GB2163566A - Thermal imagers - Google Patents

Abstract

A free standing or non-enclosed liquid crystal 20 forms the sensitive element of a thermal imager. The liquid crystal is a chiral smetic C in which the pitch of the helix and therefore the optical activity is strongly dependent on temperature. The liquid crystal is preferably associated with a heat retaining body 22. To read-out information from the liquid crystal a polarized beam of light 31,32 is passed through the film several times. <IMAGE>

Description

SPECIFICATION Thermal imagers This invention relates to thermal imagers which use liquid crystals as the sensing elements.

It is known to use liquid crystal cells for thermal imaging. The crystals are contained by transparent plates which form the walls of the cell. In use, the liquid crystal is held at its cholestric-isotropic phase transition temperature. At this temperature the crystal has a large temperature coefficient of optical rotatory power. When infra-red radiation falls on the crystal held at this temperature, it alters the crystal temperature locally, producing changes in phase within the crystal, ie from cholesteric to isotropic. These changes can be measured using a scanning polarimeter which gives an output electronic signal which is proportional to the incident infra-red radiation.

However, the sensitivity of the liquid crystal cell is reduced due to the walls of the cell absorbing some of the incident radiation and locally heating the crystal. Problems with the flow of the liquid crystal may also occur due to the localised changes in phase caused by this local heating. Impurities on the internal walls of the cell may act as centres of nucleation, ie points onto which the crystals may attach themselves and produce undesirable molecular orientation effects.

Free-standing chiral smetic C liquid crystal films, having thicknesses between 10 to 100 molecules (500-5,000 A), are known to exhibit a strong temperature dependence in which the pitch of the helix alters with temperature. The term "free-standing" is applied to a liquid crystal film and is used to define those liquid crystal materials which have sufficient surface tension to form a film without the necessity of containment plates.

According to one aspect of the invention, there is provided a thermal imager in which the sensitive element includes a free-standing liquid crystal film.

Naturally, the film has to be supported and this may be affected by an apertured plate or frame.

The thermal imager also includes means for directing the incident radiation onto the film, and readout means for reading out the image formed on the film.

Preferably, the readout means is a polarised light system, but depending on the type of liquid crystal film used a capacitive technique may be used to give a direct readout.

If the image area needs to be large, a reticulated structure may be used.

According to a second aspect of the invention, there is provided an imaging systems in which the sensitive element includes a liquid crystal film and heat retention means positioned adjacent one surface of the film and operable for retaining radiation incident on the film for a given time period.

Advantageously, the film is mounted inside a hole formed in the retention means.

According to a third aspect of the invention, there is provided an imaging system including: A liquid crystal film; readout means which includes a polarised beam projected through the film, the change in properties of said beam being related to incident radiation; and enhancement means for enabling said beam to pass through the film several times to enhance the change in properties of said beam.

Preferably, the enhancement means comprises first and second members which are at least partially reflective, one member being mounted on each side of the film.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic diagram of a chiral smectic C liquid crystal; Figure 2 is a schematic diagram of a thermal imager comprising a free-standing smectic liquidwcrystal film mounted on its support member; Figure 3 is a schematic diagram showing a reticulated smectic liquid crystal imager; Figure 4 is a schematic diagram of a thermal imager comprising a free-standing smectic liquid crystal film mounted in an insulating block; and Figure 5 is a schematic diagram of the fig.

4 imager mounted in a Fabry-Perot resonant cavity.

The smectic structure of a liquid crystal is divided into two groups: structure A and structure C. In both groups, the molecules are long and thin, and form layers with their long axes across the layers. The smectic A structure has the molecules aligned with their axes normal to the layers, whereas the smectic C structure has the molecules tilted away from the layer normal at some tilt angle, but in both cases the molecules within a layer lie parallel to one another. The molecular forces between lyers are relatively small but the forces with each layer can be quite large.

If an optically active compound or similar dopant is added to smectic C structure, the liquid crystal structure is modified to give chiral smectic C structure in which the molecules are still parallel to each other in each layer, but in passing from one layer to the next, the molecular axis is rotated or twisted.

In Fig. 1, a liquid crystal 1 having the chiral smectic C structure is shown. The crystal 1 comprises several molecular layers (only seven of which, numbered 2 to 8, are shown). The molecules in each layer are shown as short lines-these lines also represent the molecular orientation within the layers. If a column, such as 9, is taken through the crystal structure, a 'helix' 10 can be seen which is made up from the rotation of the molecular orientation when passing from one layer to another-a characteristic of this liquid crystal structure. The pitch of the helix 10 is strongly temperature dependent and will change when the temperature is altered due to the rotation of the molecules in the helix as they absorb energy.This change in the pitch of the helix changes the optical activity of the crystal 1, and this can be monitored to form an optical image corresponding to the thermal image formed on the crystal.

The thermal imager shown in Fig. 2 comprises a support member 11 having a hole 12 formed in it. A suitable smectic liquid crystal film 13 (of the sort described above) is stretched over the hole 12. The size of the hole 12 is chosen that the surface tension of the film 13 is large enough to allow the film to be stretched over the hole in this manner.

The support member 11 and the film 13 are maintained at a reference temperature T, and polarised light 14 is passed through the film 13. The light 15 leaving the film 13 has different optical properties due to the optical effect of the liquid crystal film 13 on the incident polarised light 14 at the reference temperature T. When an infra-red image is impressed onto the film 13, the molecules in the film rotate altering the pitch of each helix (as mentioned previously) These changes in the crystal modulate the incident polarised light 14 to give a visual image corresponding to the thermal or infra-red image impressed on the film 13.

The imager shown is thought to work only while the liquid crystal film stays in its appropriate smectic phase and this may limit the scene temperature range which can be imaged in this way.

The largest size of film area over which a thermal image may be impressed (having the required thickness) is determined by surface tension effects as the film is pulled across the hole in its isotropic state. A reticulated structure 16 comprising several liquid crystal films 17, as shown in Fig. 3, may be used to provide a larger film area than is possible with one aperture or hole. The effects of reticulation can be removed during the optical processing of the image.

However thermal imagers of this type are required to have a response time which is less than 50 ms and a sensitivity which enabies temperature changes of iess than 1K in the scene to be detected. In order to meet the required response time, a film thickness of 0.1 ,um is needed. This however produces the problem that the change in the read beam may not be large enough to be detectable after it has passed through the film. This may be overcome by the imager shown in Fig. 4.

The imager shown in Fig. 4 comprises a smectic liquid crystal film 20 mounted in a hole 21 formed in a block 22 of insulating material. The hole 21 is between 6 to 10 ,um deep ie the film is 60 to 100 times thicker than that for a free-standing film relying on surface tension effect, and retains the film 20 in a preferred molecular orientation. The insulating block 22 is transparent to radiation in the visible portion of the electromagnetic spectrum, thereby allowing a read beam (not shown) of polarised light from a helium-neon laser to pass through it, and has the purpose of retaining incident infra-red radiation so that the properties of the film can change in response to the incident radiation.The read beam is plane-polarised and emerges from the film 20 elliptically-polarised in response to the temperature change sensesd by the film 20.

However, this arrangement again has the disadvantage that the read beam may not change sufficiently to enable the temperature change due to the incident radiation to be detected as the beam only passes through the film once.

In order to overcome this, the imager may be mounted in Fabry-Perot resonant cavity as shown- in Fig. 5.

The resonant cavity shown in Fig 5 comprises a apair of substrates 23, 24 on which are deposited dielectric mirrors 25, 26 respectively. The substrates 23, 24 are required to transmit both infra-red and visible radiation, and if required each one may be made of a different material to the other. The mirrors 25, 26 are partially-reflective and partiallytransmissive having a reflectivity, R, of about 94-95%. A thermal imager 27 (as described previously) is mounted in the cavity between the mirrors 25, 26 adjacent a conpensator 28 which is in contact with the mirror 26. The compensator 28 nulls out the natural birefringence of the film 20 and the insulator 22 when the imager is in the OFF state ie a dark background is obtained if the cavity is detuned or a light background if the cavity is turned under ambient conditions.

An iris 29 is positioned in front of the imager 27 so that the read beam only passes through the active region ie the film 20 of the imager.

In operation, the incident infra-red radiation 30 and the read beam 31 are both incident on the substrate 23 and pass through it and the mirror 25 onto the film 20. The infra-red beam 30 passes into the insulator 22 and exits the cavity via compensator 28, mirror 26 and substrate 24. The read beam 31 also passes through the film 20, insulator 22 and compensator 28, but is partially reflected at the mirror 26 and is directed back through the imager 27 to mirror 25 where it is again partially reflected. The beam 31 passes across the cavity many times but only two reflections are shown in the Figure for clarity. The nonreflected parts of the beam 31 ie the transmitted portions 32, pass through the mirror 26 and substrate 24 and are summed to produce a representation of the image which is stored on the film 20 at that time.As mentione previously, in the OFF state, the background is either dark or light depending on whether the cavifty is detuned or tuned.

The image stored on the film 20, then acts either to tune or detune the cavity ie the representation containing the image is either a dark background containing a light image or a light background containing a dark image.

The polarisation changes observed are from linear to elliptical and back again ie the film 20 changes the plane-polarised beam into an elliptically polarised beam when the beam is passing from left to right and vice versa.

The substrates 23, 24 may be made of a material IRTRAN-2 and the dielectric mirrors 25, 26 may be made from zinc sulphide and cryolite deposited on the substrates.

The enhancement of the read beam 31, which is obtained is related to the mirror reflectivity R ie the enhancement is (1 +R)/(1 R). However, this enhancement has to be offset against the amount of radiation absorbed in the substrate 23 and mirror 25.

The response time of the imager 27 when mounted in a Fabry-Perot resonant cavity, depends on the thickness of the insulator 22 behind the film 20 and the heat sinking properties of the compensator 28.

Ferroelectricity (or other smectic effects) may also be used to sense changes in temperature and naturally an appropriate readout system would be used to obtain the image.

Claims (13)

1. A thermal imager in which the sensitive element includes a free-standing liquid crystal film.

2. A thermal imager according to claim 1, wherein the film is supported at its edges by a frame.

3. A thermal imager according to claim 1, wherein the film is supported at its edges by an apertured plate.

4. A thermal imager according to claim 1, wherein the film is supported in a reticulated structure to provide a large image area.

5. A thermal imager according to any preceding claim, including means for directing incident radiation onto the film and readout means for reading out the image formed on the film.

6. A thermal imager according to claim 5, wherein the readout means is a polarised light system.

7. A thermal imager according to claim 5, wherein the readout means uses a capacitive technique to give a direct readout.

8. An imaging system in which the sensi tive element includes a liquid crystal film and heat retention means positioned adjacent one surface of the film and operable for retaining radiation incident on the film for a given time period.

9. An imaging system according to claim 8, wherein the film is mounted inside a hole formed in the retention means.

10. An imaging system including: a liquid crystal film; readout means which includes a polarised beam projected through the film, the change in properties of said beam being related to incident radiation; and enhancement means for enabling said beam to pass through the film several times to enhance the change in properties of said beam.

11. An imaging system according to claim 10, wherein the enhancement means comprises first and second members which are at least partially reflective, one member mounted on each side of the film.

12. A thermal imager substantially as hereinbefore described with reference to the figs.

2 and 3 of the accompanying drawings.

13. An imaging system substantially as hereinbefore described with reference to the figs. 4 and 5 of the accompanying drawings.