue pump light from the LED is partially absorbed upon transmission through the phosphor, with the phosphorescent emission combining with residual blue light to create the desired broadband white light illumination.

Different phosphor formulations affect the spectral power distribution of the phosphorescent emission, while the thickness of the phosphor layer impacts upon the amount of residual blue light that is transmitted. By varying the phosphor composition and thickness, the spectral power, colour temperature and colour rendering of the LED lamp can be tuned to that which is required.

Integrating Spheres for Quantum Efficiency Measurements

A key metric that is used to quantify the optical performance of a phosphorescent material is the quantum efficiency, also referred to as the luminescent quantum efficiency. This is simply the ratio of the photons emitted to the photons absorbed by the sample.

The fluorescent emission is diffuse, therefore an integrating sphere coupled to a fibre optic spectrometer is the obvious choice to capture the 4π fluorescent emission. It is important to note however that the integrating sphere introduces an error arising from reillumination excitation. Any light that is not absorbed when the sample is first illuminated by the excitation beam is either reflected or transmitted. This light can in turn reflect from the sphere wall and diffusely reilluminate the sample. Some of this light will again be absorbed, causing a higher level of fluorescent emission that would be the case without a sphere present. Fortunately, this re-excitation error can be corrected. For a complete explanation of the five measurement steps – including the correction for reillumination excitation – please refer to the technical note that you can find in the resources tab.

QE-060-SF Integrating Sphere Features

The QE-060-SF is coated as standard with Labsphere’s Spectraflect diffuse reflectance coating, a barium sulphate formulation that provides a peak reflectance of 98% and >95% reflectance from 300-1300nm.

The QE-060-SF integrating sphere features three orthogonal ports. A 50mm diameter sample port is located at the sphere’s north pole position. A 25mm detector port is located at the 0° position on the sphere’s horizontal equator, while a second 25mm input port is positioned at the 90° position on the equator. All ports are equipped with rugged port frames to simplify the mounting of port accessories.

The all-important baffle is positioned to screen a centre-mounted sample from the detector port to prevent direct illumination of the photodetector or spectrometer from the sample under test. The sphere is fitted with a ¼-20 tapped mounting boss at its south pole position and comes with a height adjustable post mount, post mount holder and sturdy base.

An optional 25mm diameter SMA-style port adaptor is available for fitment to the detector port to couple light into an optical fibre for collection by a spectrometer.

Higher Performance QE-060-SL Spectralon Sphere for Enhanced Signal-to-Noise Measurements

An alternative model is available, the QE-060-SL, that features Labsphere’s Spectralon diffuse reflectance material instead of the Spectraflect coating. At its peak, Spectralon has a 99% reflectance, and maintains >95% reflectance from 250-2500nm. While a 1% difference in reflectance may not seem very much, it represents half the loss per reflection inside the integrating sphere cavity. Consequently, the Spectralon sphere provides at least 50% higher throughput over the entire wavelength range, with the gain increasing to up to 400% in the near infrared. This improved throughput translates directly into a higher signal reaching the photodetector or spectrometer, to the benefit of signal to noise and the ability to resolve low level fluorescent signals. The enhanced performance of the Spectralon QE sphere is important when you consider that fluorescent emissions can be quite weak in comparison to the excitation signal.

The other benefit of Spectralon is its thermal stability, resistance to UV damage and ease of cleaning, although accidental contamination of either Spectraflect or Spectralon coatings with phosphorescent chemicals should be avoided at all costs.

Centre Mount Sample Holder

The material under test is held in the centre of the sphere using the suppled centre mount sample holder that attaches to the sphere at the 50mm port at the north pole. The material’s quantum efficiency is simply determined by recording the detector signal with the sample held inside the sphere but not directly in the beam, compared to the detector signal when the sample is rotated into the probe beam.

The centre mounted sample holder assembly switches between two clearly marked positions which allows for rotation of the sample in and out of the excitation beam for baseline referencing. The centre mount sample holder can be supplied with interchangeable sample holders. Choose from a clip-style for thin delicate samples, a jaws-style for more rigid materials, a cuvette holder for liquid samples and a powder sample holder.

System Requirements

Your complete experimental setup for quantum efficiency measurements should include the QE integrating sphere, a collimated external monochromatic light source or laser for sample excitation and a photodetector or spectrometer. Typically, the detector used will be an array spectrometer (also available from Pro-Lite), for which you will need to purchase an SMA port adaptor for the detector port on the sphere.

For absolute flux measurements, and for normalisation of spectral responsivity, a standard lamp of spectral flux is available on request. To avoid self-absorption errors, the calibration lamp is best left mounted on the sphere at an optional fourth port location. Please enquire if you are interest in purchasing a QE sphere for absolute flux measurements.

Please note that we welcome requests for modified standard products or out-and-out bespoke integrating sphere designs to suit your exact needs. Please use our enquiry form to share your requirements with us.