GB2125573A - Thermal imaging system - Google Patents

Abstract

In a thermal imaging system employing a thermochromic film 2 supported on a temperature stabilised radiation absorbing membrane 1 increased sensitivity is obtained by exposing the film 2 to light in the visible region of the spectrum after exposure of the membrane to the thermal image. The intensity and spectral distribution of this light is arranged to provide a measure of positive optical feedback. <IMAGE>

Description

SPECIFICATION Thermal imaging system This invention relates to thermal imaging systems, and in particular to such systems employing a thermochromiccholesteric layeras the display medium.

Thermochromic devices using cholesteric materials are very sensitive temperature indicators. Mixtures are available forwhich the selectively reflected wavelength moves through the entire visible range of the spectrum over only one degree Celsius. A con struction of thermal imaging system based on this effect is described for instance in the specification of United States Patent No.3527945. Typically such a device uses a cholesteric liquid crystal film about 15 microns thick coated on a polyesterfilm about6 microns thick, which is blackened on the opposite surface to provide efficient absorption ofthe thermal radiation incident upon it.Such a film is placed in a carefully designed enclosure and its temperature is accurately controlled. Thesensitivityfor such a device can be such as to provide forthe detection of a temperature difference of 0.02"K at 300"K with a resolution of 0.05 line pairs per mm.

The limiting factors governing the performance of this type of system are the sensitivity ofthe cholesteric in terms of reflection wavelength change against I temperature, the accuracy of the thermostatic control, and thermal spreading effects. This invention is concerned with the use of positive optical feedback to provide enhancement ofthe effective sensitivity ofthe cholesteric.

According to the present invention there is provided a method of operating a thermal imaging system in which the thermal image is focussed upon a radiation absorbing membrane supporting athermochromic cholesteric film which by temperature dependent selective reflection in the visible range of the spectrum displays the image, wh ich method of operation employs positive optical feedback, and includes the steps of stabilising the temperature of a region of the membrane, of irradiating itwith a thermal image, of ;uniformily irradiating the region via thefilm with light at relatively higher energy density having a spectral distribution providing positive optical feedback, and of viewing thefilm with illumination of relatively lower energy density.

The invention also provides a thermal imaging system having an optical system for focussing a thermal image upon a radiation absorbing membrane that supports a thermochromic cholesteric film, means forstabilising the temperature of the mem ; brane at least overthe area of the focussed image, an optical source for illuminating the film with relatively higher intensity uniform light in the visible range of the spectrum having a spectral intensity distribution in relation to the spectral reflectivity of the film such as to provide positive optical feedback of the visible light image created inthefilm by differential heating ofthe membrane by the thermal image, and an optical source for illuminating the film with relatively lower intensity light to enable the visible light image to be seen.

The application ofthe invention to thermal imaging will now be described and explained in more detail.

The description refers to the accompanying drawings in which: Figure 1 depicts a schematic representation of a thermal imager embodying the invention in a preferred form, and Figure 2 is a graph depicting the reflection characteristics of certain cholesteric mixtures.

Referring to Figure 1, a thin plastics membrane 1 blackened on both sides is supported in a radiation controlled environment (notshown) and carries a thermochromic cholestericfilms 2 and 3. In orderto reduce lateral heat spreading the membrane may be apertured in the region beyond the extent of the films 2 and 3 in the manner described in the United States patent specification previously referred to.

Film 2 faces a wavelength reflective mirror4which allows imaging optics, represented in the figure by lens 5, to form a thermal image ofthe same under observation. Light from two sources 6 and 7 is directed via first and second beam splitters 8 and 9 and a further optical system, represented by a lens 10, to allow illumination and inspection ofthefilm 2 with light in the visible range of the spectrum. The mirror4 may be provided by a slice of germanium, which will transmit light in the wavelength range ofthe thermal radiation but reflect light in the visible range, whereas beam splitters 8 and 9 are conventional beam splitters, provided for instance by multilayer dielectric stacks, that will divide light in the visible range ofthe spectrum into two components having substantially the same spectral distribution.Preferably a circular polariser 11 of appropriate handedness is placed between the sources 6 and 7 and the film 2 to remove, from the light incident upon the film from those sources, that component which is not selectively reflected when its wavelength matches the pitch ofthe cholestericmaterial of which the film is made. (This reference to wavelength refers to the wavelength in the material of the cholesteric.) On the other side ofthe membrane 1 ,film 3 faces a source 13 of monochromatic lightwhose light is spread substantially evenly over the surface of the fibre by an optical system represented by a lens 14.

Preferably a circular polariser 15 of appropriate handedness is placed between the source and the film to remove from the light incident upon film 3 that componentwhich is not selectively reflected when its wavelength matches the pitch ofthecholesteric material ofwhich the film is made.

The thermal imaging system operates cyclically with four distinct periods comprising temperature stabilisation ofthe sensor priorto exposure to the thermal scene, irradiation with an image ofthe thermal scene, enhancement of the resulting image by irradiation with visible light, and finally observation of the enhanced image.

The temperature stabilisation period will be discus sed later. During the irradiation period the cholesteric film is exposed to a thermal radiation pattern which produces differential heating across the surface of the film. The composition of the film may be such that its reflections wavelength shifts from red to blue with heating orofthe alternative kind in which shift is in the opposite direction, from blue to red. Taking the case of a film providing a shiftfrom blue to red, the slightly warmer areas will reflect green light and stand out against a blue background of the colder areas.During the optical feedback period exposure of the film to blue light from source 6 will leave the blue background areas substantially unchanged because they reflect this illumination, but the green areas will absorb the light, heat up, and change to colours of longer wavelength. The effect of the illumination is thus to provide a measure of positive optical feedback in which the cooler areas are left cool but the warmer areas are made yet warmer.It is not necessaryforthe illumination during the positive optical feedback enhancement period to be single coloured, and ifthe source is strong in blue, but has a steady diminution of optical energy towards the red, a wider range of enhanced colours can be achieved by virtue ofthe distinction it makes between the different colours presented priorto enhancement. Typicallythether- mal image irradiation period lasts about 1 OOms, while the enhancement period last a few tens of milliseconds. This is followed by the observation period which is typically somewhat longer and generally a few hundred milliseconds long.For this a substantiallywhite lightsource 7 is required in orderforthe developed colours to be distinguished, but the intensity is less than that used during the enhancement phase so that it shall not disruptthe enhancement. It may however, be adjusted in spectral distribution and strength so as to tend to compensate at least a part of the thermal decay.

Reverting attention to the initial temperature stabilisation period, a preferred way of achieving this stabilisation is by illumination with a substantially monochromatic source 13, and by making use of the selective reflection ofthe cholestericfilm 3. Light from this source is circularly polarised so thatthe light incident upon thefllm is selectively reflected when its wavelength is matched by the pitch ofthe cholesteric material of the film 3. At all other pitches the light is transmitted through the film to be absorbed by the blackened membrane 1. The reflection characteristics oftypical cholesteric films is shown in Figure 2.This plots peak reflected wavelength as a function of temperature forfour different mixtures of cholesterol nonanoate (CN) and cholesteryl chloride (CC). Curve 1 is the most typical of the type of behaviour required.

The aim is for the incident radiation from source 13 to heat film 3 until that radiation is selectively reflected.

Underthese conditions the membrane 1 no longer receives the full radiation and the temperature tends to stabilise at an equilibrium value. It is to be noted however, thatthis is a state of metastable equilibrium only, since an overshoot will tend to lead to runaway heating as, with highertemperatures, the membrane once again begins to receive the full radiation. The limits to stability and the accuracy of stabilisation can be estimated by considering the bandwidth and sensitivity of the reflected wavelengths. Selective reflection by a cholesteric film is not strictly monochromatic, but has a finite bandwidth related to the principal refractive indices and given for planar samples by the expression.

Bandwidth = 2A (nO- ne)/(nO+ ne).

Fortypical cholesteric mixtures the bandwidth is about one tenth ofthe reflected wavelength and thus is about 50nm forwavelengths in the near infra-red.

Narrow band pass filters or coherent sources are available with much narrower bandwidths, and hence it is not difficultto provide a narrow region of equilibrium using a level of illumination that ensures thattheequilibrium point is reached beforethe peak reflection wavelength, so that further heating ofthe film produces greater reflection, and hence stability of operation.Sensitive cholesterics move their reflection wavelength through the entire visible spectrum in about 0.5 C, providing a mean reflection wavelength coefficient of about 800nm"C-', and therefore with a monochromatic source stabilised to + 5nm, the line width of a typical interference filter, the temperature stabilisation is of the order of + 0.005 C. In practice the coefficient characterising the rate of change with temperature ofthe wavelength of peak reflection is itself a function of temperature as can be inferred from Figure 2, and hence improved stability can be achieved by choosing to operate at a frequency at which the coefficient is at or near a maximum. For low thermal massthethicknessofthefilm should be minimised, but a competing consideration is the need to provide high reflectivity which increases with film thickness. Reflectivity depends upon the birefringence ofthe cholesteric, and typically it is found that a 90% reflectance level is reached with film thicknesses in the range 10 to 25 microns.

Alternativelythethermal imagersystem may con sist of onethermochromic layerwhich is acted on in sequence by the stabilisation, irradiation, enhancement and viewing systems.

Claims (10)

1. Amethodofoperating athermal imaging system in which the thermal image is focussed upon a radiation absorbing membrane supporting a thermo ch romic cholestericfilm which by temperature dependent selective reflection in the visible range of the spectrum displays the image, which method of operation employs positive optical feedback, and includes the steps of stabilising the temperature of a region of the membrane, of irradiating it with a thermal image, of uniformly irradiating the region via the film with light at relatively higher energy density having a spectral distribution providing positive optical feedback, and of viewing the film with illumination of relatively lower energy density.

2. A method as claimed in claim 1, wherein light with which the cholesteric film is illuminated is circularly pqlansed light.

3. A method as claimed in claim 1 or 2, wherein said step of stabilising the temperature of the membrane is effected by uniformly irradiating it with substantially monochromatic light th rough a second thermochromiccholestericfilmsupported on the opposite side of the membrane.

4. A method as claimed in claim 3, wherein the second cholesteric film is irradiated with monochromatic lightthat is circularly polarised.

5. Amethodofoperating athermal imaging system which method is substantially as hereinbefore described with reference to the accompanying drawings.

6. Athermal imaging having an optical system for focussing a thermal image upon a radiation absorbing membranethatsupportsathermochromiccholesteric film, means for stabilising the temperature of the membrane at least overthearea of the focussed image, an optical source for illuminating the film with relatively higher intensity uniform light in the visible range ofthe spectrum having a spectral intensity distribution in relation to the spectral reflectivity of the film such as to provide positive optical feedback ofthe visible light image created in the film by differential heating ofthe membrane bythethermal image, and an optical source for illuminating the film with relatively lower intensity lightto enable the visible light image to be seen.

7. A system as claimed in claim 6, wherein the sources illuminate the cholestericfilm with circularly polarised light.

8. A system as claimed in claim 6 or 7, wherein the meansforstabilising thetemperature ofthe membrane consists at least in part of a substantially monochromatic light source directing energy to the absorbing membranethrough a secondthermochro mic cholestericfilm supported on the face of the membrane oppositethat supporting the first cholestericfilm.

9. Asystem as claimed in claim 8, wherein the monochromatic light source illuminates the second cholestericfilm with circularly polarised light.

10. Athermal imaging system substantially as hereinbefore described with reference to the accompanying drawings.