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A. Ikehata
9.1.5 Other Optical Materials
9.1.5.1 Transparent Materials for NIR Light
Materials transparent in the visible region are often transparent in the NIR region.
Although ordinary glass, such as BK7, can be used in the short wavelength NIR
range, because it contains O–H groups due to impurities, it exhibits absorption near
1400 and 2200 nm. For longer NIR wavelengths, fused silica (fused quartz), which
is pure silicon dioxide glass, is most often used. However, fused quartz also contains
O–H groups, depending on its purity. To avoid this problem, infrared grade highpurity quartz should be used. In terms of durability, sapphire is a very good material.
However, attention should be paid to the possibility of loss and interference due to
reflection, because sapphire has a refractive index of 1.7 or higher, even in the NIR
region. CaF 2 and MgF 2 crystals used in the mid-infrared region are completely transparent in the NIR region, but they are rarely used because of problems of mechanical
strength except, in cases where the low refractive index is essential.
9.1.5.2 High- and Low-Reflection Materials
The most popular mirror in the visible range has an aluminum-coated surface, but
the reflectivity of Al decreases near 850 nm. Therefore, gold is used as a standard
reflector in the NIR region. The reflectivity of gold consistently exceeds 96% in
the NIR region (700–2500 nm). To prevent reflection, black materials tend to be
used. However, even objects that appear black in the visible spectrum may reflect
NIR light. For example, black alumite that is often used for optical mounts has a
reflectance of nearly 50% in the NIR region, and this may cause stray light. Carbon
black paint without lacquer or black flock paper can achieve lower reflectivity than
black alumite. Black bakelite (phenolic resin) is also a good low-reflection material.
An anti-reflective cloth with flocked fibers dyed black having a reflectance of only
2% is available at camera stores.
9.1.5.3 Polarizer
In normal use, a polarizer made by monoaxially stretched dye polymer film is easy
to use in a large area. It can transmit light with electric field amplitude perpendicular
to the stretching direction. However, transmittance is about 30–40% due to the dye.
When using a laser, the polarizing film may be destroyed, so a polarizer based on
the birefringence of the crystal is required. A typical Rochon prism polarizer made
from quartz or MgF 2 is available in the NIR region.
A. Ikehata
9.1.5 Other Optical Materials
9.1.5.1 Transparent Materials for NIR Light
Materials transparent in the visible region are often transparent in the NIR region.
Although ordinary glass, such as BK7, can be used in the short wavelength NIR
range, because it contains O–H groups due to impurities, it exhibits absorption near
1400 and 2200 nm. For longer NIR wavelengths, fused silica (fused quartz), which
is pure silicon dioxide glass, is most often used. However, fused quartz also contains
O–H groups, depending on its purity. To avoid this problem, infrared grade highpurity quartz should be used. In terms of durability, sapphire is a very good material.
However, attention should be paid to the possibility of loss and interference due to
reflection, because sapphire has a refractive index of 1.7 or higher, even in the NIR
region. CaF 2 and MgF 2 crystals used in the mid-infrared region are completely transparent in the NIR region, but they are rarely used because of problems of mechanical
strength except, in cases where the low refractive index is essential.
9.1.5.2 High- and Low-Reflection Materials
The most popular mirror in the visible range has an aluminum-coated surface, but
the reflectivity of Al decreases near 850 nm. Therefore, gold is used as a standard
reflector in the NIR region. The reflectivity of gold consistently exceeds 96% in
the NIR region (700–2500 nm). To prevent reflection, black materials tend to be
used. However, even objects that appear black in the visible spectrum may reflect
NIR light. For example, black alumite that is often used for optical mounts has a
reflectance of nearly 50% in the NIR region, and this may cause stray light. Carbon
black paint without lacquer or black flock paper can achieve lower reflectivity than
black alumite. Black bakelite (phenolic resin) is also a good low-reflection material.
An anti-reflective cloth with flocked fibers dyed black having a reflectance of only
2% is available at camera stores.
9.1.5.3 Polarizer
In normal use, a polarizer made by monoaxially stretched dye polymer film is easy
to use in a large area. It can transmit light with electric field amplitude perpendicular
to the stretching direction. However, transmittance is about 30–40% due to the dye.
When using a laser, the polarizing film may be destroyed, so a polarizer based on
the birefringence of the crystal is required. A typical Rochon prism polarizer made
from quartz or MgF 2 is available in the NIR region.