Proceedings Article | 16 November 2011
KEYWORDS: Prisms, Cryogenics, Infrared materials, Error analysis, Infrared radiation, Mirrors, Light sources, Refractive index, Refractometry, Temperature metrology
In order to enable high quality infrared lens designs at cryogenic temperature, we decided to study the IR materials’
optical properties, such as the refractive index, the thermo-optic coefficient (dn/dT) etc. As a matter of fact, accurate
precision refractive index data for infrared optical materials at cryogenic temperatures is scarce, so we decided to design
a cryogenic absolute prism refractometer for infrared optical materials.
Considering the most accurate and precise measurements of the real part of the refractive index, n, optical materials are
obtained through minimum deviation refractometry, we decided to choose this classic refractometer for our cryogenic IR
materials index measuring. Given the thermo-optic coefficient of many IR materials, which was reported by NASA, the
measurement precision is at least 10-3. In order to achieve this precision, the error of apex angle of sample prism and
deviation angle must be less than 20 arc-second. The thermal stress of the prism must be in control, or the volume
change of the prism may lead to stress birefringence (photo-elastic effect). The bandwidth of IR source must be less than
20nm and the error caused by dispersion (dn/dλ) will generally be negligible in our system.
The hardware system consists of 5 subsystems: the rotating sample chamber subsystem, the rotating plat mirror
subsystem, the cryogenic vacuum subsystem, auto control subsystem, exit image collimation subsystem. Finally, the
system is designed to measure IR materials’ indices between 1-4, at the wavelength of 1.0-12μm, at room (300K) and
cryogenic temperature (100K), with a precision of 10-4.
