GB2125982A

GB2125982A - Thermal imaging system

Info

Publication number: GB2125982A
Application number: GB08223590A
Authority: GB
Inventors: Peter William Ross
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-08-17
Filing date: 1982-08-17
Publication date: 1984-03-14
Also published as: GB2125982B

Abstract

In a thermal imaging system in which the thermal image is focussed on a temperature stabilised membrane to produce a temperature distribution representative of the thermal image, temperature stabilisation is achieved at least in part by covering the membrane over the area of the focussed image with a thermochromic cholesteric film and heating the membrane by radiation directed through the film from a monochromatic light source.

Description

SPECIFICATION Thermal imaging system This invention relates to thermal imaging system and in particular to such systems in which the thermal image is focussed on a temperature stabilised membrane to produce a temperature distribution representative of the thermal image.

Image conversion means convert this temperature distribution into an optical effect which can be observed in the visible range of the spectrum.

A cholesteric material may be used for this purpose in the form of a film supported on the membrane. The optical properties of certain cholesteric materials are very sensitive to changes in temperature.

One way of making use of his temperature dependence of cholesterics is to adopt a thermochromic approach in which the pitch of the cholesteric is arranged to be such that it selectively reflects light in the visible range of the spectrum. With an appropriate cholesteric the wavelength peak of this reflection can move through the entire visible range of the spectrum with a temperature change of the order of 0.50C.

With a film thickness of about 1 5 microns this peak reflection is typically about 90%.

Alternatively a thinner film may be used and a longer pitch, in which case temperature differences produce a visible effect by altering the amount by which the plane of polarisation of light transmitted through the layer is rotated. The amount of this rotation is converted to a visible effect by directing plane polarised light through the film and viewing it through a polarisation analyser. Under these circumstances the supporting membrane needs to the opaque to the thermal radiation in order to be heated by it, but needs to be transparent in the visible range of the spectrum in order to enable the film to be viewed in transmission.

With the thermochromic approach the membrane likewise need to be opaque to the thermal radiation and can also be opaque in the visible. A construction of thermochromic thermal imaging system is described for instance in the specification of United States Patent No.

3527945. Typically such a device uses a cholesteric film about 1 5 microns thick coated on a polyester film about 6 microns thick which is blackened on the opposite side to provide efficient absorption of the thermal radiation incident upon it. Such a film is placed in a carefully designed enclosure and its temperature is accurately controlled. The sensitivity for such a device can be such as to provide for the detection of a temperature difference of 0.020K at 3000K with a resolution of 0.05 line pairs per mm.

The limiting factors governing the performance of these types of system are sensitivity of the cholesteric pitch length to changes in temperature, the accuracy of the thermostatic control of the temperature of the membrane, and thermal spreading effects. This invention is concerned with thermostatic control of the temperature of the membrane and employs a cholesteric film on the membrane to act in conjunction with a monochromatic light source to control the temperature by making use of the thermochromic selective reflection of such a layer. In a device in which the thermal image is developed with the aid of a thermochromic film it is possible under appropriate circumstances to use the same film for both functions, alternatively the membrane supports a first film on one side for image development and a second on the other for thermostatic control.

According to the present invention there is provided a thermal imaging system that includes an optical system for focussing a thermal image upon a thermal radiation absorbing membrane, and includes means for stabilising the temperature of the membrane at least over the area of the focussed image, which means consists at least in part of a substantially monochromatic light source directing energy absorbed by the membrane at the membrane through a thermochromic film supported by the membrane.

The invention also provides a method of operating a thermal imaging system in which the thermal image is focussed upon a radiation absorbing membrane, wherein temperature stabilisation of the membrane at least over the area of the focussed image is effected at least in part by directing substantially monochromatic light at the membrane through a thermochromic film supported by the membrane.

The application of the 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 (not shown) and carries thermochromic cholestric films 2 ms 2 and 3. In order to 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 face a wavelength reflective mirror 4 which allows imaging optics, represented in the figure by lens 5, to form a thermal image of the same under observation. Light from two sources 6 and 7 is directed via first and second beam splitters 8 and 9 and further optical system, represented by a lens 10, to allow illumination and inspection of the film 2 with light in the visible range of the spectrum. The mirror 4 may be provided by a slice of germanium, which will transmit light in the wavelength range of the thermal radiation but reflects 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 of the 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 of the cholesteric material of which the film is made. (This reference to wavelength refers to the wavelength in the material of the cholesteric).

On the other side of the membrane 1, film 3 faces a source 13 of monochromatic light whose light is spread substantially evenly over the surface of the fibre by an optical system represented by a lens 14. Preferably a circular polariser 1 5 of appropriate handedness is placed between the source and the film to remove from the light incident upon film 3 that component which is not selectively reflected when its wave length matches the pitch of the cholesteric material of which the film is made.

The thermal imaging system operates cyclically with four distinct periods comprising temperature stabilisation of the sensor prior to exposure to the thermal scene, irradiation with an image of the thermal scene, enhancement of the resulting image by irradiation with visible light, and finally observation of the enhanced image. If desired, the second period, the enhancement, may be omitted.

The temperature stabilisation period will be discussed 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 heating or of the alternative kind in which shift is in the opposite direction, from blue to red. Taking the case of a film providing a shift from blue to red, the slighly 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 necessary for the illumination during the positive optical feedback enhancement period to be single coloured, and if the 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 of the distinction it makes between the different colours presented prior to enhancement.Typically the thermal image irradiation period lasts about 100 ms, 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 substantially white light source 7 is required in order for the developed colours to be distinguished, but the intensity is less than that used during the enhancement phase so that it shall not disrupt the 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, this is achieved by illumination with a substantially monochromatic source 13, and by making use of the selective reflection of the cholesteric film 3. Light from this source is circularly polarised so that the light incident upon the film is selectively reflected when its wavelength is matched by the pitch of the 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 of typical cholesteric films is shown in Figure 2. This plots peak reflected wavelength as a function of temperature for four different mixtures of cholesterol nonanoate (CN) and choiestery chloride (CC).

Curve 1 is the most typical of the type of behaviour required. The aim is for the incident radiation from source 1 3 to heat film 3 until that radiation is selectively reflected. Under these conditions the membrane 1 no longer receives the full radiation and the temperature tends to stabilise at an equilibrium value. It is noted however, that this is a state of metastable equilibrium only since an overshoot will tend to lead to runaway heating as, with higher temperatures, 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=2(no-ne)Ano+ne).

For typical cholesteric mixtures the bandwidth is about one tenth of the reflected wavelength and thus is about 50 nm for wavelengths in the near infra-red. Narrow band pass filter or coherent sources are available with much narrower bandwidths, and hence it is not difficult to provide a narrow region of equilibrium using a level of illumination that ensures that the equilibrium point is reached before the peak reflection wavelength, so that further heating of the film produces greated reflection, and hence stability of operation. Sensitive cholesterics move their reflection wavelength through the entire visible spectrum in about 0.50C, providing a mean reflection wavelength coefficient of about 800 nm0C-1 and therefore with a monochromatic source stabilised to +5 nm, the line width of a typical interference filter, the temperature stabilisation is of the order of +0.0050 C. In practice the coefficient characterising the rate of change with temperature of the wavelength 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 mass the thickness of the film should be minimised, but a competing consideration is the need to provide high reflectivity which increases with film thickness. Reflectivity depends upon the birefringence of the cholesteric and typically it is found that a 90% reflectance is reached with film thicknesses in the range 10 to 25 microns.

Claims

1. A thermal imaging system that includes an optical system for focussing a thermal image upon a thermal radiation absorbing membrane, and includes means for stabilising the temperature of the membrane at least over the area of the focussed image, which means consists at least in part of a substantially monochromatic light source directing energy absorbed by the membrane at the membrane through a thermochromic film supported by the membrane.

2. A system as claimed in claim 1, wherein the emission of the monochromatic light source lies in the near infra-red.

3. A thermal imaging system as claimed in claim 1 or 2, wherein the membrane supports a second cholesteric film covering the area of the focussed image.

4. A thermal imaging system as claimed in claim 3, wherein the second cholesteric film is a thermochromic film whose reflection peak lies in the visible range of the spectrum at the temperature at which the reflection peak of the first is matched with the emission of the monochromatic light source.

5. A method of operating a thermal imaging system in which the thermal image is focussed upon a radiation absorbing membrane, wherein temperature stabilisation of the membrane at least over the area of the focussed image is effected at least in part by directing substantially monochromatic light at the membrane through a thermochromic film supported by the membrane.

6. A method as claimed in claim 5, wherein the light is in the near infra-red.