Development of an Augmented Reality system using ARToolKit

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Development of an Augmented Reality system using ARToolKit and user invisible markers 4.1.1 Ink Pen Now will be described in more detail the properties of the before mentioned ink. The ink is invisible to ultraviolet light and the human eye. Ultraviolet light is below 400nm. The human eye can see light between 400nm and 750nm. The ink emits 840nm frequency light and has a 793nm absorption frequency, which lies in the IR range. There exist three classifications for IR light, near-IR, mid-IR and far-IR. This classification is based on ascending light frequencies. The ink fits into the near-IR spectrum, 750nm to 3000nm. The light spectrum is graphically displayed in Figure 4.4. Figure 4.4 The light spectrum The ink itself is delivered in the form of an ink pen, the so called IR1PenSM. The tip of the pen is green fluorescent to the human eye. The manufacturer of the ink notifies that the ink is influenced by ultraviolet light. Estimated is that the ink will last for about a week, but keeping it out of ultraviolet light will increase this period. The manufacturer mentions a price of $18,43. This is without handling costs, administration costs and shipping costs. The pen is displayed in Figure 4.5. Figure 4.5 IR1PenSM Anything written on white paper can not be seen by the human eye. But, when viewed with a modified camera with IR functionality the ink becomes visible. An external light source is not required. This is being shown in Figure 4.6 and Figure 4.7. Figure 4.6 Image as seen by the human eye Figure 4.7 Image captured by IR camera 4.1.2 IR Powder Not only the manufacturer delivers ink that is visible in the infrared range, but it also has at its disposal powder with similar properties, the so called IRDC2. This IR fluorescent powder is visible at both 880nm and 1050nm. Therefore it needs to be stimulated by an external light source, that produces either blue or red light, of 39
Development of an Augmented Reality system using ARToolKit and user invisible markers frequencies 450nm respectively 630nm. The powder is said to have very high long term stability, although there exists no accurate definition of period. Just as the ink is visible green fluorescent, the same holds for IR fluorescent powder. The price for this powder is reasonably higher than that for the ink. A quantity of 10 grammas has a price of $150,40. Also this is without handling costs, administration costs and shipping costs. The higher price, together with the need for an external light source, which also needs to be obtained, makes it less cost attractive to apply. Moreover there is no specific information available on application of the powder, as is provided with the ink. Although the powder may be used as an invisible marker, the external light source will be visible. This may interfere with application requirements. 4.2 Optical filters Now that is described the invisible marker tracking medium, the visualization aspect is explained next. For the visualization it is necessary to have a camera that can detect the ink described above. Normal filters let pass through visible and IR light. For the ink to be detected a camera should be infrared sensitive, that is, the standard IR blocking filter should have been removed. This can be done manually but also the company at http://maxmax.com offers a service to adapt cameras. Prices for modification range from $150 to $350. This is without shipping, insurance and administration costs. For several Sony cameras this is known to work. For other cameras it is not sure, and it can result in a damaged camera. Also, the size of the filter may not fit with other cameras. For the mentioned Sony cameras the filter are known to fit, but the weight is the limiting factor. 4.2.1 IR filters After the IR blocking filter has been removed, a special optical filter should replace the removed filter. This is necessary in both cases; for viewing visible light and to view infrared light. It appears that the manufacturer of the ink delivers two kinds of special optical filters. Those either filter out visible light and block IR light, or filter out IR light and block visible light. The ones that filter out the IR light come in different length waves. The available optical filters have different cut off values; these are 630nm, 665nm, 715nm, 780nm, 830nm, 850nm and 1000nm. Not only are several cut off values available, but also different filter sizes; 25mm, 30mm, 37mm, 58mm, 72mm and 82mm. Moreover, there are several possibilities to choose for the thickness of the filters; 1mm, 2mm and 3mm. The filters are constructed from precision made and ground polished German glass. They appear completely black to the human eye. Such a filter is shown in Figure 4.8. In Figure 4.9 the transmittance characteristics of several filters are displayed. 40
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Development of an Augmented Reality system using ARToolKit

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