Termohram quality stabilisation by the way of thermal defocus compensation

Abstract

This article considers the problem of the thermograph lens defocusing under the influence of ambient temperature. The gradient of the external ambient temperature leads to a change of the design parameters of the optical system and, consequently, to thermoresponsive and the emergence of thermobarics image, which cause a sharp deterioration in the frequency and energy characteristics of the lens of the thermograph. One method of solving this problem is to compensate the influence of temperature fields on the image quality of the focusing assemblies operating in the infrared spectral range for the case of uniform heat distribution in the system.

Elimination based on the characteristics of the lenses from the temperature is advantageously carried out at the design stage of the optical system by using methods of passive optical termales, which is based on the application schemes termaltake infrared (IR) lens components, made of different optical materials, transparent in the infrared region of the spectrum and have different signs of thermo-optic constants.

<p >Based on the proposed method of filling thermograph thermoresponsive lens  and further optimization was designed ternary diagram of temperature-compensated infrared lens for use with thermographs modern matrix receivers emitters with a size of 340×240 pixels with a size of one pixel 25x25 mkm.

Thanks to this method, and the calculations in the mathematical software Mathcad have been received a number of combinations termaltake triplets infrared spectral range of 8-12 microns. At the same time it was managed to achieve thermal stabilization and system achromatization. However, this approach is only oncovin because was made from absolutely fine components and without taking into account the nonlinearity of the dependence of materials optical parameters from temperature. Therefore, for the design of termaltake working system with good performance requires further optimization of the triplet. It was implemented in the environment of computer-aided design Zemax, which allows to conduct computer modeling with the following factors: nonlinear dependence of characteristics of optical materials from the wavelength of the radiation and temperature; the analysis of passing through focusing system a large number of rays involved in image formation; obtaining a graphical representation of image quality of the optical system.

The proposed lens have the following characteristics: relative aperture of 1:1 angular field of view 2ω=20°, focal length f'=38,2 mm. High image quality of the lens is confirmed by the level of modulation transfer function of 50% at a spatial frequency of 20 mm-1 for the edge of the system view field and the level of the function of energy concentration of 66% in the spot size dispersion of 25 mkm for the edge of the system view field.

Back focal length of this triplet at variations of temperature in the range from 0 °C to +50 °C changes only by a fraction of a micrometer, which in turn gives the opportunity to obtain thermogram with high resolution. The stability characteristics of the lens make it possible to more accurately determine the temperature gradient adjacent portions of the object surface and the diagnosis when the temperature of the external environment changes.

Ref. 11, fig. 1, tabl. 2.