Thetametrisis - Technology: Reflectance measurements

_Reflectance measurements

In the reflectance mode the signal from the reflection probe is collected and analyzed by the spectrometer embedded in the FR-tool and subsequently is processed by FR-Monitor software. The reflection probe consists of tight bundle of 7 parallel optical fibers. The outer six fibers are connected to the light source and the central fiber is coupled to the spectrometer through SMA connectors.

The reflectance spectrum represents the light reflectance from a solid sample vs. wavelength, figure 1. For more detailed description on how to perform the reflectance measurement please refer to section on FR-Monitor manual, in Download section.

Figure 1. Reflectance set up. The sample is placed on the working area and the reflection probe is attached on X-Y-Z manual positioning holder, that allows for meauserments over the whole sample by moving the reflection probe over the point of interest.

The definition of Reflectance (R(λ)) is:

where:

I_o is the reference light intensity, and

I_o is the light intensity reflected from the measurement sample .

For precise measurements the dark spectrum should be subtracted from both sample and reference spectra, so eq. 1a becomes:

where:

I_ref is the reference light intensity, i.e. the light intensity of the light source used as it is recorded by the spectrometer when reflected at a shinny and of ultra high reflectivity surface.

I_sum is the light intensity reflected from the sample i.e. the light intensity recorded by the spectrometer when the light from the light source incidents on the sample of interest.

I_dark is the dark intensity i.e. the light intensity recorded by the spectrometer with the light source switched-off or blocked and without sample.

The reflectance value is in the [0, 100]% range. Reflectance measurements of semi-transparent and transparent samples can be also performed, as well as and of un-supported (suspended) films. Furthermore reflectance measurements could be performed in liquid environments.