Shimadzu IRAffinity-1 Instruction Manual page 25

Assume that the light source emits monochromatic light of wavelength λ (cm). When the distance l
the fixed mirror and the beam splitter is equal to the distance l
splitter, the optical path difference between the two beams, χ = 2 (l
phase. While in phase, the beams interfere constructively with each other
is displaced λ/4 cm, the optical path difference becomes λ/2 cm, and the two beams are out of phase,
interfering destructively (Fig. 1.2
nλ and destructively when χ = (n + 1/2) λ where n is an integer.
Light from the
fixed mirror
Light from the
moving mirror
Light from the
moving mirror
Equation 1.1, extrapolated from the above principles, calculates the intensity I*(χ) of light (wavelength λ)
incident to the detector.
4RTS λ ( ) 1
I* χ ( )
= -- -
2
where R: energy reflected by the beam splitter
T: energy transmitted by the beam splitter
S (λ): radiation energy from the light source
The intensity of the light observed by the detector is a function of Equation 1.1. I(χ) denotes the light intensity,
and the wavenumber σ (cm
1
I χ ( ) 4RTS λ ( ) • -- -
=
2
B σ ( )
= cos 2πσχ
where B σ ( ) 4RTS λ ( ) 1 2 ⁄
=
The signal I(χ) observed by the detector is called an interferogram, and 4RT is labeled beam splitter efficiency.
If polychromatic light is emitted instead of monochromatic light, I(χ) is given by the integration of 1.2 with
respect to wavenumber.
∞
∫
I χ ( ) B σ ( ) cos 2πσχ σ
=
0
A, C). Thus, the two beams interfere constructively with each other when χ =
Fig. 1.2 Interference
χ
1
-- - -- -
+ cos 2π
......................................................................................................... (1.1)
2 1
-1
) replaces the wavelength λ.
cos 2πσχ
............................................................................................................................ (1.2)
•
δ
.................................................................................................................... (1.3)
1.1 Principle and Construction
between the moving mirror and the beam
2
− l
), is equal to zero, and the beams are in
1 2
(Fig. 1.2 A, B). As the moving mirror
The difference of the resultant
interfering light
bright
dark
IRAffinity-1
between
1
λ light wave
2 λ light wave
1-3