Variable Angle Reflection Accessory Tech

Figure 1. Three Models of the Variable Angle Specular Reflection Accessory. Center model is most commonly used for FT-IR spectrometers, the other two for UV-VIS spectrometers.

The Variable Angle Reflection Accessory is ideal for specular reflection studies of films on metallic substrates, coatings, contaminants on reflective surfaces, and measurements of film thickness. This accessory is the industry standard for variable angle specular reflectance studies. Three different models, shown in Figure 1, are available to suit the various optical designs of commercial UV-VIS and FT-IR spectrometers.

For polarization measurements, Harrick Scientific's Wire Grid (infrared) or Glan Taylor (UV-VIS) Polarizers can be easily mounted onto the sample stage. For sampling versatility, there are four different specular reflectance sample stages available: a continuously variable angle rotational stage, included with the accessory; 12° absolute reflectance stage; 45° absolute reflectance stage and a 12° reflectance stage.

Variable Angle Specular Reflectance Sample Stage

Figure 2. The Variable Angle Reflectance Stage.

This sample stage, shown in Figure 2, is included with the Variable Angle Reflection Accessoy and is the industry standard for variable angle specular spectroscopy studies. The incident angle can be continuously varied from approximately 30° to 80°. Samples with a minimum size of ½" x ½" and up to 0.5" thick can be readily examined. This RMA works best with samples up to 0.5" thick.

The incoming infrared or UV-VIS radiation is directed by mirrors to the sampling stage, where it reflects from the sample stage mirror and the sample. This mirror and the sample are coupled to rotate together. This configuration ensures that, once the Variable Angle Reflection Accessory is aligned, it remains aligned for all incident angles. Some typical applications are shown in Figures 3 and 4.

Figure 3. Specular Reflectance of an SiO₂ Coated Mirror at an Incident Angle of 75°.

Figure 4. Specular Reflectance of a Mylar Film at an Incident Angle of 20°.

Absolute Reflectance Sample Stages

Figure 5. The 12° Absolute Reflectance Accessory.

Figure 6. The 45° Absolute Reflectance Stage.

Two absolute reflectance stages are available. One, particularly well suited for examining highly reflective samples, features a 12° incident angle and retains the polarization of the incident beam (see Figure 5). The other, shown in Figure 6, offers a 45° incident angle.

The two fixed angle absolute reflectance sample stages available feature incident angles of 12° and 45°. Both utilize the 'V-W' double reflection technique illustrated in Figures 5 and 6. Using this technique, the reference spectrum is obtained in the 'V' mode. The beam is directed by mirrors from the sample stage mirror to the detector. To collect the sample spectrum, the stage is rotated (12° model) or inverted (45° model) to the 'W' mode. In this mode, the beam is directed from the sample to the stage mirror. The sample stage mirror reflects the beam back to the sample. The beam is then directed via mirrors to the detector of the UV-VIS or FT-IR spectrometer.

This configuration maintains the alignment and optical pathlength for both the sample and the reference spectra. Note that the quantity measured is the ratio of the spectrum with the sample to the spectrum without the sample, is the square of the reflectance, R2.

Horizontal Reflection Stage

The Horizontal Reflection Stage (see Figure 7) is used in combination with the Variable Angle Reflection Accessory for specular reflectance measurements at a 12° angle of incidence (near normal). The sample rests sample side down on a horizontal stage, making surface scanning and sample exchange straightforward.

Figure 7. Horizontal Reflection Stage.

This sample stage incorporates a single mirror that reflects the radiation from the sample stage mirror to the horizontal sample and then directs the light reflected from the sample back to the sample stage mirror.