Integrating sphere efficiency

When light is directed onto a sample it may either be transmitted or reflected. Hence, one can obtain the spectra by either transmission or reflection. Since some of the light is absorbed and the remainder is reflected, study of the diffuse reflected light can be used to measure the amount absorbed. However, the low efficiency of this diffuse reflectance process makes it extremely difficult to measure 120) and it was speculated that infrared diffuse reflection measurements would be futile 120). Initially, an integrating sphere was used to capture all of the reflected light121) but more recently improved diffuse reflectance cells have been designed which allow the measurement of diffuse reflectance spectra using FT-IR instrumentation 122). 

Figure VB-2 Comparison of QEj (EL) (solid dots) with QE ,(PL), both measured under the same conditions for devices with different luminescent polymer thicknesses. Open squares PL efficiency as obtained when the incident light intensity was corrected by directly measuring the sum of unabsorbed and reflected light for each individual device outside of the integrating sphere. Solid x s PL efficiency as obtained when the incident light intensity was corrected by measuring the transmittance and reflectance of EL polymer films of different thickness on the glass substrate, following the procedure of Greenham et al. (Taken from ref 158)...

Figure 6.22. (A) Path length enhancement for a clear sample inside an integrating sphere. (B) Additional mirrors to provide a double laser pass through the sample and increased collection efficiency.

Integrating spheres are also prevalent in NIR spectroscopy and have the advantage of improving the efficiency of signal collection. Workman and Burns [126] have reviewed NIR instrumentation. 

Time-resolved PL measurements were also performed using time-correlated single-photon counting (TCSPC) and photoluminescence upconversion (PLUC) spectroscopies. Descriptions of the setups can be found in refs. [14, 65], respectively. All measurements were taken in continuous-flow He cryostats (Oxford Instruments OptistatCF) under inert conditions. Finally, PL efficiency measurements were performed on simple polymer thin films spin coated on Spectrosil substrates using an integrating sphere coupled to an Oriel InstaSpec IV spectrograph and excitation with the same Ar+ laser as above. 

Fig. 4.21 shows the thickness dependence of the external quantum efficiency for electroluminescence, QFext(FL) as a function of the film thickness for two closely related PPV derivatives, OCIClO-PPVand MEH-PPV. The quantum efficiencies of these devices were measured using an integrating sphere. 

The authors used an integrating sphere to measure the external efficiencies based on the total light emitted. A correction factor of 3 is applied to make the values comparable to the other external efficiencies listed here (see Section 12.04.2.2). 

As in the case of DT, diffuse reflectance (DR) can be measured in the mid-IR region with an integrating sphere [131, 141]. However, this technique is used mainly in reflectometry, where repeatable measurements with full hemispherical collection of radiation are needed [142]. Better performances, up to 12% efficiency, can be obtained using accessories with the so-called biconical optical configuration, which focuses the reflected radiation onto the IR detector. The SNR of such spectra is sufficient if such an accessory is used in conjunction with a FTIR spectrometer. The first DRIFT S accessory was described by Fuller and Griffiths in 1978 [143]. After that many forms of the DRIFTS accessories... 

Nevertheless, quantum efficiency, measured by the integrated sphere technique, was 32%, not yet good enough for the luminescent collection application, which requires near 60% conversion of absorbed to luminescent energy. Also optical scattering should be lower. 

Another method of measurement of diffuse reflectance does not require an integrating sphere. In this method, the sample beam is focused onto the sample by means of ellipsoidal or spherical mirrors and collected by another ellipsoidal mirror, at either 180 or 90° from the incident beam. This method has been used for many years in the infrared, since proposed by Fuller and Griffiths (1978, 1980). The advantage is very high collection efficiency and the ability to measure very small samples. Such geometry is known by a number of names, including biconical, Praying Mantis (a trade-... 

The throughput (t) of an integrating sphere, sometimes referred to as another measure of sphere efficiency, is given by the following expression... 

For any integrating sphere, the sphere error (Eg) due the variance in sphere efficiencies between the sample and the standard when both are measured at the identical port is given by... 

Three types of devices have been described for the measurement of mid-infrared spectra on-axis accessories, off-axis accessories that are mounted in the sample compartment of the spectrometer, and integrating spheres. On-axis accessories for diffuse reflection spectrometry are very much like highly efficient specular reflection accessories. They usually have a higher optical efficiency than either of the other two designs. A typical design of a DR accessory with on-axis geometry is shown in Figure 16.2. 

There are also different and efficient solutions to locate solid samples inside the spectrophotometer to study their transmittance or their total reflectance. These housing systems allow to orient the sample at different angles with respect to the incident light to measure both the incident and diffuse reflectance. It is also possible to use the so called integrating spheres that completely surround the sample and are able to collect all the reflected light to measure the value of the total reflectance. 

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