Keywords

1 Introduction

Video projection on glass or other transparent materials is a key piece of technology for augmented reality applications. By overlaying the real view with the projected image, we can add information related to the real world. Imagine projecting navigation arrows on the front glass of a car. With additional sensors, we can overlay the navigation markers on the physical road. Such a solution is inherently safer and more convenient than having to switch our focus between a traditional LCD panel and a front car window. Similar to the car window, the actual glass panel is usually not flat but curved due to aerodynamic or ergonomic demands. Even more problematic is a video projection on hand-blown glass with the curved surface in more than one direction. We have to consider the geometry of the projection surface to achieve non-deformed projections from the point of the viewer. Furthermore, we have to deal with the light distribution as we want to achieve uniform brightness. Another problem that can arise with projection on transparent surfaces is reflection. We have to plan the placement of the video projector to avoid the glare affecting the viewer. Additionally, we have to reckon with reflections inside the material itself.

However, to project on a glass one needs to treat the surface so it can reflect the projected light. There are numerous solutions ranging from projection films, special materials inside the glass to special light sources. The goal is to let most of the visible light pass through but at the same time be able to project on discrete surfaces. That means we need the material to be clear for the human eye but at the same time opaque for the video projection. Most commonly available projection films sacrifice a bit of transparency and also video projection gain to achieve this. Generally speaking, more opaque the projection film is, the better gain can be achieved but worse transparency.

Our solution keeps the original transparency of the material and at the same, we can selectively emit light directly from the surface. We achieved this using a special light source - LED in UVA range combined with a special clear UV fluorescence varnish. As the projected UV light only excites the surface, the actual light that the viewer sees emanates directly from the surface where the UV is transformed into a visible light spectrum. Our projection system can be applied by spray painting, which is way more convenient to apply than projection films. It can also be easily applied in the case where projection films could not be used such as complex organic shapes. Another major benefit is maintaining the original material transparency.

2 State of the Art

We could use an active transparent LCD for video projection. Transparent LCD displays can achieve 80% light transmission [8]. However, for curved or large-scale surfaces this approach is not feasible. For such use-cases, video projection is a preferred solution.

There are numerous methods for projecting video onto clear glass. One of the solutions might be PDLC [3] also known as a smart glass film. PDLC consists of a sheet of liquid crystals that are randomly oriented and opaque. When the high voltage is applied, PDLC crystals align and turn the surface transparent. Unfortunately, commercially produced PDLC films have numerous disadvantages. It cannot accommodate organic shapes, it cannot be dimmed selectively, it is expensive and requires high voltage generators. However, it also gives good light gain.

Closer to our proposed solution are various semitransparent video projection films that are applied onto glass [10]. There are numerous technologies used, the most commonly available use reflective nanoparticles dispersed inside the film. The more particles are inside the film, the more gain it gives, but it also reduces the transparency. A disadvantage of such films is that they have to be applied on flat or curved surfaces in one direction only. It might be impractical to apply a film on a surface that is curved in more than one direction. Such films are typically cheaper than PDLC, but they suffer from worse gain compared to PDLC.

Our proposed solution is similar to the one developed by T.X. and Cheng, B [9]. In their paper, they discuss the system of three UV fluorescent films merged in a thin sheet and applied onto a glass and excited by a custom-made UV light source. However, the materials used are not described in the paper and thus it is not possible to reproduce the results. Moreover, the UV fluorescence is achieved using a film, so it is limited to flat glass applications.

Unlike T.X. and Cheng, B. solution, our system is monochrome only, but this could be extended by using different wavelength dyes with specific light sources. We also discuss DIY light source modification and use cases. Fluorescent dyes are widely available, can be mixed with different varnishes depending on your use case. Overall, the solution is cheaper compared to using films or PDLC.

3 Use Case

We intend to use the coating to achieve a dynamic light design using hand-blown glass. The final product is a chandelier consisting of multiple glass components. Such glass components are typically not flat, but they have volume and varying shapes. To project video on components, sandblasting was previously required. UV coating preserves the transparency of glass but enables video projection on the surface. Thanks to curves in more than one direction, it is not possible to use traditional projection films glued on the surface. On top of that, each component is original and scaling would also be an issue. If we would include the UV pigment inside the molten glass directly, we would lose the transparency as we have verified in a separate experiment. That is why our coating is the best solution for a given use case.

Another proposed use case is a volumetric 3D display (see Fig. 4). We have created a prototype consisting of a grid of glass tubes hanged from the ceiling. Tubes are arranged in a way to not overlap from the point of view of the video projector. Thanks to calibration using camera video projector pair [7] we matched the 3D model of the display with the video projector point of view. Therefore, we can render virtual 3D objects - when the virtual objects collide with the tubes in the display, the intersection is highlighted. By using clear glass with applied UV varnish solution, we can further improve the overall transparency of the display. See video documentation of the proposed physical 3D display prototype [6].

There are many art installations in which the semi-transparent layered surface is used for video projection. Such as a piece for Laterna Magika exhibition by Michael Bielicky [2] in Meetfactory gallery [11]. Bielicky uses flexible plastic strips hanging from the ceiling as a projection surface. Unlike the proposed 3D volumetric display, it does not work with depth or display discrete 3D images. Rather the video image is broken by the projection surfaces. Proposed UV coating enables artists to work in the third dimension while not losing transparency.

Moreover, the transparent display is also usable for augmented reality use cases. Another setup we will test the UV projection on is a glass cylinder, 3 m in diameter, where the viewer can stand inside (see Fig. 5). Using video projection, we can enable the viewer to see through the glass and display additional information on the glass surface. Such a display can be also useful for air traffic control or car HUD.

4 Projection System

Fig. 1.
figure 1

Left: light source, Right: reflected light from the UV coating on clear glass.

Full size image

4.1 UV Dye Varnish Applied to Glass

We have mixed Aragurad 109 UV dye made by Aralon with VA 177 TH epoxy varnish by ElChemco originally designed to protect PCB. The coating is first mixed as 45% UV dye and 55% varnish. The solution is then diluted to 10% dye and 90% varnish. We have manually mixed the solution using a wooden stick for about 30 s until the dye is uniformly distributed in the varnish. The solution can be spray painted using a regular paint gun or applied directly with a brush. The coating can also be applied selectively to paint a concrete motive on the glass (see Fig. 2) or applied to the whole surface and mask the light source or use video projection to create dynamic content. UV light 365 nm is converted into mainly blue light 445 nm. The optimal wavelength for the highest luminescence of the dye was measured at 320 nm, but we had available a 365 nm light source only for the prototype. Therefore, we anticipate that the performance of the varnish can be further improved by using an optimal light source. By using different dyes we can achieve different colors as well. More readily available fluorescence compounds can be also used. Such as quinine diluted in the appropriate varnish solution.

Fig. 2.
figure 2

Left: 3 mm clear glass with motive painted with UV coating under ambient light, Middle: same as left but in dark, Right: overhead projector with masked output. Text behind glass to demonstrate transparency.

Full size image
Fig. 3.
figure 3

Left: modified overhead projector with UV LED, Middle: overhead projector optics (Fresnel lens and adjustable mirror), Right: electrical schema.

Full size image
Fig. 4.
figure 4

3D glass tube display prototype with one video projector as a light source.

Full size image
Fig. 5.
figure 5

Left: modular glass cylinder for video projection. Right: car front window video projection illustration

Full size image

4.2 Safety

Powerful UVA light sources such as used LED can cause skin aging and eye damage when exposed for prolonged periods of time [1]. To remedy this, we suggest using UV blocking varnish applied on the other side of the glass to ensure no or minimum of UV light passing through. However, even without it much of the UV light is turned into visible light thanks to the proposed UV coating. In any case, the light source should be placed in a manner that it will not spot directly on the person. For example, it could point from above at an angle to the glass surface or be used as a back projection. Light spill outside the targeted area should be minimized by masking the light source.

4.3 UV Light Source

We are using readily available SMD LED (10 * 10 grid) UV 365 nm 100 W by OTdiode driving it at 34 V 3000 mA. We have measured the light characteristics using a spectrometer (see Fig. 1 left) Lighting passport by Asensetech with 8 nm optical resolution. The LED is fitted inside an overhead projector (see Fig. 3). We have removed the original light source and replaced it with our LED, a heatsink with an active cooler rated for 180 W (Sunon LM310-001A99DN), an appropriate 100 W LED driver and multirotary potentiometer for dimming. The overhead projector also includes a large Fresnel lens, an optical lens, and a mirror that direct the image to the screen. Such a system can be used to apply various masks that can be created from paper or by painting with a black marker onto a cellophane sheet. We have chosen an overhead projector for the simplicity of the light source modification. Have a look at the dynamic projection of bubbles inside water placed over the projector [5].

Other methods that can be used are modifying or creating a custom DLP projector. The LED projector is not suitable as the LED display typically also includes a UV filter that would block the light. Even if you remove a UV filter, the LED display will suffer from UV light exposure and eventually degrade. Therefore, DLP technology is much more suitable. We are currently working on modifying the DLP projector by replacing the projector lamp. Sensors detecting the open cover have to be bypassed and it is also necessary to simulate the circuit communication with the video projector motherboard to bypass the lamp manufacturer detection. You could also construct your own DLP projector to avoid these issues.

Another option is to use a moving headlight with gobo wheels. Gobos are circular stencils made from metal or glass mounted on light optics that can rotate. With such a light source you can selectively cover a 360\(^{\circ }\) view and with custom gobo wheels you can project images or text. By using two gobo wheels rotating in opposite directions you can achieve effects similar to an animated fire or water surface. Such projectors are already commercially available [4]. Motorized light barn doors can be used as well.

5 Future Research

We will focus on measuring the relative gain of the fluorescence varnish we have created. We will also try to come up with a formula for a suitable UV dye. The next step is to create an actual prototype for each use case mentioned, such as augmented reality, hand-blown glass design chandelier, and a physical 3D display consisting of multiple Plexiglas sheets. We also want to switch from using an overhead projector to a custom-made DLP video projector.

6 Conclusion

We have developed a new projection system for clear glass or other transparent materials. The main benefit is that the coating can be applied in the form of spray paint and thus allow for non-flat video projection surfaces. Another advantage is that the light source is nearly invisible (especially when used with a UV pass-through filter) and the light appears to emit from the projected surface. Different UV dyes and varnishes can be used to adapt to a particular use case. The proposed solution is also cheaper compared to PDLC or projection films. We have measured the light characteristics of the coating and documented our application.