Absolute Total Reflectance Spectra of Very Small Objects Using The ISR-2600Plus Integrating SphereShimadzu Scientific Instruments, Columbia, Md.

Introduction

Often reflectance spectra of very small samples are required.

Typical sphere openings and sampling accessories are designed for samples in the 25mm X 25mm range with maximum thicknesses of 1 cm.

Consequently, correct positioning of such samples to acquire accurate spectra can be very challenging.

This application note demonstrates an easy-to-fabricate holder that is suitable for many small samples. In addition, it shows the measurement of absolute total reflectance using this sample holder.

Small samples requiring special sampling techniques

For accurate analysis of samples in the Shimadzu ISR-2600Plus integrating sphere, the sample must be positioned level with the plane of the sphere. This can be very challenging for small samples. Success has been demonstrated by fabricating a small sample holder out of a white Compact Disk fiberboard sleeve. The fiberboard sleeve offers three advantages.

1. There is a manufactured fold that aids in preparing a “binding” action for small sample positioning.

2. The sleeve is white, which can mimic the sphere’s internal BaSO4 coating and help to maintain a high S/N for measurements.

3. The thin nature of the fiberboard allows the positioning of the small sample close to the plane of the sphere’s opening.

Sample Analysis

It is important that the sample be placed as close to the sphere opening as possible as any depth can cause a tunnel effect which acts like a light trap and can reduce overall S/N resulting in poorer quality spectra.

Sample cards ready for measurement with samples attached

Small sample holder fabricated from CD fiberboard sleeve

Sample Analysis

For small flat samples, the sample can be easily sandwiched between the two sides of the sleeve. The sleeve can then be positioned against the sphere using the sphere’s spring retaining arm for analysis.

For thicker samples, the sample can be fixed to the back side of the sleeve and easily positioned on the sphere in a similar manner. A mark was made on each card that could readily be used to reproducibly position the modified holder so that the sample port was in the same position in the sphere opening for each measurement.

For background measurements, a working (un-calibrated) mirror was placed behind the modified sample holder so that the sampling port of the holder was covered by the mirror. In a similar fashion, acquisition of the spectrum of the calibrated reference mirror was accomplished by placing the reference mirror behind the modified holder.

Sample Analysis

Alignment marks used to reproducibly align the sample in the sphere port for analysis

Sample Analysis

Calibrated standard mirror in position for measurement, behind the sample holder card

Sample Analysis

Spectra of typical small samples acquired with the modified small sampling accessory card are shown. A blank spectrum (a scan of the working mirror used for baseline acquisition) is shown below and demonstrates the excellent linearity of the fiberboard sample holder throughout the full working range of the sphere (1400 to 220 nm).

Blank of working mirror and new fiberboard small sample holder showing excellent linearity throughout the full working range of the instrument

Spectra

The graph below shows acquired relative reflectance spectra of typical small samples. Notice that portions of two scans exceed 100% reflectance. This is not uncommon as the resultant reflectance values are “relative” to the working mirror used for background acquisition.

Acquired spectra of three typical small samples: a glass optical component (black), a plastic lens (blue), and an optical filter (magenta)

Spectra

Acquired spectrum of a calibrated standard reference mirror

The relative reflection data from the acquired spectra above can be converted to absolute spectra values by acquiring the spectrum of a calibrated standard reference sample and the following equation:

corrected =((Sample - 0) / (100 - 0)) Reference

Spectra

In this example, the spectrum of the calibrated mirror becomes ρ100.

In theory, with highly specular samples, ρ0 is typically insignificant, and therefore was not used in these measurement.

The Data Table feature of UVProbe was used to truncate the acquired scans to match the 25nm pitch of the supplied reference mirror absolute reflectance data.

In addition, the Data Table allows for easy copying of the spectral data directly to a spreadsheet program where the absolute (corrected) calculations were performed, table 1.

Spectra

Table 1: Spreadsheet data for all scans

Spectral Data Table

Spectral Data Table

Table 1, continued

Acquired spectrum of a calibrated standard reference mirror

The final absolute results are shown in the graphic:

Spectra

Conclusion

With proper care to sampling holders and sample placement, the relative reflectance of small samples can readily and accurately be measured using a Shimadzu Integrating Sphere accessory.

In addition, with the Data Table features of the UVProbe software, Absolute (corrected) reflectance spectra can be easily calculated with little extra effort.

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