# FTIR Moving Mirror Travel Distance Calculation

## Issue

What is the distance that the moving mirror in an FTIR spectrometer travels during a data collection.

FTIR

## Resolution

The chart below shows the distance that the moving mirror will travel during a data collection at various resolution settings.

Resolution

(cm-1)

Calculated Retardation

(cm)

Calculated Mirror Travel Distance

(cm)

Actual Mirror Travel Distance

(cm)

32 0.03125 0.015625 0.03125
16 0.0625 0.03125 0.0625
8 0.125 0.0625 0.125
4 0.25 0.125 0.25
22 0.5 0.25 0.5
1 1 0.5 1
0.5 2 1 1
0.25 4 2 2
0.125 8 4 4

## Analysis

In an FTIR spectrometer, the travel distance required by the moving mirror is dependent on the optical resolution of the data collection. These values are related through the following equation:

Optical Path Difference (or Retardation) = 1/Resolution

This equation gives the required difference in optical path between the fixed and moving mirrors to achieve a given optical resolution. Because the IR light travels past the point of equal distance and then back, the distance that the moving mirror travels is actually one half of the Retardation value calculated from the equation above.

In a Thermo Fisher Scientific FTIR system, this general rule holds true for the resolutions of 0.5 cm-1 and higher because the instrument collects a single-sided interferogram. For resolutions of 1 cm-1 and lower, the instrument will collect a double-sided interferogram. This means that the instrument collects data equidistant on both sides of the zero path difference, and therefore the actual travel distance is twice the calculated mirror travel distance.