High refractive index material such as ZnSe creates fringing, which is also known as channeling. Fringes are useful for path length calibration, but fringes can also interfere with spectral analysis, particularly when the refractive index of the sample is not well matched to the refractive index of the sample. Fringing can occur in a single window where the light is reflected between parallel surfaces of window and in a cell containing a pair of windows where the light is reflected off of both windows. The smaller the path length, the more dramatic the fringing effect is. Accordingly, thinner windows will have more dramatic fringes than thicker windows. Fig. 1 shows the fringe pattern of a properly assembled ZnSe 0.1mm path length liquid cell. This cell can be calibrated to a high degree of accuracy and has utility for well matched samples. But, if the sample is not well matched and ZnSe or some other high index material is chosen for reasons such as spectral range or material compatibility, then the spectroscopist may be well advised to consider means useful to limit fringing.
A simple way to limit fringing is to wedge the spacers in liquid cells. The downside is that the path length cannot be calibrated with the same high degree of accuracy that would otherwise be possible. This method works because the wedged spacer insures the windows are not parallel. There is an additional charge for the wedged spacer and the part number is 0001-9228.
Another simple way to limit fringes is to fog the windows. This of course only works with deliquescent windows which typically have a low refractive index anyway. Most windows that comprise the sample compartment seal in FTIR spectrophotometers are wedged KBr with a moisture barrier coating.
In gas cells, we provide windows that are both wedged and anti-reflective (AR) coated on one side. Prices for stock AR/wedged windows are shown below.
Broadband AR coatings also can be applied on both sides of a window, and are typically designated as AR/AR. Although AR coatings do have some spurious bands, they can generally be tolerated and can be compensated by means of a background scan of the sampling accessory containing the AR coated windows.
We routinely provide wedged windows in many materials. Wedged windows are the traditional and most inexpensive means of limiting fringing.
