1. Introduction

Electromagnetic waves emitted from the objects having the temperatures greater than absolute zero are called thermal radiation. Thermal radiation has a number of advantages over visible detection methods, since it may be possible to image through the fog, smoke, rain, and at night [1]. Infrared (IR) thermal imaging, also often briefly called thermography, is a very rapidly evolving field which has been found to be extremely useful in scientific research, as well as in industry and in clinical applications [2–4]. Thermal imaging is a technique that converts the invisible IR radiation pattern of an object into visible images. There are many types of thermal image-forming systems which utilize various physical properties to convert an IR radiation image into a visual one. For instance, the evaporograph is a thermal imaging device which converts an IR image into a visible image by differential evaporation or condensation of oil on a thin membrane [5]. The ferroelectric detectors use a ferroelectric phase transition in particular dielectric materials. At this phase transition, small variations in temperature cause large changes in electrical polarization. Barium strontium titanate is used as the material in ferroelectric detectors. A specific type of resistor consisting of vanadium oxide and amorphous silicon is used in microbolometer. The electrical resistance of a material is altered by IR radiations, which can be converted to electrical signals and processed into an image [6]. Nevertheless, the thermal imaging devices have many drawbacks. For example, the use of semiconductors in thermal imaging systems requires cooling to maintain their results. Most of the detectors having cooling operation can operate from 60°k to 100°k range. In addition, such type of sensing equipment is not only bulky compatibly expensive due to additional hardware, but they are also considered more energy consumption and costly [7]. The bolometer is a thermal infrared detector which employs an electrical resistance thermometer to measure the temperature of the radiation absorber. Bolometers are complicated and costly devices because they combine various elements in various ways. In some bolometers, a single element is used for several functions. In composite bolometers, these functions are accomplished by separate elements so that they can be optimized independently [8]. Evaporograph has a low resolution (10 L/mm) and slow reaction time. Liquid crystal thermography has received great attention due to its capability to quickly reconstruct temperature fields in surfaces and volumes. Liquid crystals (LCs) are organic substances which behave, from a mechanical point of view, as liquids while they simultaneously exhibit optical properties similar to crystalline materials. Among different types of liquid crystal structures, the cholesteric liquid crystals (CLCs) exhibit optical properties sensitive to temperature variations. The presence of a supramolecular chiral structure leads to a periodical modulation of the refractive index inside the CLC and it behaves as a one-dimensional photonic crystal. CLCs exhibit a selective reflection of light, with the presence of the typical photonic bandgap in their reflection spectra that is the effect on which many of their applications are based. Since the supramolecular helical structure is temperature sensitive, liquid crystal substances are known to shift the band gap position when subjected to temperature changes. For instance, thin coatings of thermochromic CLCs at surfaces are utilized to obtain detailed heat transfer data of the steady or transient process [9–13].

In many CLCs the transition between the cholesteric and isotropic phases occurs through a cascade of weakly first-order transitions to intermediate structures, known as “blue phases” (BPs) and consisting of self-assembled disclination networks. The first set of differential scanning calorimetry studies that proved the stability of the blue phases were able to capture a very small but finite change in density at the transition point [14–16]. BPs which are believed to consist of double-twist cylinders are classified into three categories, depending on the cylinders' packing structure: blue phase I (BPI), blue phase II (BPII) and blue phase III (BPIII). BP I has a body-centered unity cell formed of double-twist cylinders, BP II is a simple cubic, and BP III is a foggy phase whose structure is considered as an amorphous network of disclinations. Due to their exotic structures BPs have attracted attention in the field of photonics and optoelectronics and possesses a several advantageous properties for photonic applications, such as ultra-fast switching speeds, existence of 3D photonic band gap manifesting in Bragg reflections related to cubic lattice structure, no need of alignment layers in display devices, thermal and electrical laser tuning, conversion of one blue phase to another upon exposure to UV light [17–23]. The most noticeable characteristic of BPs is that it demonstrates the selective reflection of incident light. Therefore, BPs are particularly attractive for display and photonics applications where fast response time is needed [24,25]. However, due to the narrow range of temperature where the BP exists, it is still far from being realized in practical applications [26].

In this work, we demonstrate for the first time that the “weakness” of BPs related to the narrow temperature range of the phases existence, can be beneficially used for the fabrication of novel type of thermal imaging devices, granted with distinguished properties, such as real-time room temperature operation capabilities. Furthermore, we propose a strategy to synergistically combine the attributes of weak first-order phase transitions between BPI and BPII to the brilliant colors of BPI and BPII caused by the selective reflections.

2. Sample preparation and experimental description

CLC mixture was prepared mixing the chiral dopant CB15 and the nematic liquid crystal BL-036 (both from Merck), with the following concentration ratio: 53 wt% CB-15 + 47 wt% BL-036. Prepared mixture was stirred in the isotropic phase at ~90 °C for ~10 minutes to make the constituents uniformly mixed. A hot stage with 0.05°C accuracy was used to control the cell temperature. The reflection spectra were collected by using an optical fiber coupled spectrometer (Avantes Ava-Spec.AVS-2048-2) having 1 nm resolution. To show the CLC and BPI and BPII phases and the transitions between them, we took the images of the samples under a digital microscope. As a thermosensitive receiver, we fabricated a flexible thin film, which is made up of three layers: a black carbon paper-based substrate with 20 μm thickness, a liquid crystal layer with 3-4 μm thickness and an upper polymer sheet with 2.6 μm thickness. The paper substrate is suitably chosen in order to prevent the leaking of the liquid crystalline material through it and to align the BP phases uniformly to grant a perfect reflectivity of the final device. The carbon paper is water- and oil-resistant and black. In addition, the pit-like morphology of the surface of this substrate acts as a quasi-encapsulating system which holds and stabilizes the BP mixtures. As a polymer sheet, we used a teflon film which has outstanding properties such as resistance to many chemicals, to UV irradiation and to extreme temperatures, low coefficient of friction, low wettability, and excellent optical properties. Once LC droplets are sandwiched between the paper substrate and the polymer sheet, a three-layer system is evenly and gently pressed to obtain a homogeneous 3-4μm thick layer [9]. The prepared thermosensitive film was mounted in a hot stage and the reflection spectra of BPI and BPII phases were recorded using a spectrometer. The selective reflection peaks are shown in Fig. 1(a), have symmetrical shapes and high reflectivity, very close to the maximum reflectance of perfect BPs structures. The thermosensitive film in the CLC phase at room temperature is shown in Fig. 1(b).

 

Fig. 1 Reflection spectra of the BPI and BPII phases of the mixture 53 wt% CB-15 + 47 wt% BL-036, (a). The curve of BPI is stabilized at 26.1 °C, and a curve of BPII is stabilized at 26.2 °C. The thermosensitive film in CLC phase, at room temperature (b).

Download Full Size | PDF

3. Results and discussions

To investigate the temperature-dependent phase transitions between BPs, and the tuning of selective reflection wavelengths, at first, the reflection spectra of CLC, BPI, and BPII phases were measured as the functions of temperature. In cooling, the prepared mixture has the following phase sequence: isotropic, BPII, BPI, and cholesteric. After that, a thermosensitive film embedded in the hot stage was placed under an optical microscope coupled to a spectrometer. Using a halogen lamp equipped with an IR band-pass filter and with a continuously variable iris diaphragm, a film was exposed with a spatially modulated thermal irradiation. The selective reflection wavelengths over the temperature ranges for which these phases are stable are shown in Fig. 2(a), and the optical response of thermosensitive pixels upon thermal irradiation is shown in Fig. 2(b). Red pixels correspond to BPI phase and the green pixels correspond to BPII phase. The red pixels display an optical response upon the relatively low thermal intensity emitted from the object, while the distribution of green pixels shows the optical response upon the higher thermal intensity emitted from the object, as shown in Visualization 1.

 

Fig. 2 Temperature-dependent selective reflection spectra and the phase transitions between BPI, BPII and CLC phases for the 53 wt% CB-15 + 47 wt% BL-036 mixture, (a). A thermal field controlled non-hysteretic reversible phase transitions between BPI and BPII phases lead to the reversible interconversions of selective reflections of BPI and BPII phases, which are seen as the bright red and green pixels respectively, (b). See Visualization 1.

Download Full Size | PDF

To produce a thermal image on the thermosensitive film, we built a setup shown in Fig. 3. The thermal irradiation emitted from the object is collected and focused on the thermosensitive film embedded inside the hot stage, located in the focal plane. To visualize the image projected on the film, an array of light emitting diodes was used. The obtained image is displayed by a CCD camera coupled to the computer.

 

Fig. 3 Schematic of the experimental setup. object emitting thermal radiation (1), reflective mirror (2), hot stage (3), an array of LEDs (4), CCD camera (5).

Download Full Size | PDF

Note that the proposed thermal device belongs to passive thermography, which enables to visualize the objects, having a higher or lower temperature than the background. For this purpose, we selected different objects with different surface/background temperatures ratio and exposed the thermosensitive film. Therewith, the ambient temperature where experiments were carried out was permanent, equal to 23-25°C, and free from any secondary infrared sources. In Fig. 4, are displayed thermal images of the objects having different temperatures. In particular, Fig. 4(a), demonstrates a thermal image of a tungsten light bulb filament, whereas in Fig. 4(b), shown a fragment of medical instrument cooled down to −2 °C.

 

Fig. 4 Thermal images of a filament of tungsten light bulb (a), and a medical instrument (b)

Download Full Size | PDF

The most amazing result we have obtained is the visualization of the temperature distribution on the surface of a human hand palm with the next evaluation of obtained thermal images over time, (Fig. 5).

 

Fig. 5 The visualization of the thermal image of a human hand palm and the evaluation of the temperature distribution on the surface over time. Altered colors correspond to the different temperatures on the palm.

Download Full Size | PDF

It should be noted that further work on the development of a resolution quality is necessary to respond to the growing demand for the modern thermal imaging systems. In this respect, we are going to replace the hot stage by a vacuum chamber to eliminate the temperature gradient caused by the thermal turbulence of air.

4. Conclusions

In this paper, we fabricated and described a thermal imaging apparatus, which combines the attributes of weak first-order phase transitions between BPI and BPII to the brilliant colors of BPI and BPII caused by the selective reflections. The presented thermal imaging device offers several advantages over traditional thermal imaging systems. First and foremost, it does not require the cooling systems and can operate at room temperature. Furthermore, due to the high spatial resolution, small size, lighter weight, low cost, and real-time operation capabilities the proposed device can be used successfully in many different fields, ranging from medicine to the artworks inspection.