Thermo Scientific Nicolet Continuum, User Manual

Page: 134 / 195

Thermo Scientific 130
Refractive
index compensation
When light traveling through one material enters another material
with a different refractive index, the path of the light changes. This is
called refraction.
In transmission experiments, the sample is often installed on or
between an infrared-transparent substrate, such as potassium
bromide (KBr), barium fluoride (BaF
2
), sodium chloride (NaCl) or
diamond. When placed in the optical path between the sample and
the objective or condenser, these (and other) substrates displace the
focus and cause optical aberrations.
Reflachromat objectives and condensers are adjustable and can
correct for focal displacement and aberrations introduced by a
sample substrate. The compensation rings have five marked
positions: 0, .17, 1.0, 2.0 and 3.0. For substrates with a refractive
index equal to 1.49, the compensation ring settings correspond to the
thickness of the substrate between the sample and the objective.
Check with the manufacturer if you are unsure about the refractive
index of your substrate. Thus, if the sample is between 2 mm salt
(NaCl) plates, set the compensation rings to 2.0 If the sample is in a
diamond compression cell, set the compensation rings to 1.0. If the
substrate and superstrate are of different materials, set the objective
compensation to match the superstrate and the condenser
compensation to match the substrate.
Note Many commonly used substrates are 2 mm thick.

If you use fixed-compensation objectives and condensers, be sure to
use an objective and condenser with the appropriate compensation
values. If the sample is installed on a substrate, the condenser
compensation value should match the substrate thickness. If a
window is placed on top of the sample, the objective compensation
value should match the window thickness.