Abstract: A dispenser for dispensing ozone containing foam. The dispenser comprises an ozone generating chamber having an air inlet in communication with a source of air and an air outlet. The ozone generating chamber contains a corona discharge generator that generates a corona discharge to produce: (a) ozonated air, by converting oxygen in the air into ozone, and (b) an ionic wind. The ionic wind draws the air from the air source into the ozone generating chamber through the air inlet, and expels the ozonated air from the ozone generating chamber through the air outlet. A foam generator receives the ozonated air expelled from the ozone generating chamber and mixes the ozonated air with a foamable liquid to generate the ozone containing foam.

Abstract: Disclosed herein is provide an electrostatic precipitator capable of allowing a charger to be thin while suppressing ozone generation. The electrostatic precipitator 1 includes a charger 10 provided with a high voltage electrode 11 receiving a high voltage from a high voltage generating circuit 40 and a counter electrode 12 facing the high voltage electrode 11 and receiving a reference voltage from the high voltage generating circuit 40, and configured to charge suspended particles by generating a discharge between the high voltage electrode 11 and the counter electrode 12; and a dust collector 20 disposed in the downstream side of an air flow direction of the charger 10 and configured to collect the suspended particles charged by the charger 10.

Abstract: An electric dust collecting device and an air conditioner having the same are disclosed. The electric dust collecting device includes a first filter unit including a plurality of discharge electrode plates which are spaced apart from each other and have spaces defined therebetween, a high voltage generator electrically connected to all of the plurality of discharge electrode plates, and a second filter unit disposed downstream of the first filter unit in an air flow direction and connected to a ground to cause corona discharge between the discharge electrode plates and the second filter unit, whereby particles electrically charged by corona discharge are collected at the second filter unit. The first filter unit has a simplified structure, and cleaning effect is improved.

Abstract: An electrically enhanced air filtration system is disclosed which uses rear fiber charging is disclosed. In particular, a fibrous filter may be placed in an upstream position within the system with one or more ionization arrays positioned downstream or to the rear of the fibrous filter in terms of airflow direction. The fibrous filter may include a grounded side and fiber side with the grounded side being upstream of the fiber side. The ionization array may include a plurality of electrodes each extending unidirectionally toward the fibrous filter.

Abstract: An electrostatic precipitator having a collecting electrode plate assembly, including at least two electrode plates disposed substantially in a parallel to each other in the vertical plane within the electrostatic precipitator, forming a space between the collecting electrode plates, and a discharge electrode assembly interposed in said spaces, wherein the electrode assembly passing at least a supporting structure of the collecting electrode plate assembly. The supporting structure is provided with an electrical screening device at least in the area of the supporting structure facing said electrode assembly.

Abstract: The invention is an air pollution control apparatus for cleaning exhaust gases from motor vehicles using both electrostatic precipitation and a non-thermal plasma. The precipitator is especially useful in cleaning exhaust gases from diesel engines. The precipitator provides for easier cleaning in between uses, as well as a more efficient cleaning of exhaust gases. The air pollution control apparatus of the present invention is configured as two pass system whereas each pass is designed as multistage horizontal plate type electrostatic precipitator. The first pass applies a direct current, consistent voltage for particle collection through electrostatic precipitation. The second pass uses spiked voltage, non-thermal plasma generation for the production of free radicals to oxidize toxic gases, and with the addition of a diesel oxidation catalyst to transform harmful molecular compounds in the exhaust gas into safe gases.

Abstract: An electric precipitator to collect contaminants, such as dust, using electrical attraction. The electric precipitator includes high-voltage electrode plates and low-voltage electrode plates alternately stacked to form an electrification region and a collection region in an air flow direction, wherein each of the high-voltage electrode plates includes a discharge electrode to generate discharge between the discharge electrode and an opposite electrode so that contaminants are electrified in the electrification region and a collection electrode disposed over the electrification region and the collection region to collect the electrified contaminants in the collection region.

Abstract: An electric precipitator includes a charge unit disposed at an upstream part and a dust collection unit disposed at a downstream part, the charge unit includes charge electrodes and a discharge wire disposed between two neighboring charge electrodes and separated from the charge electrodes, the dust collection unit includes high voltage electrodes, front ends of which are opposite to the charge unit, and low voltage electrodes, front ends of which are opposite to the charge unit and which alternate with high voltage electrodes, and the front ends of high voltage electrodes protrude toward the charge unit as compared to the front ends of low voltage electrodes, thereby guiding electrons to the discharge electrodes due to an electric field formed between the front ends of the high voltage electrodes and the discharge electrodes and thus reducing current leakage through the low voltage electrodes.

Abstract: The invention relates to a system wherein the fluid stream from an object to be evacuated is filtered before it enters into the vacuum pump with the maximum purity to the level of 99% and up to the particle size of 0.3 microns with minimum pressure difference thereby improving the life cycle of pump and performance of system.

Abstract: An electrostatic precipitator is provided with a ionizing unit and a collecting unit placed on the downstream side of the ionizing unit, and the ionizing unit has discharge electrodes each of which generates a corona discharge and a ground electrode plate connected to the earth, and in this structure, the ground electrode plate is provided with an insulating substrate, a resistor formed on the surface of the insulating substrate and a conductive section that is electrically connected to the resistor on the surface of the insulating substrate, and the discharge electrodes face the resistor of the ground electrode plate at a predetermined interval.

Abstract: Disclosed herein are an electric precipitator, which efficiently charges dust particles without lowering dust collecting efficiency, although a velocity of an air current is not uniform, and an air cleaner having the same. The electric precipitator includes a charger unit including at least two charge cells to charge dust particles contained in air, and a collector unit to collect the dust particles charged by the charger unit. The at least two charge cells include a first cell, through which the dust particles pass at a first velocity, and a second cell, through which the dust particles pass at a second velocity different from the first velocity.

Abstract: An electrostatic precipitator including a charger to charge dust particles in air and a collector to collect the dust particles. The collector includes a collector case including high-voltage electrodes, to which high-voltage is applied, low-voltage electrodes alternately stacked with the high-voltage electrodes so as to be grounded, and first electrode support elements to support the high-voltage and low-voltage electrodes with a distance therebetween. The first electrode support elements include electrode contact terminals to support extreme edge portions of the high-voltage and low-voltage electrodes. The high-voltage and low-voltage electrodes are formed of a conductive material, or a non-conductive material, the surface of which is subjected to conductive treatment. The electrode contact terminals for the high-voltage electrodes are formed of a semiconductive material.

Abstract: An electrostatic precipitation air cleaner to reduce ozone output is provided. The electrostatic precipitation air cleaner includes a housing with an air inlet and outlet. Located in the housing are an air mover for moving a stream of air along an airflow path between the inlet and the outlet, an ion emitter electrode positioned in the airflow path downstream of the inlet for ionizing particulates entrained in the stream of air, a collector electrode having an inlet downstream of the ion emitter electrode, and an intermediate element intermediate the ion emitter electrode and the collector electrode. The collector electrode is comprised of a plurality of collector plates spaced apart in a direction transverse to the airflow path. The plates are electrically biased to create and maintain an electric field in the space therebetween to precipitate ionized particulates entrained in the stream of air onto a confronting surfaces of the plates.

Abstract: An electric precipitator prevents dielectric breakdown by ensuring the dielectric distance among a plurality of electrodes. The electric precipitator includes a charging section for charging dust particles in air, and a collecting section for collecting the dust particles charged by the charging section. The collecting section includes a high voltage electrode having a conductive layer coated with a dielectric layer, and a low voltage electrode having at least one protrusion that maintains a gap between the high voltage electrode and the low voltage electrode. The conductive layer includes at least one cutting part formed in an area corresponding to the protrusion.

Abstract: The invention is an air pollution control apparatus for cleaning exhaust gases from motor vehicles using both electrostatic precipitation and a non-thermal plasma. The precipitator is especially useful in cleaning exhaust gases from diesel engines. The precipitator provides for easier cleaning in between uses, as well as a more efficient cleaning of exhaust gases. The air pollution control apparatus of the present invention is configured as two pass system whereas each pass is designed as multistage horizontal plate type electrostatic precipitator. The first pass applies a direct current, consistent voltage for particle collection through electrostatic precipitation. The second pass uses spiked voltage, non-thermal plasma generation for the production of free radicals to oxidize toxic gases, and with the addition of a diesel oxidation catalyst to transform harmful molecular compounds in the exhaust gas into safe gases.

Abstract: This device for inactivating and finely filtering viruses and microorganisms in a flow of air is intended for cleaning air or gas. The device comprises a high voltage power supply (4) and the following components arranged in series in the direction of flow: a means (1) for pre-treating a flow of air, a two-section inactivation chamber (2) and a precipitator (3). The pre-treating means (1) consists of oppositely charged conductive filtering elements (7, 7?). The first conductive filtering element (7) in the direction of flow is in the form of a cylindrical electrode (6) and a plate (9) which is made of a current conductive, highly porous material and is positioned at a distance from the free end face of the cylindrical electrode (6). A needle electrode (8), electrically associated with the plate (9), is adjacent to said plate.

Abstract: In an electrostatic precipitator for filtering particulate matter from a gas, a fan (10) draws the gas (5) through an ioniser section (7) and a collector section (6). Ioniser blades (3) with sawtooth spikes (8) create a corona discharge (4) when charged to a high voltage, so that gas-borne particles are charged when passing through the corona area. The collector section (6) consists of a set of metallic plates (1, 2) of which every other plate (1) is connected to ground, while the remaining plates (2) receive a charge by induction from the ioniser blades (3), and act as repeller plates, pushing charged particles in the gas flow (5) over to the grounded plates (1).

Abstract: Particulate emission reducing devices reduce particulate emissions from a vehicle. A housing is attached to a vehicle's exhaust pipe downstream of its muffler by an intake funnel having a desiccant bed and check valve. An ionizer is connected to the intake funnel. A collection unit is removably inserted into the housing through a collection unit opening with support bar notches in the housing's top downstream of the ionizer. Contacts in the bottom of the housing negatively charge collector plates, which collect positively charged particulates from the exhaust stream. Exhaust gases are emitted by an exhaust funnel attached to the housing downstream of the collection unit. A handle attached to the access panel of the collection unit facilitates removal of the collector plates for cleaning. The vehicle may include an indicator light to remind the user to clean the collector plates. The vehicle's trunk may include an access opening.

Abstract: An air purifier includes a housing, a high voltage power supply, a first electrode assembly in which a wire-like first electrode is either the only first electrode or, alternatively, is spaced sufficiently far from any other such first electrodes so as to avoid undesirable effects upon each other, and a second electrode assembly in which there are a plurality of blade-like second electrodes. The air purifier can be of the type in which air flows through the housing as a result of electro-kinetic effects. To increase air flow velocity, the voltage between first and second electrodes is relatively high, such as 23-50 kV, and the first and second electrodes are accordingly spaced apart a relatively great distance, such as at least 30 mm.

Abstract: An electrostatic dust collector includes a dust charging section and a dust collecting section. The dust charging section includes: a discharge electrode which is made up of a plurality of paralleled discharge wires, and a counter-electrode which is made up of a metal honeycomb net. The dust collecting section includes: positive metal electrodes and negative metal electrodes disposed alternately, and insulating spacers disposed between each of positive metal electrode and negative metal electrode.

Abstract: An electrostatic fluid accelerator includes an electrode array including an array of corona discharge electrodes and an array of accelerating electrodes. A detector is configured to sense a constituent component of the fluid as present in an output from the electrode array. A control circuit supplies power to the electrode array, the control circuit being responsive to an output from the detector for operating the electrode array responsive a level of the constituent component.

Abstract: A system useful for electrostatic deposition (ESD) of aerosol particles and methods of depositing the aerosol particles onto a substrate are disclosed. The ESD system and the method of the present invention are useful for electrostatically depositing nanoparticles produced by gas-phase synthesis, using an induction particle generator, onto a substrate. Direct current may be used with minimized corona leakage in the system, which would otherwise be damaging to the induction particle generator.

Abstract: A portable air cleaner having a vertical orientation with a small footprint that uses an electrostatic precipitator to remove small particles from the air in combination with forced air for increased air flow and improved particle removal.

Abstract: A flexible bag for receiving flowable material is suspended, as from a gantry, and is tethered transversely and yet has substantially free movement along three sides for complete filling. The length of the tethers may be selected to prevent complete collapse of the bag when empty, and tubular perforated members having an “H” configuration within the bag may be provided for this purpose. Auxiliary structures provide for cooling of materials provided to the bag, their diffusion on inlet to the bag, their recirculation in the bag, and their removal from the bag. Accessories for use with wet gas provide for separation of inlet liquid and drainage of condensate within the bag.

Abstract: An electrostatic air cleaning device includes an array of electrodes. The electrodes include corona electrodes connected to a suitable source of high voltage so as to generate a corona discharge. Laterally displaced collecting electrodes include one or more bulges that have aerodynamic frontal “upwind” surfaces and airflow disrupting tailing edges downwind that create quite zones for the collection of particulates removed from the air. The bulges may be formed as rounded leading edges on the collecting electrodes and/or as ramped surfaces located, for example, along a midsection of the electrodes. Repelling electrodes positioned between pairs of the collecting electrodes may include similar bulges such as cylindrical or semi-cylindrical leading and/or trailing edges.

Abstract: Electrostatic precipitator (ESP) systems and methods are provided. A system includes at least one corona discharge electrode and at least one collector (and likely, at least a pair of collector electrodes) that extend downstream from the corona discharge electrode. An insulated driver electrode is located adjacent the collector electrode, and where there is at least a pair of collector electrodes, between each pair of collector electrodes. A high voltage source provides a voltage potential to the at least one of the corona discharge electrode and the collector electrode(s), to thereby provide a potential different therebetween. The insulated driver electrode(s) may or may not be at a same voltage potential as the corona discharge electrode, but should be at a different voltage potential than the collector electrode(s).

Abstract: An electrostatic precipitator includes a frame, several parallel square metallic rods securely disposed in the frame, and several metallic boards positioned parallel in the frame for charges to be deposited on, and spacing elements; the metallic boards have through holes thereon, and the square metallic rods are passed through corresponding through holes of the metallic boards; the spacing elements are in the shape of a tube having a lengthwise extending gap, and they are positioned one on top of another around a whole length of the metallic rods for spacing the metallic boards equidistantly apart with; the spacing elements are flexible and resilient so as to tightly embrace the metallic rods when they are fitted around the rods.

Abstract: An electrostatic precipitator includes several dividable supporting rods, which are passed through corresponding big holes, and tightly fitted in corresponding small holes of metallic boards so as to support the metallic boards equidistantly spaced apart in position. Each supporting rod is made of several connectable parts, each of which has an insertion end portion, and a hollow holding end portion. Each supporting rod is assembled by means of joining insertion end portions to adjacent hollow holding end portions, and is apart from the edges of corresponding big holes of the metallic boards. Thus, the metallic boards are firmly held in position by means of tops of hollow holding end portions of those connectable parts that are joined to small through holes thereof.

Abstract: A laminar flow, wet electrostatic precipitator (ESP) with planar collecting electrodes preferably made of membranes, such as a woven silica fiber. The collecting electrodes are spaced close to planar discharge electrodes to promote laminar flow (Re<2300). Charging electrodes are positioned upstream of the wet ESP to charge the particulate entering the wet ESP to promote collection. The wet ESP is preferably downstream from a conventional turbulent dry ESP for collecting a substantial portion of the larger particulate in the gas stream prior to the gas stream entering the wet ESP.

Abstract: A dust is electrostatically charged using an ion-releasing device adapted to release only ionized air with occurrence of a corona discharge, thereby reducing the amount of power consumed and the amount of ozone generated to the utmost. An electric dust collector includes a charging section having a discharging electrode and an earthed electrode, a dust-collecting section having a voltage-applied electrode and an earthed electrode and an air feed fan. Dust introduced into the dust collector is electrostatically charged by breaking the air insulation by a corona discharge occurring in the charging section to produce ionized air and then removed in the dust-collecting section where an electric field is formed. However, because the corona discharge is generated, there is a problem that the discharged current is large, and the amount of power consumed and the amount of ozone generated are large.

Abstract: An electric dust collecting unit disclosed herein includes a corona discharging portion for giving rise to corona discharge to electrically charge cigarette smoke particulates, toner, and other floating particulates in air, an electric collecting portion disposed on a leeward side of this corona discharging portion, for electrically collecting the floating particulates charged at the corona discharging portion, and a box-type frame with its opposite surfaces opened, in such a configuration that the corona discharging portion has a plurality of needle electrodes which is disposed in such a way that their respective needle tips may be directed toward the leeward side (that is, with their backs directed toward a front face opening in the box-type frame).

Abstract: An electrostatic air cleaner includes a corona charging section and a precipitation section. The charging section includes a first and a second array of substantially parallel earth wires, each array being disposed in a respective plane substantially perpendicular to the direction of air flow, and a third array of substantially parallel corona wires sandwiched between the first and second arrays. With this design, the spacing between earth wires and the spacing between corona wires can be selected independently to obtain the most favorable corona discharge conditions. The arrangement also enables easy cleaning of the earth wires.

Abstract: Germanium needles as emitter electrodes for use in low particle generating gas ionizers in general and static eliminators in particular. They oxidize with minimal incorporation of nitrogen, and the negative polarity emitters oxidize at a greater rate than the positive polarity emitters. Negative polarity silicon emitters generate several orders of magnitude greater particle emissions than either positive or negative germanium electrodes. The mean particle size is about 0.015 &mgr;m. No particles were shed from these needles. Lowest particle generation is achieved when germanium electrodes are purged with dry air.

Abstract: A process and apparatus for reducing particulate, nitrogen oxides ("NOx"), sulfur dioxide ("SO.sub.2 "), and mercury ("Hg") emissions from the combustion exhaust of fossil fuel fired plants while producing an end product that is commercially useful, comprising the steps of oxidizing Hg, NOx and SO.sub.2 using a barrier, pulse, corona, or electron beam electrical discharge apparatus (100) to produce HgO and the acids HNO.sub.3 and H.sub.2 SO.sub.4 collecting the HgO, acids and particulates in a wet ESP (120), and separating the particulates from the collected acid mixture, then separating and concentrating the acids for industrial use.

Abstract: An electrostatic precipitator for purifying air from charged particles. The precipitator includes at least a pair of electrode elements (R and A) spaced apart by a gap distance (a). The electrode elements are moisture-proof. At least one of the electrode elements is composed of a high ohmic cellulose material and is coated with a thin moisture-proof layer.

Abstract: In a two-stage, electrostatic precipitator for extracting airborne particles, charged plates are electrically connected to each other and physically separated from each other by ferrite spacers so that an impedance of the spacers limits an amount of arc discharge current that will flow when an arc discharge occurs from one of the charged plates. Ferrite spacers can also be provided to electrically connect and physically separate grounded plates in the precipitator. In addition, aluminum spacers can be provided to adjust the impedance of the series of spacers through which an arc discharge current flows, so that the current is a minimum necessary amount for an arc detection circuit to detect.

Abstract: An electrostatic filter for the separation of particles includes a particle charging unit arranged upstream, including at least one corona wire for the charging of particles and a particle collector unit arranged downstream relative to it. The particles collecting unit includes two groups of plate-like elements (1, 1') arranged essentially parallel between each other and the flow direction through the device at a predetermined distance from each other, wherein a first group of elements (1)is connected to earth (3) and a second group elements (1') is electrically insulated with reference to the device and to the first group of elements (1) and that the elements (1, 1') essentially are made of one or more non-hygroscopic polymer materials with a mass resistivity which preferably lies within the region of 1.times.10.sup.2 .OMEGA.cm to 1.times.10.sup.8 .OMEGA.cm.

Abstract: A two-stage electrostatic filter includes an ionization section which is arranged in an upstream part of a throughflow passage (28) and includes a wire-like corona electrode (31) which is disposed in an ionization chamber (29) and connected to one pole of an electric high voltage source (16). The filter further includes a target electrode (21;37) which is spaced from the corona electrode (31) and connected to another pole of the high voltage source. A capacitator separator (30) is located in a downstream part of the throughflow passage (28) and includes a first and second group of electrode elements (32,33) which are placed side-by-side in spaced-apart relationship. The electrode elements (32) of the first group are placed alternately with the electrode elements (33) of the second group and are adapted to lie on a potential which is different from the potential on which the electrode elements (33) of the second group lie.

Abstract: An air cleaning apparatus, especially for cleaning of room air, includes an ionizing device (14) having a unipolar ion source formed by a corona discharge electrode, an electrostatic precipitator (12) connected to a high-voltage source (15) and having a flow-through passageway (18) for air to be cleaned and two groups of electrode elements (16, 17) disposed in the flow-through passageway, the electrode elements (16) of one group being interleaved with and spaced from the electrode elements (17) of the other group and arranged to be at a potential different from that of the said other group. The corona discharge electrode (21) is arranged such that the ions generated at the electrode can diffuse essentially freely away from the electrode and thereby diffuse substantially freely throughout the room in which the ionizing device (14) is positioned.

Abstract: An electrostatic precipitation system (100) utilizes laminar flow of a particulate-laden gas in order to enhance the removal of sub-micron sized particulates. The system incorporates a vertically oriented housing (105) through which the gas flows downwardly therethrough to a lower outlet port (110). The gas, which may be a flue gas enters the laminar flow precipitator (102) through an inlet port (108) for passage through a charging section (104). The charging section (104) imparts a charge to the particulates carried by the flue gas. The flue gas and charged particles then flow to a collecting section (106) which is downstream and below the charging section (104). The collecting section (106) is formed by a plurality of substantially parallel tubular members, each tubular member defining a collecting passage therein.

Abstract: A novel electrostatic precipitator includes an electrostatic collector section with discharge electrodes positioned between pairs of grounded collector electrodes, a gas entry port located upstream of said electrostatic collector section, and a transition section between the gas entry port and said electrostatic collector section into which an aqueous acid gas neutralizing agent is sprayed into a gas stream. An additional collector section may be interposed between the gas entry port and the point where the acid gas neutralizing agent is injected into the gas stream. The collector section may comprise alternating charging and short collection sections in which the grounded electrodes of adjoining charger and collector sections are connected. A liquid spray removes particulates collected on the grounded electrodes of the collector sections.

Abstract: An electrostatic precipitator has a plurality of arrays of spaced parallel dust-collecting electrodes, and at least one array of spaced parallel discharge electrodes, each array of discharge electrodes is arranged between an adjacent pair of arrays of dust-collecting electrodes. Each array of dust-collecting electrodes and each array of discharge electrodes are arranged such that the faces of the dust-collecting and discharge electrodes are parallel to each other, and parallel to the flow direction of a gas to be treated. The spacing between the dust-collecting electrodes of an array of dust-collecting electrodes, and the spacing between the discharge electrodes of an array of discharge electrodes may be sequentially narrowed from the upstream side of the electrostatic precipitator to the downstream side of the electrostatic precipitator.

Abstract: An electrostatic precipitator, and more particularly an electrostatic precipitator having a sets of improved plate type electrodes having opposing surfaces defining collecting and ionization zones therebetween, and with one of the electrodes in each pair of electrodes including a corona source thereon.

Abstract: A light sensitive printing plate comprising a support having provided thereon a light sensitive layer and a matted layer, said matted layer being removable upon development and containing a light absorbing agent possessing an absorption in the spectral region to which the light-sensitive material coated on the light sensitive printing plate is sensitive.

Abstract: A shallow relief printing plate is disclosed having a polymerized layer of less than about 0.020 inch and which includes an array of small protuberances in non-image or background areas to prevent bottoming. Photopolymerizable elements, as well as processing techniques, are also disclosed for making such printing plates.

Abstract: A shallow relief printing plate is disclosed having a polymerized layer of less than about 0.020 inch and which includes an array of small protuberances in non-image or background areas to prevent bottoming. Photopolymerizable elements, as well as processing techniques, are also disclosed for making such printing plates.

Abstract: A support suitable for spin coating which is transparent to near ultraviolet and visible light and has a flat surface to be coated, wherein the periphery of the surface is removed in an amount of at least the excess thickness at the periphery of a coated layer to be formed by spin coating so that the surface of the coated layer at the periphery is not raised as compared to the surface of the coated layer at the center. A mask blank which has a mask layer formed on the support is also disclosed as is a light-sensitive photographic material which has a silver halide emulsion layer formed on the support or the mask blank. The process is also claimed.

Abstract: A channeled photosensitive element is described. In particular, the element has a substantially solid photosensitive, thermoplastic layer having grooves or channels therein, the layer being applied with pressure to a surface, e.g., having raised areas such as in a printed circuit board without entrapping air bubbles.

Abstract: Integral negative-positive diffusion transfer reflection prints are provided with a non-planar, e.g., lenticular, surface through which the transfer image is viewed.

Abstract: A method of producing a surface having a reduced reflectance to electromagnetic radiation in a predetermined wavelength band which comprises arranging on the surface a regular array of protuberances having a height which is not less than one third of the length of the longest wavelength in the band and at a spacing which is less than the length of the shortest wavelength of the band divided by the refractive index of the material of which the protuberances consist.