GB2371629A - Light diffuser of foamed polymer - Google Patents

Abstract

Light diffusers, used for optical screens and the light-reflecting surfaces of integrating spheres, are made from certain foamed polymers such as polystyrene, normally used for non-optical packaging and thermal insulation applications. The material can be simply machined using knives, saws and hot wires. In addition, these foams can be moulded into complex shapes, even encapsulating other elements such as flow-cells, at very low cost. The diffuser may form an integrating sphere.

Description

Patent Application : LIGHT-DIFFUSER TECHNICAL FIELD OF THE INVENTION The invention relates to the fabrication of light diffusers, such as those used in display-screens, back-lights, optical instruments and integrating spheres.

BACKGROUND OF THE INVENTION Many types of optical system require the provision of surfaces which are used in conjunction with light sources for illumination. The way in which light is reflected from the surface is then an important characteristic. Most real surfaces exhibit some degree of specular reflection character (like a mirror) combined with some light diffusion character (like matt paint), giving a complicated directional characteristic to the reflected light (Fig. 1). For special uses, this may not be ideal. Hence there is a need for materials and fabrication methods for surfaces with well-controlled optical reflection properties.

For example, in some applications a high retro-reflecting efficiency is required (narrow angle of back-reflection), as in the reflective markings on road-signs. In other applications, a very diffuse reflection is required, as when a very wide viewing angle for a projection screen is required. In this case (Fig. 2) the distribution of output rays should have no discernible specular nature, and should give an angularly uniform reflected distribution independent of the input rays'directions (this is often called "Lambertian"reflection). This characteristic is required in the fabrication of integrating spheres, on which the following description will focus.

The integrating sphere (Fig. 3) is a hollow cavity made from or internally coated with a material with high diffuse reflectivity in the light wavelength region of interest. The diffuse character means that after a few reflections, almost any input light distribution will be transformed into a uniform optical field, with the power density being the same at all positions, and in all directions throughout the cavity. The detected power density at any point on the cavity surface is then approximately equal to the input power divided by the surface area of the cavity. This assumes that there is no localised absorption within the cavity and that power losses through apertures, cracks etc. are small.

These"spheres"are widely used to measure, or integrate, the total light output from sources such as incandescent bulbs and light emitting diodes (LEDs), whose total emitted power is to be determined. In practice they do not need to be even approximately spherical, but are used in whatever cubic, cylindrical or arbitrarilyshaped, more or less enclosed cavities, as the application requires.

Integrating spheres are also used to determine the optical absorption of solid, liquid and gas samples. The absorption can be measured by placing the sample, either in a substantially low-loss container or flow-through cell in the cavity, or contained in the cavity itself, noting the reduction in cavity power-density caused by the introduction of the sample.

With the introduction of an absorbing object or material, the average power-density in the cavity is reduced, and this reduction can therefore be measured to determine the object's total volume*absorption coefficient product. As the radiation is diffuse, the power-density reduction is very similar, independent of the shape or spatial distribution of the absorption, and independent of any loss-less scatter. Only the volume of absorbing material plays a role. The sphere can therefore be used to conveniently determine the absorption of odd-shaped objects, faceted gemstones, and static and flowing fluid samples. By way of example, using a glass flow-cell passing through the cavity, we have demonstrated that absorption measurements of environmental waters can be determined to very high resolution and with greatly suppressed interference from the turbidity of the water. By directing a free-flowing stream of liquid through the cavity, the measurement can even be performed without physical contact.

Note that the sphere makes possible the use of transparent containers of arbitrary shape and poor optical quality, for example low-cost moulded plastic containers and flow-cells. The container should exhibit low optical loss, but can without detriment be strongly scattering, even translucent.

A related use of the reflecting material, formed into more or less planar sheets, is to increase the optical intensity between two such sheets which bound a substantially planar sample, in order to sensitively measure its optical absorption. This has been named an"integrating sandwich". In this case the material is used both in reflection and in transmission, with the reflected and transmitted fractions being controlled by the material thickness (Fig. 4).

It is clear that the material used to fabricate the integrating sphere should exhibit unusual and well-controlled properties. As described, these include a stable, high and/or controllable intensity reflection coefficient in the wavelength region of interest, and negligible specular reflection (highly diffuse).

The material conventionally used is an inorganic salt, evaporated metal, or lightscattering polymer chosen for a high power reflection coefficient (R 1) and highly diffuse reflection. Even white domestic emulsion paint has been put to use for these applications. Several machinable polymers based on fluorinated polymers (such as PTFE: poly-tetrafluoroethylene) loaded with light scattering materials are also available, which have R > 0.90 over much of the visible and ultraviolet wavelength range.

These materials perform well optically, but exhibit a number of disadvantages. For example, most commonly used coatings, especially of barium sulphate and other chemicals, are fragile and cannot be cleaned without damage. Low-cost emulsion paints exhibit a poor reflection coefficient. The solid materials based on loaded PTFE perform superbly, but must usually be machined from solid blocks, or rolled into a sheet. They are also very heavy and expensive, especially in large sizes. For low-cost products it would therefore be very desirable to use a much cheaper material, and to be able to mould the cavities at low cost and in a wide range of sizes and shapes. Low cost and weight are especially important for large-size integrating spheres, required for some applications with large light sources and samples. Low weight is also desirable for portable instruments using the spheres and in aerospace applications.

All the above information is known to those skilled in the art of optical instrument design, and available in the technical literature.

The basis of my claims of inventive steps is described now.

According to the present invention there is provided a light diffuser made of polymer foam. I have discovered that properly prepared polymer foams, for example of polystyrene typically used in packaging for its mechanical and thermal properties, can also offer a range of optical properties useful in the above-described applications.

Their diffuse reflection coefficient is almost as high as the best loaded PTFE-based

polymers. The material is available already foamed in large blocks (larger that 2m3 are available to us), and machining with saws and hot wires/knives is fast and economical.

In addition, the material may be foamed and moulded into complex shapes in suitable dies in a single operation, at very low cost. Foaming is usually performed with steam or gas injection; the gas flow and process parameters should be chosen to give the bead-density and bead-size optimised for optical properties. Similarly, the correct surface texture of cut or moulded surfaces must be effected by correct choice of fabrication parameters, for example the cutting velocity and temperature of hot-wire cutters."Virgin"polystyrene, that is without any contaminated or recycled material, is used to advantage for lowest absorption loss and hence to obtain the best optical performance.

The low production cost enabled by these materials means that use in low-cost instruments becomes attractive. Disposable integrating spheres are even viable in some applications, which would be useful in some applications, for example to reduce cross-contamination. In this case the moulding process further enables the encapsulation of other elements, such as fluid-containing optical cells, multiple cells, calibration, opto-electronic and optical wave-guide components within the light diffuser. This bestows superior and repeatable optical performance. The material is very light, and it may be recycled after use, including for packaging.

Note that these materials are not normally used as an optical material. The designs I have tested were fabricated by a manufacturer of packaging material and foam drinking cups; these are the typical current uses of this material.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION A preferred embodiment of an integrating sphere for static liquid absorption measurements, fabricated from hot-wire-machined polystyrene foam, is shown in Figure 5.

The integrating sphere used here consisted of a cube 9 of white"virgin"polystyrene polymer (i. e. made from pure polystyrene without recycled material) 250mm on each side with a close-fitting lid 10 formed from the same material (shown in the figure in a partially open state). A 120mm diameter cylindrical cavity was cut into the block 9.

A sample container 17 of transparent material including glass and plastic was placed in the cavity for the measurement.

Two plastic encapsulated LEDs 11 with wavelengths 430nm and 525nm were used to illuminate the inside of the cavity through a hole 15 cut in the cavity wall. Optical fibres can alternatively be used to guide light into and out of the cavity. In an effort to minimise all light leakage from the cavity all holes were made as small as possible and all absorbing surfaces were clad in diffuse reflecting material. The LEDs were electrically driven with modulation electronics 13. In a second hole 16 a 1 mm2 silicon photo-detector 12 was mounted, with which the average cavity light powerdensity was measured using synchronous detection electronics 14.

The measurement consisted of four stages. The cavity intensity was measured first without any sample container, then with an empty sample container, then with a full container of a liquid of known volume and absorption coefficient (calibration standard), and finally with a measured volume of the unknown sample. It is only necessary to use the standard liquid periodically if, for example, the cavity reflectivity has changed due to contamination or damage, or if a different sample container is used. The measured intensities allow calculation of the absorption of the unknown sample. The two LEDs for this measurement were chosen with wavelengths at which the sample has a high and a low optical absorption coefficient. Sensitivity was stabilised with conventional intensity referencing and temperature stabilisation electronics. Other light sources can be used instead of LEDs, and even full spectra measured in similar fashion.

A small relief angle was designed in to the cylindrical sides of the sphere cavity to simplify removal from the die if moulded parts are required in the future.

A preferred embodiment of an integrating sphere for flowing fluid samples fabricated from moulded polystyrene foam is shown in Figure 6. The foam body material 18 is moulded as shown here onto a spacer shell 20 of frosted transparent material. The spacer 20, shown here as spherical, allows for an air-space around the fluid channel 19, improving the uniformity of the light distribution, and forcing it to be incident on the fluid from all directions. The fluid channel 19 and spacer shell 20 should be fabricated from a transparent material exhibiting low optical loss, but can with advantage be multifaceted or even"frosted"with texturing on their surfaces to improve light diffusion. Alternatively, the foam can be moulded directly onto the fluid channel 19.

The fluid channel 19 is arranged to lead into the cavity through narrow-diameter, bent or zig-zag tubes in order to avoid direct optical channels through which light can pass, and so to minimise light loss. The fluid channel 19 is also formed with sloped surfaces arranged so that it may be filled without capturing bubbles, and emptied without retaining significant fluid. This allows the cavity intensity to be conveniently determined with and without sample, which aids the absorption calculation. By forming the flow-cell and cavity together their optical characteristics can be better controlled, and the low cost can make replacement, for example when fouled, an economical option. This does not preclude the use of cleaning agents. Not shown in the figure, filling of the fluid channel with liquids is aided by offsetting inlet and outlet pipes, so that a fountain of liquid is not pumped directly out of the main sample volume.

Light source (s) 11 and detector (s) 12 are here inserted into moulded passages in the foam. Alternatively they may be moulded permanently into the main body, if this is economical. Optical fibres are useful alternative means to inject and extract light.

The above embodiments are described by way of example only.

Summary of the Invention Optical diffusers are fabricated from foamed polymers such as polystyrene, either moulded into approximately the final form, or carved out of pre-foamed solid blocks, or assembled from cut pieces. Given the correct material purity, bead-size and surface finishing this material exhibits a high diffuse reflectivity, and can be used in all applications of reflecting and transmitting screens and illuminating surfaces, and especially for the fabrication of one or more of the reflecting surfaces used in optical integrating spheres and integrating sandwiches.

Brief Description of the Drawings Fig. 1: When illuminated by a beam of light 1, most surfaces 3 scatter light 2 with some specular (mirror-like) character, some diffuse character. The ellipse shows the approximate envelope of the distribution of rays in the various directions for this particular sample. The distribution need not be elliptical. This is well-known in the prior art.

Fig. 2: A diffusely reflecting or"Lambertian"surface 3 scatters light 4 uniformly distributed in angle, independent of the direction of the input light 1. This is wellknown in the prior art.

Fig. 3 : The integrating sphere is a cavity 7 with an internal surface which scatters and reflects light. Independent of the directions of light entering an aperture 8, after several reflections the light achieves a high degree of diffusion and uniformity. This is well-known in the prior art.

Fig. 4: Sheets of diffusing material 3 can also be used to both reflect rays 5 and transmit rays 6, both with a diffuse angular distribution. This is well-known in the prior art.

Fig. 5: Embodiment of a practical static-sample machined or moulded integrating "sphere"fabricated from foamed polymer. The cavity is formed from a main body 9 and a close-fitting lid 10, depicted here in the open position before being inserted to give a light-tight seal. Light from a source 11 driven by modulation electronics 13 is input through the small aperture 15 and detected after diffuse reflections through aperture 16. The power density at the detector 12 is measured in electronic system 14 A sample in a transparent container 17, which however need not be of optical quality, is contained within the cavity, which sample modifies the cavity optical power density, allowing determination of the optical absorption coefficient. This is an example of the use of the foamed polymer material of the Claims.

Fig. 6: Embodiment of a continuous-flow measurement cell for fluids, in which a fluid channel 19 and spacer shell 20 are moulded into the integrating sphere 18, meant as a low-cost, disposable item. This is an example of the use of the foamed polymer material of the Claims.

Claims (5)

1. CLAIMS What is claimed is: 1. A light diffuser comprising a surface, comprising a foamed polymer material.
2. 2. A light diffuser as in Claim 1 comprising polystyrene foam.
3. 3. A diffuser as in Claims 1 and 2 formed form"virgin", non-recycled polystyrene foam.
4. 4. An integrating sphere comprising one of more surfaces of light diffuser formed as in Claims 1 or 2 or 3.
5. 5. An integrating sphere as in Claim 4 moulded around a fluid channel to provide a complete measurement cell.