A number of acousto-optic figures of merit and elasto-optic constants of a mercury bromide (
H
g
2
B
r
2
) crystal were measured. The key characteristics of the wide-angular acousto-optic tunable filter based on a
H
g
2
B
r
2
crystal in the (
1
1
¯
0
) plane were calculated from the measurements carried out and previously known data. It is demonstrated that the crystal makes it possible to create effective devices operating in the 5–25 µm wavelength range. In addition, we proposed the design of a filter operating in the long-wavelength infrared range of 8–12 µm, providing spectral resolution up to
R
=
300
and a field of view outside the crystal up to 10°. Its diffraction efficiency can reach up to 2% per 1 W of continuous driving power.
Measurements of the refractive indices and the full set of longitudinal acousto-optical figures of merit in an orthorhombic single-crystal indium iodide (InI) were carried out. The high acousto-optic characteristics (
M
2
up to
1100
×
10
−
15
s
3
k
g
−
1
) and strong optical anisotropy (
Δ
n
=
0.47
) make it possible to design various types of high-performance acousto-optic devices of the middle and far-infrared spectral range. In combination with a wide transparency range (
0.62
−
51
µ
m
), the obtained characteristics make the crystal extremely promising in comparison with most existing analogs.
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