GB2200987A - Ultraviolet radiation detector - Google Patents

Abstract

An ultraviolet radiation detector (10) incorporates photoluminescent element of thin section in the form of a sheet (12) of fluorescent material. The sheet (12) has upper and lower surfaces (12A and 12B), and sheet edges together with the lower surface (12B) have reflective coatings (14A and 14B). The upper surface (12A) is overlaid by a UV transmissive, visible non-transmissive filter (16). A light output port (18) is formed through the edge coating (14A), and has a much smaller area than that of the upper surface (12A). UV radiation transmitted by the filter (16) passes through the upper surface (12A) and is partially absorbed in the sheet (12). Unabsorbed UV radiation passes to the lower surface (12B) and its coating (14B) for reflection, and returns for a second transit of the sheet (12) and further absorption. This produces fluorescence at visible wavelengths. A substantial proportion of the fluorescence is guided by internal reflection at the upper and lower sheet surfaces (12A and 12B) to the light output port (18). This provides light concentration because UV radiation is gathered over the area of the upper surface (12A), but luminescence is guided to the much smaller output port area. The light output intensity may be compared with a pre-calibrated graduated colour scale for UV intensity measurement. This provides a UV detector of great simplicity and cheapness. Focussing optics and electronic detectors are not necessary, although the latter may be provided if electronic readout is required.

Description

ULTRAVIOLET RADIATION DETECTOR This invention relates to an ultraviolet radiation detector.

Detection of ultraviolet (UV) radiation is well known in the prior art. One known detection technique involves the use of a monochromator to isolate UV radiation wavelengths of interest, and their detection by a phQtomultiplier or other UV sensitive detector. Apparatus of this nature is complex, expensive, fragile and bulky.

British Patent No 522,402 relates to an alternative form of UV detector of layer or multiple sheet construction. It comprises four successively disposed layers.

These consist of a UV transmissive first filter layer, a fluorescent material layer, a fluorescence transmissive second filter layer and a fluorescence sensitive photodetector layer. UV radiation transmitted by the UV filter layer is absorbed in the fluorescent material, which responds by fluorescing. Light emitted by the fluorescent material is transmitted by the second filter to the photodetector layer, which provides a current to a micro-ammeter. This device suffers from the disadvantage that it requires a photodetector of substantially the same area as the fluorescent layer. Such a detector is not a commercially available device and must be specially manufactured, which increases expense. Furthermore, the photodetector layer only receives light from one face of the fluorescent layer, which will be a minor proportion of the total fluorescence emission.Since in addition the photodetector layer will not absorb - all radiation incident on it, detection efficiency will be low. The device also requires an external micro-ammeter, which increases bulk and expense.

British Patent No 1,067,615 discloses a fluoroglass dosimeter for the detection of gamma rays. This comprises a rectangular glass block in which luminescent centres are created by gamma rays, and which are then induced to luminesce by UV illumination. The luminescence or fluorescence is in the visible, and is detected by a photomultiplier to provide an indication of gamma ray intensity.

This patent discloses a collimating optical lens system and stops to direct UV light from a source on to the glass block, and a mirror with bevelled edges to redirect back to the block light transmitted by it. Further collimating optics direct fluorescent light from the block to a photomultiplier. The system is designed to detect fluorescent emission from the block while rejecting stray light.

The optical stop before the block and the - bevelled mirror following it are carefully arranged to reject UV light not passing straight through the block.

However, this arrangement is complex, bulky and expensive. It requires collimating optics between the UV light source and the block, beyond the block and between the block and detector. Furthermore, the photomultiplier detector requires a power source and some form of counting or metering device.

United States Patent No 4,061,922 to Last relates to a UV sensing device for use in a water purifier. It discloses a cylindrical housing with a window to admit UV light, which is incident on a fluorescent screen. A photocell is mounted in an aperture in the side wall of the housing, and receives fluorescent emission from the screen in a narrow cone of angles. The centre of the cone is inclined at a small angle to the incident UV light, which corresponds to a backscattering direction. This optical geometry is required to ensure that the photocell is shielded from direct UV light. Inevitably, the photocell captures comparatively little of the fluorescent emission. The photocell output is fed to a signal amplifier and a subsequent power amplifier, and is then employed to operate a solenoid valve controlling water flow in a purification system.As in previously described prior art, the device of Last is relatively bulky and expensive. It requires a cumbersome housing for the fluorescent screen and photocell, and also a substantial amplification circuit for the photocell output.

There is a need for a small, portable ultraviolet detector of inexpensive construction for use by the general public. One particular application relates to a device for sunbathers to use to provide a measure of solar ultraviolet light intensity. This would enable individual sunbathers to achieve a suntan while avoiding dangerous over-exposure and burns. A further application relates to the use of conventional sun lamps for suntan purposes. These provide a combination of ultraviolet and visible radiation, but the ultraviolet component diminishes with time without obvious reduction in the visible component. As a result, users of conventional sun lamps cannot detect degradation in ultraviolet output, and continue to use them after they have become useless for suntan purposes.

In order to provide the general public with a means for monitoring the ultraviolet component of solar or sun lamp radiation, it is important to provide a detector of extreme simplicity and cheapness. The prior art relates to complex and expensive devices which do not satisfy this long-felt want.

It is an object of the present invention to provide an ultraviolet radiation detector of simplified construction as compared to the prior art.

The present invention provides an ultraviolet detector including a photoluminescent element response to ultraviolet radiation and means for determining luminescent intensity emitted from the element, wherein the element is at least partly of thinsection construction defined by a first surface for receiving ultraviolet light and a second surface separated therefrom by the element thickness dimension, the element being arranged to guide luminescent radiation by internal reflection at its first and second surfaces to an output edge surface region or rim of the element having less than a tenth the first surface area to produce luminescence concentration and adjacent to which the means for determining luminescent intensity is disposed.

It will be appreciated that the expression "photoluminescence" embraces both fluorescence and phosphorescence, the latter being normally associated with a longer response time and larger shift between the wavelengths of absorbed and emitted radiation.

The invention provides the advantage of being an ultraviolet detector of extreme simplicity and cheapness. In its simplest form, it may consist of a thinsection photoluminescent element having a comparison chart of graded colour tones adjacent the output edge surface region or rin, the chart simulating differing luminescent intensities and being precalibrated in terms of intensity. In this embodiment a user compares output luminescent intensity with the chart to identify the most closely matching colour shade, thereby determining both luminescent and ultraviolet intensity. As will be described, the photoluminescent element may be of plastics material, and the simplest form of the invention is capable of mass production for a unit cost in the order of or less than one penny. This is absolutely ideal for use by the general public.

The invention employs a thin-section photoluminescent element which has light guiding properties directing luminescence preferentially transverse to its section thickness dimension by virtue of internal reflection at element surfaces. The element receives incident ultraviolet light at a surface of comparatively large dimensions, and then acts as a light guide to guide luminescent radiation preferentially to the much smaller output surface area of an element edge surface or rim. This has the effect of concentrating the luminescent radiation and increasing its intensity. Such amplification of luminescent intensity increases as the ratio of the light collection area to that of the output surface increases, ignoring variation in light guiding efficiencies.This provides a very convenient design variable, since the fluorescent intensity amplification may be adjusted to accommodate the sensitivity of the means for determining luminescent intensity.

The invention provides the further advantage there is no necessity for expensive, cumbersome, non-portable light collimating optics as described in the prior art of British Patent No. 1,067,615. Furthermore, the invention is capable of providing much greater detection sensitivity than the prior art of British Patent No.

522,402. This latter patent discloses an ultraviolet detector incorporating a luminescent sheet with a luminescence detector layer adjacent one sheet surface.

The detector layer receives only luminescence output from one sheet surface and generated in directions near the normal to that surface, which is only a small proportion of the total luminescence produced. This prior art device is therefore fundamentally less efficient in gathering luminescence than the present invention, since it does not provide for light guiding.

In one embodiment, the invention may incorporate a photoluminescent element in the form of a sheet having first and second surfaces separated by the sheet thickness dimension. The first surface is exposed to ultraviolet light, and luminescence emerges at an output edge of the sheet. Other sheet edges are surfaced with reflecting material to inhibit unwanted emergence of luminescence.

The output edge of the sheet may be partly surfaced with reflecting material, so that luminescence emerges preferentially from a small output port region having an area less than one tenth that of the first surface. The output port region area is preferably less than one hundredth that of the first surface. The second surface of the sheet may be covered with reflecting material to reflect ultraviolet light for further absorption in the photoluminescent element and to enhance luminescence concentration. The reflecting material may be a metallisation layer, or alternatively it may be a Lambertian reflector such as a paint layer producing diffuse reflection as opposed to specular reflection.

The light output port region may be optically coupled to an electronic detector such as a photodiode. The invention may include filtering means to inhibit non-luminescent radiation reaching the detector. The detector may be coupled to the output port region by a light guide having infrared attenuating properties and providing at least part of the filtering means. By virtue of the light guiding properties of the photoluminescent element, the electronic detector need not be of high sensitivity.

The detector of the invention may be arranged as a cap for a fluid receptacle such as a suntan lotion container. In this case the curved cap wall is the photoluminescent element, and luminescence emerges from the upper cap rim.

The cap closure surface bears a graded set of colour tones conveniently of segmental form and located within the rim.

In an alternative aspect, the invention provides a method of measuring ultraviolet radiation intensity comprising arranging a thin section element of photoluminescent ultraviolet responsive material to receive ultraviolet radiation via a first element surface separated by the element thickness dimension from a second element surface, the element being arranged to guide luminescence by internal reflection at the first and second element surfaces to an element edge or rim of area less than one tenth that of the first surface, and determining the intensity of luminescence concentrated at the said edge or rim.

In order that the invention might be more fully understood, embodiments thereof will be described, by way of example only, with reference to the accompanying drawings, which: Figure 1 is a perspective view of an ultraviolet radiation detector of the invention; Figure 2 is a sectional view of the Figure 1 ~detector; Figure 3 shows a graph of the absorption and emittance of a fluorescent material versus wavelength; Figure 4 shows graph of the transmission coefficient of a visible light filter versus wavelength; Figure 5 schematically illustrates a colour tone scale for intensity measurement; Figure 6 illustrates photodiode detection of fluorescent intensity; Figure 7 shows a graph of the normalised intensity of radiation emitted by a sun lamp versus wavelength; Figure 8 shows a section of the graph of Figure 7;; Figure 9 shows a graph of the normalised intensity of radiation emitted by a Halogen lamp versus wavelength.

Figure 10 shows a further embodiment of the invention of tapered form; and Figure 11 illustrates an embodiment of the invention arranged as a cap for a fluid container.

Referring to Figures 1 and 2, an ultraviolet radiation detector 10 is shown in perspective and in section respectively. These drawings are not to scale and some dimensions have been exaggerated to aid clarity. The ultraviolet radiation detector 10 consists of a sheet 12 of plastics material incorporating a fluorescent dye, for example Perspex Green 6609 manufactured by ICI Chemicals Ltd, a British company. The sheet 12 is 5 mm thick, and has square planar upper and lower surfaces 12A and 12B with side dimensions of 50 mm. An evaporated aluminium coating 14A, approximately 50 microns thick, is deposited on the sides of the sheet 12. A further evaporated aluminium coating 14B, also approximately 50 microns thick, is deposited on the lower surface 1 2B. A filter 16 is attached to the upper surface 1 2A by UV transmitting cement. The filter 16 transmits ultraviolet radiation, but attenuates visible light, and may consist of material designed OX1 and supplied by Ealing Optics Division. A light output port 18 is located adjacent the centre of one edge of the sheet 12. It consists of a circular opening in the coating 1 4A which exposes a region of the underlying sheet 12. The port 18 is 1.5 mm in diameter, and accordingly has an area which is less than one thousandth that of sheet surface 12A.

Referring to Figure 3, there are shown graphs 30 and 32 illustrating respectively the absorption and emission properties of the material of the sheet 12. ~ They represent respectively the absorption and emission spectra as a function of wavelength (nm). The graphs 30 and 32 show that the wavelength of the emitted light is directly related to, and longer than, the radiation absorbed, ie the response to excitation is at a lower photon energy. When ultraviolet radiation is incident on and absorbed by the fluorescent material of the sheet 12, visible fluorescent light is emitted. An increase in ultraviolet radiation intensity absorbed leads to a corresponding increase in fluorescent visible light intensity emitted.

Referring now to Figure 4, there is shown a curve 34 which illustrates the transmission of the filter 16 as a function of wavelength (nm). The curve 34 shows filter transmission wavelength intervals 250 nm to 405 nm (UV) and 660 nm onwards (IR). The filter 16 is opaque in the visible between 405 nm and 660 nm.

The ultraviolet radiation detector 10 operates as follows. The filter 16 transmits ultraviolet radiation but attenuates the visible. The transmitted ultraviolet radiation passes to surface 12A, and is absorbed by the fluorescent sheet 12.

Any ultraviolet radiation which is not absorbed on a first transit through the sheet 12 is reflected by the aluminium coating 14B for a second transit to produce enhanced absorption. Infrared radiation transmitted by the filter 16 at wavelengths near 700 nm (see Figure 4) is not absorbed by the sheet 12.

The surfaces 12A and 12B of the sheet 12 produce total internal reflection of fluorescent radiation incident at angles above the critical angle. This directs the radiation preferentially towards sheet edges. In effect, the sheet 12 has light guiding properties since it is of thin-section construction. The aluminium coatings 14A and 14B retain within the sheet 12 fluorescent radiation which would otherwise emerge at sheet edges. Ignoring possible minor optical imperfections in the sheet 12 and coatings 1 4A and 14B, fluorescent radiation is preferentially emitted via the light output port 18. The detector 10 accordingly gathers (UY) light over the large area of surface 12A, and guides fluorescent light preferentially to the output port 18, which is more than one thousand times smaller in area.Even allowing for losses such as fluorescent light escaping by virtue of being near normal incidence at the surface 12A, the output port 18 receives light which is greatly concentrated in intensity.

The intensity of fluorescent light emerging from the port 18 may be measured by a number of methods to provide an indication of the intensity of ultraviolet radiation incident on the device 10. Tests on this embodiment of the invention demonstrate that the fluorescent light is sufficiently concentrated to be observable by the naked eye.

Two examples of luminescence measurement techniques will now be described with reference to Figures 5 and 6. Figure 5 provides a chart 40 having a number of different shadings representing colour intensities. An observer compares the fluorescent light intensity from the port 18 with each colour shade on a chart simulated by the chart 40, and selects the most closely matching shade. Each shade corresponds to a respective predetermined UV intensity, and provides a measurement of this intensity when matched to observed fluorescence.

This demonstrates that one embodiment of the invention provides a UV detector which does not require collimating optics or electronic detection. Furthermore, it has been surprisingly found that it is possible to estimate luminescent intensity with the naked eye in the presence of other background visible radiation.

Accordingly, filters such as 16 may be omitted from embodiments of the invention not requiring high discrimination characteristics.

Figure 6 illustrates use of the invention in conjunction with electronic measurement of fluorescent light intensity. A photodiode 42 receives light from the port 18 via a light guide 44. Photodiodes respond to infrared radiation as well as visible light, and moreover infrared light is transmitted by the filter 16 but not absorbed by the sheet 12. Accordingly, to avoid a photodiode response to infrared, the light guide 44 is chosen to have infrared attenuating properties.

An electronic circuit 46 produces a readout (not shown) corresponding to the intensity of ultraviolet radiation incident on the device 10.

The measuring-techniques described with reference to Figures 5 and 6 may be calibrated using ultraviolet sources of known intensity.

An experimental embodiment of the device 10 was constructed to test operation of the filter 16, sheet 12, reflective coatings 14A and 14B and photodiode 42.

It differed from that described with reference to Figures 1 and 6 in that the filter 16 was removable and the photodiode 42 was connected directly to the port 18. A Halogen lamp and a commercially available sun lamp were used to illuminate this device under various experimental conditions.

Referring now to Figures 7 to 9, these provide emission spectra of the sun lamp and the Halogen lamp. Figure 7 shows a curve 50 illustrating the normalised emission radiation intensity of the sun lamp as a function of wavelength (nm).

Figure 8 shows a section of the curve 50 of Figure 7 on an enlarged scale and in the ultraviolet region. Figure 9 shows a curve 52 illustrating the normalised emission radiation intensity of the Halogen lamp as a function of wavelength.

Table 1 shows the output voltage of the photodiode 42 under various conditions of lamp illumination of the device 10. It indicates the source used, and also the photodiode output voltage under the following four conditions: (a) no illumination, (b) sheet illuminated with filter 16 removed, (c) sheet illuminated with filter 16 present, and (d) sheet illuminated with filter 16 and an ultraviolet absorbing - filter present.

The UV absorbing filter was that designated 2A and supplied by Ealing Optics Division. The UV transmissive/ visible opaque filter 16 was type OX1 from the same supplier.

Comparison of the voltages in columns (a) and (d) demonstrates that the effect of IR radiation on the detector is negligible. Subtraction of column (d) figures from those of column (c) indicates that ultraviolet radiation in the sun lamp spectrum produces a photodiode voltage change of 1.1 mV. This demonstrates adequate sensitivity for electronic measurement purposes despite the simplicity of the device 10.

TABLE 1

PHOTODIODE OUTPUT VOLTAGE LAMP LAMP OFF LAMP ON (a) (b) (c) (d) DARK NO OX1 FILTER BOTH LEVEL FILTERS 16 ONLY FILTERS Halogen Lamp 0.90 mV 6.4 mV 1.0 mV 0.90 mV Sun Lamp 0.90 mV 6.3 mV 2.3 mV 1.20 mV As has been said, the light output port 18 of the device 10 has a cross-sectional area less than one thousandth that of the UV incidence surface 12A. The ratio of these areas is in fact about 7 x 10-4 to 1. This ratio may be altered to provide any desired degree of light concentration.In climates with a high UV intensity, or embodiments of the invention incorporating a sensitive electronic photodetector, a lesser degree of light concentration may be required. This ratio therefore provides a convenient design variable to adapt the invention for different operating regimes. In general however, it will be necessary for the port 18 to have less than one tenth the area of the surface 12A, and preferably less than one hundredth.

The device 10 of the invention may be further adapted to improve visible light gathering efficiency. The cross-sectional area of the fluorescent sheet 12 may -be tapered so that it reduces towards the light output port 18. It may taper sufficiently to match the output port area. Since plastics materials are easily moulded, production in tapering form to enhance light guiding properties is straightforward to implement.

Referring now to Figure 10, there is schematically shown a further embodiment 60 of the invention. The device 60 is a wedge-shaped sheet of photoluminescent plastics material, and tapers to a luminescence output edge face 62 acting as a light output port. The face 62 is coated with a layer of CdS photoconductivematerial indicated by shading. Electrical leads 64 make ohmic contact to either end of the layered edge face 62. All other edge faces and the undersurface of the device 62 (of which only an edge face 66 is shown) are coated with white paint acting as a diffuse Lambertian reflector. The device 60 has an upper surface 68 for reception of ultraviolet radiation.

The upper surface 68 is configured (not shown) to act as a diffuser of incident ultraviolet radiation. The device 60 is 3 mm in thickness, and tapers from 20 mm in width at an edge 70 to 10 mm at the face 62. Its length between the face 62 and the edge 70 is 16 mm. In this example, the device 60 is of constant thickness, but alternatively the thickness may be reduced progressively from above 3 mm at edge 70 to below 3 mm at face 62.

The electrical leads 64 are connected to an external circuit (not shown) providing bias current to the photoconductive layer, comparing the voltage across it with a reference level, and amplifying the difference. The amplified difference voltage is subsequently converted to a digital signal indicating ultraviolet intensity incident on the upper surface 68.

Referring now to Figure 11, there is schematically shown a perspective view of an embodiment 80 of the invention in the form of a cap for a fluid container (not shown). The cap 80 has a hollow cylindrical wall 82 of thin section construction moulded from photoluminescent plastics material. The wall 82 is arranged to accommodate within in it the neck of a fluid container. The wall 82 terminates at an upper edge region or rim 84 within which is located a cap closure surface 86. The closure surface 86 bears a graded set of six colour tones indicated by varying shading, each tone such as 88 being of segmental form.

The embodiment 80-operates as follows. Ultraviolet light incident on the wall 82 creates luminescence within it. The luminescence is guided by internal reflection at the wall surfaces, and a substantial proportion of it emerges at the rim 84 producing a glow. An observer compares the glow with the colour tones on the surface 86, and selects that which most closely matches. The tones bear respective indicia (not shown) such as numerals, which are associated with ultraviolet intensities. The indicia may be associated with legends on the fluid container relating to degree of ultraviolet intensity. The container may be a suntan lotion receptacle.

The length of the cap 80 may be increased to increase luminescence output, and the lower rim (not shown) of the wall 72 may be arranged to reflect luminescence to the upper rim 84.

The invention relates generally to an ultraviolet detector of thin section properties. Luminescence is guided to an output edge or rim of much smaller dimensions than those of the ultraviolet radiation receiving surface. This amplifies the luminescent intensity, which greatly increases sensitivity and greatly reduces the expense, bulk and weight of luminescence detecting means.

Embodiments of the invention such as 80 have output surfaces 84 whose widths are equal to the thickness of the photoluminescent element in each case.

Generally speaking, this width should be less than one tenth of a typical element linear dimension, such as the length of the wall 82 between upper and lower rims. This is because ultraviolet absorption has an exponential dependence on element thickness, whereas output light amplification reduces linearly with thickness. If a particular element thickness produces 50% ultraviolet absorption, doubling that thickness increases absorption to 75% but reduces light amplification at the output face by half. Doubling element thickness accordingly reduces luminescent output intensity by 25% in this particular instance. However, embodiments of the invention which do not require constant cross-section may be tapered to reduce output surface area and achieve any degree of light amplification.

Claims (11)

1. An ultraviolet radiation detector including a photoluminescent element responsive to ultraviolet radiation and means for determining luminescent intensity emitted from the element, wherein the element is at least partly of thin section construction defined by a first surface for receiving ultraviolet light and a second surface separated therefrom by the element thickness dimension, the element being arranged to guide luminescent radiation by internal reflection at its first and second surfaces to an output edge surface region or rim of the e:ement having less than a tenth of the first surface area to produce luminescence concentration and adjacent to which the means for determining luminescent intensity is disposed.

2. A detector according to Claim 1 wherein the photoluminescent element is a sheet of plastics material incorporating a photoluminescent dye and the said first and second surfaces are separated by the sheet thickness dimension, the said edge surface region being a sheet edge region and the sheet having other edges surfaced reflectively to inhibit emergence of luminescence.

3. A detector according to Claim 2 wherein the sheet edge region is partly reflectively surfaced and partly transmissive to define a light output port having an area less than one hundredth that of the first surface.

4. A detector according to Claim 1, 2 or 3 characterised in that the means for determining luminescent intensity is a comparison chart of graded colour tones arranged to simulate differing luminescent intensities.

5. A detector according to Claim 2 or 3 wherein the means for determining luminescent intensity includes an electronic photosensitive device.

6. A detector according to Claim 5 including filtering means arranged to inhibit non-luminescent radiation reaching the photosensitive device.

7. A detector according to Claim 6 wherein the filtering means comprises a filter arranged over the first surface and a light guide coupling the photosensitive device to the output port, the filter being ultraviolet transmissive but non-transmissive to visible light and the light guide having infrared attenuating properties.

8. A detector according to Claim 1 arranged as a cap for a fluid container, the first and second surfaces being those of the cap wall, the edge surface region or rim being the upper rim of the cap wall and the means for determining luminescent intensity being a graded set of colour tones arranged on a cap closure surface within the upper rim.

9. A detector substantially as herein described with reference to the accompanying Figures 1 to 9.

10. A detector substantially as herein described with reference to the accompanying Figure 10 or 11.

11. A method of measuring ultraviolet radiation intensity comprising arranging a thin section element of photoluminescent ultraviolet responsive material to receive ultraviolet radiation via a first element surface separated by the element thickness dimension from a second element surface, the element being arranged to guide luminescence by internal reflection at the first and second element surfaces to an element edge or rim of area less than a tenth that of the first surface, and determining the intensity of luminescence concentrated at the said edge or rim.