Warm 3D images using thermal displays

Stereoscopic or 3D displays—such as stereoscopic LED displays with polarized glasses or multi-layer 3D displays—allow a viewer to perceive the depth of an image to provide a richer viewing experience. Much research has been dedicated to determining how to offer blind people, or individuals without stereoscopic vision, the opportunity to experience this 3D effect. A possible solution is to use the sense of heat: employ three-dimensionally localized heat spots that provide a thermal sensation when an individual walks through the 3D image.

Our research aimed to realize thermal and visual aerial signage, a technique that forms a 3D pattern composed of heat and light in the air, without any physical hardware at the position of the sign. These warm 3D images can be used, for example, to display advertisements alongside a footpath or side walk. In addition to seeing these signs, users can feel them when their faces touch the thermal images.

A reflective optical device can focus far-IR radiation from a heat source, which provides warmth as well as visible light. Crossed mirrors, sometimes called lobster's eye imaging devices, are widely used in x-ray optics.¹Unlike conventional mirrors that form a virtual image, crossed mirrors form a real image in that they converge light to image points, which can be used for 3D image formation.² However, to maintain a precise shape, the devices should require substrates that absorb far-IR radiation. This absorptive quality renders such substrates unsuitable for thermal signage applications. To overcome this limitation, we designed and fabricated a crossed-mirror array (CMA) for aerial imaging of an LED sign with a smoothing function between the LED lamps.^{3, 4} Our CMA is composed of hollow apertures surrounded by mirrors without substrates. We used the CMA to form thermal and visual 3D images.⁵

Figure 1. Image formation with the crossed-mirror array (CMA), illustrated using third-angle projection. The plan view shows light rays that emerge from the left point are reflected twice in crossed mirrors and converge into one point next to the light source position. The side and the front views show the converging position is the plane-symmetrical position of the light source about the CMA.

Figure 1 shows the composition and imaging principle of our CMA. The array is composed of comb-shaped stainless steel mirrors that perpendicularly intersect one another. After the incident rays are double-reflected, they converge into the image position because each reflection surface acts as a dihedral roof mirror. Every ray emitted from a light source converges to the position of the plane of symmetry of the source on the CMA plane, forming the aerial image in this way.

Figure 2. Experimental setup to observe convergence of heat originating from a soldering iron. D: Distance to a screen used to measure temperature increase at the image position.

To confirm that three-dimensionally localized hot spots are formed by the CMA, we conducted experiments with our device at room temperature (∼23°C). Figure 2 shows the experimental setup for a thermal 3D display. We used a CMA made of stainless steel mirrors (1mm in thickness and 8mm in height) composed of 4×4mm square apertures to form the heat image of a soldering iron. We verified that temperature increased locally at the image position of the heat source (see Figure 3).

Figure 3. (a) Temperature distribution versus distance from the CMA. The peak temperature is obtained at the image position of the heat source. (b) Thermal camera image of a screen placed on the image position.

To demonstrate a thermal and visual 3D display, we fabricated a large (40×40cm) CMA and placed a halogen heater stove next to an LED sign. Using a screen located at the aerial image of the LED sign ‘L’, we observed the aerial images of ‘L’ and of the stove: see Figure 4(a). We detected heating by replacing the screen with a thermo-chromic screen: see Figure 4(b). Although there was no change in the aerial LED sign, temperature on the image of the heater instantly increased: Figure 4(c)–(f).

Figure 4. Heating of the 3D image of a halogen heater stove is observed using a thermo-chromic sheet. The orange line in the center is the aerial image of the heater. (a) A screen is placed on the aerial image of the LED sign ‘L’ and the stove. (b), (c) The screen is replaced by a thermo-chromic sheet at the same position. (d), (e), (f) Temperature increases on the image of the stove as time goes by.

In summary, we successfully realized aerial imaging of heat and light. The proposed technique improves the attractiveness of aerial signage by stimulating the user by both vision and haptics. In the future, we will investigate the perception of aerial thermal images and optimize heat images for real-world applications.