A Perspective on Wavelength Transformation by Absorptive Optical Filters

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Dwayne Dawson
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1 A Perspective on Wavelength Transformation by Absorptive Optical Filters Steve Caldwell, FMS #614 There has been much discussion within the fluorescent mineral community recently about the functioning of absorptive filters when used in conjunction with UV LEDs. Virtually all of this dialogue is centered on USP 7,781,751 by Gardner (2010). While perhaps the ultimate embodiment of this patent would be a flashlight containing high power UVC-emitting LEDs and providing for the selective use of phosphor-loaded UV transparent film filters to additionally produce UVA, UVB, and/or other light, the use of terms such as wavelength distribution and wavelength transforming leave ample room for other embodiments within the scope of the patent. It is my personal belief that the form much of this dialogue has taken, especially on social media, is unproductive for the advancement of the field of interest we all enjoy and want to see grow both for ourselves and future generations of fluorescent mineral collectors. That thought brings me to writing this article. The Nature of This Article This article provides a simple physics demonstration that seeks to explore terms such as wavelength distribution and wavelength transforming as they may apply to absorptive UV filters such as (LW) Wood s Glass or (SW) Hoya U-325C. This demonstration can be reproduced by anyone with a well equipped basement. And this is only a demonstration, as it does not reveal any physical principle not already known to someone familiar with the basics of light interaction with matter. It is completely qualitative, as performing a definitive, quantitative characterization would require the use of a high intensity UV source in conjunction with specialized instrumentation, including spectrophotometric equipment that could function over the wavelength range of ~ nm. Only then could the light source and transmission characteristics of all filter materials be fully characterized. In the case of subtractive filters such as those specifically mentioned, there is a relatively high percentage of absorption of unwanted visible wavelengths and reasonably high transmission of the desired UV band(s) the intentional result of carefully formulated glass chemistry. Concurrent with the production of this desired wavelength transmission, through whatever set of physical mechanisms, a portion of the absorbed optical energy is converted into heat. While this effect is not always apparent due to the more aggressive heating of UV filter glass by more robust heat sources (such as a Hg vapor discharge tube), it is nevertheless present as a small component of the overall filter heating. Conservation of energy dictates that the absorbed optical energy cannot simply vanish. This demonstration illustrates a method whereby this effect can be examined in a simple, qualitative manner. What This Article Is Not It is not the purpose of the author to take sides in the current debate over the validity or applicability of any patent to any current commercial UV-generating device, but rather to illustrate readily observable effects that are relevant to the terminology contained in the patent being debated. The author by way of this article does not present or imply any legal opinion as that should be left to those trained in patent law. Rather, demonstrable physical principles speak for themselves. It is only a matter of looking in the correct manner. LIGHT SOURCE TRANSPARENT PLATE UV FILTER PLATE VIS VIS VIS filtered NIR (slight) IR, low LW LWIR the incident the transformed wavelength wavelength distribution distribution Figure 1. The arrangement of chosen components for the demonstration was based on limitations of a well-equipped basement. 1

2 The Demonstration If what is said in the 3 rd paragraph about absorptive filters is correct, then the transmitted wavelength distribution from such a filter should be different than the wavelength distribution of the incident radiation on the face of the filter. Moreover, a component of the wavelength distribution emitted from the back of the filter plate should contain a detectable heat signature resulting from optical absorption. The basic elements of the following demonstration can be illustrated by the diagram in Figure 1. For the purpose of this demonstration, a common white light LED flashlight was selected as a convenient source of intense, relatively collimated light. This unit emitted ~500 lumens. Consulting the Cree XM-L2 datasheet for the 5000K version, it was found that the diode emits slight NIR, but IR production falls to ~0% by 780 nm. There is no UV production. Next in the beam path of this crude optical bench was a 1.5 OD thin walled aluminum tube used as a collimator to limit stray light from the flashlight. The defined beam then passed through a transparent soda-lime glass plate obtained from a picture frame. Consulting several internet sites revealed that the transmission spectrum of high silica glasses (including both soda lime as well as borosilicate) have an IR cutoff of ~4.5 µm. (Other silicate glasses such as Schott KG-5 used as heat absorbing filters in slide and other film projectors have a glass chemistry engineered to result in a more rapid IR cutoff of ~0.9 µm.) Before proceeding further, a brief review of IR wavelengths is in order. There are several schemes for IR band definition and Table 1 presents only one. Atmospheric gas species absorb strongly in defined IR wavelength intervals an inescapable filtration process in open air that produces absorption bands that in turn have been used to define the IR bands. In order to look for a heat signature resulting from incident beam energy absorption, one must be able to selectively sense LWIR. Table 1. IR bands resulting from atmospheric transmission. Names Wavelength (µm) Common sources Near IR (NIR) Incandescent light sources, sun, IR LEDs, lasers, night sky. Shortwave (SWIR) Incandescent sources, sun, IR LEDs, lasers. Midwave (MWIR) 3 5 Incandescent sources, sun, IR LEDs, lasers, combustion. Longwave (LWIR) 8-13 Sun, lasers, thermal radiation from hot objects and living organisms Thermal imaging provides a convenient way to sense local low level heating. In contrast to night vision devices that employ high gain amplification of VIS and NIR wavelengths, thermal imagers look only at LWIR emitted from hot objects, with hot being displayed relative to the other objects sensed in a given field of view thus "relatively" hot. As a result, thermal viewers can be used in broad daylight and brightly lit rooms with no optical interference. The unit used for this demonstration was manufactured by FLIR, and was designed for imaging at much greater standoff distances than used in this demonstration. This fact contributed to low-res, out-of-focus images though thermal images from affordable units are never extremely sharp. The sensor in this device was a vanadium oxide microbolometer. An iphone 6 was used to capture the ~2X images appearing on the thermal viewer screen. Returning to Figure 1, the transparent filter plate is not truly transparent though clear to the eye. As such, any absorbed wavelengths would be expected to give rise to local heating of the irradiated area. In the case of this soda lime plate, it would be expected that any resultant heat signature would be small. It was discovered early in setting up this demonstration that stray heat signatures appeared. This was quickly identified as heating of components simply by handling. Not visible in any of the forthcoming pictures was a small fan used to continuously cool the optical bench to minimize any such thermal artifacts. Figure 2 shows the heat signature of a hand print on the glass plate, illustrating the sensitivity of the FLIR thermal imager. Before proceeding further, it is valuable to demonstrate the LWIR blocking capability of common soda lime glass, as this characteristic is important to this demonstration. Figure 3 shows the thermal image of a coffee cup filled with hot water. Figure 4.A shows the arrangement used to create the LWIR image in Figure 4.B. In that latter image, the glass plate effectively blocks the thermal emission of the hot cup. Figures 2-4 tell an important story. First, it is seen that a soda lime glass plate can effectively block incident LWIR radiation. However, it has also been shown that the glass can effectively emit LWIR when locally heated. 2

Figure 2. Thermal signature of hand print on glass plate in LWIR. Figure 3. LWIR image of coffee cup filled with hot water.

Thermal image showing the LWIR blocking effect of soda lime glass.

the UV filter plate. For the demonstration, a broken plate of 3/16 thick Kokomo ( Wood s) Glass was employed.

greater local heating if the incident wavelength distribution is transformed by conversion of absorbed beam energy into

Any LWIR imaged on the rear for the UV filter plate must result from transformed light energy, as the incident wavelength

3 Figure 2. Thermal signature of hand print on glass plate in LWIR. Figure 3. LWIR image of coffee cup filled with hot water. Figure 4.A. White light view of soda lime glass plate blocking ~1/2 of coffee cup. Figure 4.B. Thermal image showing the LWIR blocking effect of soda lime glass. Again returning to Figure 1, once the light beam passes through the clear glass plate, it impinges on the front surface of the UV filter plate. For the demonstration, a broken plate of 3/16 thick Kokomo ( Wood s) Glass was employed. The high optical absorption of this filter, reportedly containing ~9% NiO in a Ba-Na silicate glass, should result in greater local heating if the incident wavelength distribution is transformed by conversion of absorbed beam energy into heat as evidenced by LWIR emission in the resultant transformed wavelength distribution. Any LWIR imaged on the rear for the UV filter plate must result from transformed light energy, as the incident wavelength distribution is principally free of any LWIR from the light source, as the LWIR blocking potential of the glass plate has already been confirmed. flashlight collimator glass plate UV filter flashlight collimator glass plate UV filter resistors resistor LWIR hot spot resistor Figure 5.A. Horizontal view of "optical bench" in white light. Key components are labeled. Figure 5.B. Horizontal view as seen in the thermal viewer. Hottest object in field of view was the flashlight body after minutes of runtime. 3

The component layout corresponding to Figure 1 is seen in Figure 5.A using white light.

being chosen to each produce ~1/3W of ohmic heating when connected to 120V.

distribution not present in the incident wavelength distribution. Figure 5.B shows this setup in the LWIR of the thermal viewer. Figure 6.

4 The component layout corresponding to Figure 1 is seen in Figure 5.A using white light. In order to clearly see the position of a projected spot on the back of each glass plate in the thermal viewer image, small resistors were positioned to the right of each spot position, their value being chosen to each produce ~1/3W of ohmic heating when connected to 120V. In effect, the resistors when energized were incandescing in LWIR, providing dependable locators for any nearby LWIR beam spot that would constitute a demonstration of transformed wavelength distribution not present in the incident wavelength distribution. Figure 5.B shows this setup in the LWIR of the thermal viewer. Figure 6. View of VIS wavelengths transmitted by the Wood's Glass filter. The resistor used as a beam spot indicator is visible to the right of the beam spot. Figure 5.B clearly shows the position of the two warm resistors indicating the position of the clear glass and UV filter plates in the beam path. More importantly, this thermal image also shows a noticeable LWIR signature on the rear of the UV filter whereas there was none on the rear of the clear glass plate which absorbed only very little of its incident radiation. Absorbed optical energy caused in local heating of the UV filter with a resultant emission of LWIR. The absorptive UV filter provided a transformed wavelength distribution containing wavelengths not present to any detectable extent in the incident spectrum. Figure 6 shows the on-axis view of the VIS light transmitted through the LW UV filter with the associated resistor. Figure 7 provides an in-line view of the glass plate and UV filter for a complementary perspective. no LWIR hot spot on glass plate Figure 7.A. Vertical white light view of component layout. Note the resistor locations. Figure 7.B. Strong thermal signatures were obtained from flashlight, the two resistors, and the rear of the UV filter. It is tempting to think filters such as Kokomo Glass and Hoya U-325C are purely subtractive eliminating unwanted visible light. From a practical perspective, that view is of course correct. However, as shown in this simple demonstration from a technical point of view, such filters create new wavelengths of light. Conservation of energy requires the following generalized relationship for the absorptive filter: 4

5 incident optical energy = reflected light energy + transmitted light energy + heat The absorbed energy in the UV filter ultimately stimulates increased atomic motion, which manifests itself as heat. (This article does not address the detail of these mechanisms it is left to the reader.) The heat is then transferred from the generation site by 3 well known modes conduction, convection, and radiation. For this particular scenario, none are particularly efficient being limited by low filter thermal conductivity, small ΔT to ambient, and low T given T 4 dependence for radiative transfer, respectively. Localized heating, though subtle, in the irradiated volume of the absorptive filter gives rise to heating, and heating to localized LWIR emission. Even after minutes of run time, there was only a LWIR signature from the beam spot on the back of the UV filter plate with no suggestion of any significant heat transfer by conduction or convection having taken place. The findings of this simple, qualitative physics demonstration are therefore consistent with the "wavelength transformation" of USP 7,781,751 that is centered in the current dialogue. I cannot find where the patent presents language that would limit its coverage to specific wavelength conversion mechanisms. To the contrary, the Detailed Description of the Invention in the cited patent, lines read: "The...(wavelength transforming)...materials of any preferred embodiments may comprise any material or system that absorbs light of one wavelength or band of wavelengths and emits light of another wavelength or band of wavelengths, thus modifying the distribution of spectral density." The images obtained in this demonstration show that wavelength transformation (in this case the generation of a significantly higher emission of LWIR) occurs in a dense, absorptive optical filter in contrast to an optically clear medium in which no significant conversion was seen to occur. While such a transformation is not the primary intention of a UV filter composition and while the heating of the filter is neither useful nor even wanted, heating nevertheless occurs as an inescapable result of high percentage absorption of incident beam energy. As stated previously, the present article is not an attempt to take sides in this debate or delve into patent interpretation, but rather to let demonstrable, qualitative physical data relevant to the topic speak for themselves. 5

transmission and reflection characteristics across the spectrum. 4. Neutral density

transmission and reflection characteristics across the spectrum. 4. Neutral density 1. Interference Filters 2. Color SubstrateFilters Narrow band (±10nm),Broadband (±50nm and ±80nm), it has extremely angle sensitive, so carefully mounting is necessary. The highly selective reduce the