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High Intensity Discharge
Key Facts
 
How it works
A high intensity discharge (HiD) lamp works by means of a discharge of electricity at high voltage between two electrodes, which results in a bright light being emitted by excited molecules of substances caught in the electrical arc.
 
Light of various colours and intensities at various efficacies can be produced by discharging electricity through the arc tube which contains vapourised metals such as mercury and sodium, at various pressures. The arc tube is contained in an outer envelope where the shape is dependent on the application.
 
Ballasts
Like fluorescent lamps, the characteristics of high pressure sodium, metal halide and mercury vapour HiD lamps require aballast to start and maintain their arcs. Ignitors are additionally required for sodium and metal halide lamps. Furthermore, to compensate blind current when using magnetic ballasts, compensation capacitors must be fitted. Lamps that are not operated within the optimal performance range will not produce proper light output or experience full life. There are several ballast types to provide proper control, but offer differing lamp wattage regulation, voltage dip tolerance, power loss and cost. The method used to initially strike the arc varies: mercury vapor lamps and some metal halide lamps are usually started using a third electrode near one of the main electrodes while other lamp styles are usually started using pulses of high voltage.
 
As well as stabilising the lamp´s operating point, ballasts also influence the lamp´s output and luminous flux, the system´s light output, the service life of the lamps as well as the colour temperature of the light.
 
Electromagnetic or electronic ballasts can be used for high-pressure discharge lamps. Unlike with fluorescent lamps, lamp efficiency is not decisively altered by the use of electronic ballasts. In contrast, electronic ballasts lead to a reduction of the inherent losses and thus to an increase in system efficiency. In addition, electronic ballasts ensure gentle lamp operation, which increases the lamp´s service life.
 
Independent electronic and electromagnetic ballasts have also been developed, which in the form of control gear units then provide special advantages during application.
 
Restrike Time
As high-pressure lamps operate with a start-up phase, the lamp´s full luminous flux will only be reached after this start-up period. In the event of mains interruptions, this start-up time can be prolonged depending on the lamp´s temperature. If an additional source of light is desired or required for this start-up period for safety-relevant applications, it is possible to switch on an auxiliary lamp with the help of a start-up switch.
 
Lamp Family
Typical start-up time
Typical restart time
(mains interruption at lamp operating temperature)
HS
3 min.
5 min.
HI / C-HI
3 min.
10 min.
HM
4-5 min.
4-5 min.
LS
10 min.
5 min.
 
Lamp Replacement
Lamps of different makes are not necessarily interchangeable, either visually or electrically. Compatibility between lamp and control gear should always be checked with the individual manufacturers.
 
Low Pressure Sodium
How it works
Low pressure sodium (LPS) lamps, also known as sodium oxide (SOX) lamps, consist of an outer vacuum envelope of glass coated with an infrared reflecting layer of indium tin oxide, a semiconductor material which allows visible lightwavelengths to pass and reflects infrared back, keeping it from escaping. The lamp has two inner borosilicate glass U-pipes containing solid sodium and a small amount of neon and argon gas http://en.wikipedia.org/wiki/Penning_mixture panlang="EN-US">to start the gas discharge. When the lamp starts (i.e. when the arc strikes) it emits a dim red/pink light to warm the sodium metal and within a few minutes turns into the familiar bright yellow/orange colour, as the sodium metalvaporises.
 
Colour Rendering
 
LPS lamps produce a virtually monochromatic light averaging at a 589.3 nm wavelength (actually two dominant spectral lines very close together at 589.0 and 589.6 nm). As a result, the colours of illuminated objects are not easily distinguished since they are seen almost entirely by their reflection of this narrow bandwidth yellow/orange light.
 
This is close to the maximum sensitivity of the human eye at normal lighting levels, and the efficacy is the highest of all lamp types but with very poor colour rendering.
 
Colour Temperature
 
LPS lamps have a colour temperature of 2,000 Kelvin.
 
Application
LPS lamps are mainly used for exterior applications such as road lighting and security lighting. At low lighting levels such as secondary road lighting the eye response changes and the use of white light sources is replacing SOX lamps particularly in amenity areas and pedestrianised shopping centres. SOX-E lamps give improved efficacy with lower power consumption and SOX-PLUS lamps have a longer life. SOX-E and SOX-PLUS lamps give optimum performance only when used with appropriate control gear.
 
Efficacy
LPS lamps are the most efficient electrically-powered light source when measured for photopic lighting conditions—up to 200 lm/W, primarily because the output is light at a wavelength near the peak sensitivity of the human eye. As a result they are widely used for outdoor lighting such as street lights and security lighting where faithful colour rendition is considered unimportant.
 
Wattage
LPS lamps are available with power ratings from 10 W up to 180 W; however, longer bulb lengths create design and engineering problems.
 
Life
LPS Sodium lamps have an average life of 15,000 to 20,000 hours.
 
Lumen Maintenance
Unlike other lamp types, LPS lamps do not decline in lumen output with age. For example, Mercury vapour HiD lamps become very dull towards the end of their lives, to the point of being ineffective and yet they continue to consume full power. LPS lamps, however do increase energy use slightly (about 10%) towards their end of life.
 
Type
LPS lamps are more closely related to fluorescent than high intensity discharge lamps, since they have a low–pressure, low–intensity discharge source and a linear lamp shape. LPS lamps tend to be available in tubular form.
 
Cap
Cap types tend to be E27 or E40 for the larger wattages
 
Starters
Lamps only require 700V for reliable ignition and electronic starters are therefore compact, simple and relatively inexpensive. Although LPS Lamps can be used with a standard reactor circuit and therefore will require an ignitor, many LPS control gear circuit operate on a leak-transformer ballast system. By default, this generates the 700V sufficient to start the lamp and in this instance an ignitor is not required.
 
Restrike Time
Like fluorescent lamps, LPS lamps they do not exhibit a bright arc as do other HID lamps; rather they emit a softer luminous glow, resulting in less glare. Unlike HID lamps, which can go out during a voltage dip, low pressure sodium lamps restrike to full brightness rapidly.
 
Advantages
One unique property of LPS lamps is that, unlike other lamp types, they do not decline in lumen output with age. As an example, mercury vapor HID lamps become very dull towards the end of their lives, to the point of being ineffective, while continuing to consume full rated electrical use.
 
Disadvantages
LPS lamps do increase energy usage slightly (about 10%) towards their end of life, which is generally around 18,000 hours for modern lamps. The obvious disadvantage with LPS lamps is the yellow/orange monochromatic light output.
 
Key Properties
 
Standards
BS EN 62035: Discharge lamps (excluding fluorescent lamps). Safety specifications
BS EN 60192: Low pressure sodium vapour lamps. Performance specification
 
High Pressure Sodium
 
How it works
The light is generated by an electrical discharge in a gas containing sodium and mercury (sodium amalgam), contained in an arc-tube. Because of the extremely high chemical activity of the high pressure sodium arc, the arc tube is typically made of translucent aluminum oxide. 
 
Xenon at a low pressure is used as a "starter gas" in the HPS lamp. It has the lowest thermal conductivity and lowest ionization potential of all the non-radioactive noble gases. As a noble gas, it does not interfere with the chemical reactions occurring in the operating lamp. The low thermal conductivity minimizes thermal losses in the lamp while in the operating state, and the low ionization potential causes the breakdown voltage of the gas to be relatively low in the cold state, which allows the lamp to be easily started.
 
High pressure sodium (HPS) lamps are smaller than LPS lamps. They produce a dark pink glow when first struck, and a pinkish orange light when warmed. Some lamps also briefly produce a pure to bluish white light in between. This is formed by the mercury glowing before the sodium is completely warmed.
 
The higher vapour pressure results in a broader spectrum of colour being emitted and hence colours of objects under these lamps can be distinguished.
 
HPS lamps are known as SON lamps – SON is the variant for "Sun". Mercury-free lamps are available and provide similar performance to equivalent existing standard ILCOS S lamps.
 
Twin arc tube lamps are also available which extend lamp life and provide more rapid hot restarting. However as the arc tubes are off the lamp central axis this may alter the light output and distribution in some luminaires.
 
Colour Rendering
HPS lamps would be specified in areas where good colour rendering is important, or desired. Low wattage ILCOS SM and SH lamps operate at a higher sodium pressure. They are designed for display lighting and have significantly bettercolour rendering (CRI 85/Group 1B) but with reduced efficacy and life.
De Luxe (Comfort) versions have improved colour rendering (CRI 65/Group 2) but give slightly lower light-output.
 
Colour Temperature
LPS lamps have a colour temperature of 2,000 Kelvin. A variation of the high pressure sodium, the White SON, introduced in 1986, has a higher pressure than the typical HPS/SON lamp, producing a colour temperature of around 2700 K, with a CRI of 85; greatly resembling the colour of an incandescent light.[2] These are often indoors in cafes and restaurants to create a particular atmosphere. However, these lamps suffer from higher purchase cost, shorter life, and lower light efficiency.
 
Application
HPS lamps are used for road lighting, for floodlighting and industrial interior lighting. They also have some commercial applications, e.g., for sports halls and public concourses. Standard versions offer high efficacy and long life. Understanding the change in human colour vision sensitivity from photopic to mesopic and scotopic is essential for proper planning when designing lighting for roads. HPS Lamps are favoured by indoor-growers for general growing because of the wide colour-temperature spectrum produced and the relatively efficient cost of running the lights.


 
Efficacy
High pressure sodium lamps are quite efficient—about 100 lm/W—when measured for photopic lighting conditions. ‘Super’ or ‘Plus’ versions, for exterior and industrial applications, have a significant increase in light output and lumen maintenance compared with Standard Low Pressure Sodium lamps.
 
Wattage
Lamps range from 35W up to 1000W and are all applicable for use with Single Phase supplies. Lamps of 100W and below are also available for use on 110V supplies. From 600W and above, lamps are also available for use on Cross (X) Phase supplies.
 
Life
The rated life of HPS lamps varies depending upon wattage and also whether lamps are standard or High Output types. For standard and High Output lamps; 50W and 70W life figures up to 28,000 hours can be achieved and for lamps 150 – 600W life is increased to 32,000 hours. However, it must be noted that higher wattage/High Output lamps have a slightly reduced life by some 3,000 hours. A 1000W High Output can typically achieve 18,000 hours.
 
HPS Lamps which are designed to offer a better colour rendering Ra have a much reduced life reaching approx 24,000 hours.
 
Lumen Maintenance
Wattage and design greatly influence lumen maintenance. High Output lamps trade their increase in lumens for a reduction in Lumen maintenance. For lamps between 150W and 400W expect 90% with the 50W and 70W following close behind at 88% and the 600W at 85%.
 
For High Output lamps we can expect 80% for the 50W and 70W with 85% for the 150W – 400W range reducing further to 80#% for the 1000W. Finally high Ra lamps are reduced 78%
 
 
Type

HPS sodium lamps tend to be:
  • Elliptical
  • Tubular
  • Double Ended
  • Reflector
Cap
Cap types tend to be E27 or E40 for the larger wattages
 
Starters
SON lamps are available with 2 methods of starting:
  • lamps with an internal ignitor are marked
  • lamps requiring an external ignition device are marked
There is also a range of ‘plug-in’ high pressure sodium lamps designed to replace high pressure mercury lamps with ballasts, which comply with BS EN 60922/923. (Some ballasts may not have adequate insulation between windings.) Small changes may be required to ballast tapping, values of PF capacitor, or to some wiring. Reference should be made to the technical literature of lamp manufacturers.
 
Restrike Time
The re-strike time for all HID lamps is based solely on the technology of the lamp and the time taken to for the lamp cool down to a sufficiently low temperature that then enables the lamp to be restarted. Normally for an HPS lamp in an open fixture such as a High-Bay luminaire this will be less than a minute. Additional time should be allowed when lamps are embodied in a heavy case Floodlight, for example.
 
Advantages
HPS lamps are very much a commodity light source, readily available from numerous sources and manufacturers. They have an established history of reliability and offer long life and good lumen maintenance. They are reasonably simplistic in their design and operate over a wide temperature range.


 
Disadvantages
Although considered by some as an acceptable light source in terms of colour temperature and colour rendering, HPS has a narrow spectral distribution and therefore visual acuity is poor. Lamps can cycle towards end of life.
 
Key Properties
 
Standards
BS EN 62035: Discharge lamps (excluding fluorescent lamps). Safety specifications
BS EN 60662: Specification for high-pressure sodium vapour lamps
 
High Pressure Mercury
How it works
A  high pressure mercury discharge lamp operates in a quartz arc tube. MBF(HPL-N, HQL) lamps have an outer ellipsoidal bulb with an internal phosphor coating, which improves the colour rendering. MBFR (HPL-R, HQL-R) lamps have a shaped outer bulb with an internal reflector coating.
 
Note: MBTF (ML,HWL) is a mercury discharge tube connected in series with a tungsten filament in the same outer bulb: external control gear not required.
 
Colour Rendering
De Luxe versions with improved colour rendering have a special phosphor coating.
 
Colour Temperature
Standard HPMV lamps have a nominal colour temperature of 4000K, where other blended or De Luxe lamps have a colour temperature circa 3400K.
 
Application
Mercury lamps were used for illuminating road signs and industrial lighting but have largely been replaced by more efficient lamps now available. They are more predominantly used in mainland Europe but will soon be replaced as part of EuP Directive.
 
Efficacy
Mercury lamps offer low cost discharge lighting where high efficacy is not important. The lamps incorporate a third electrode for starting and so the control gear to operate mercury lamps is only a ballast and power factor correction capacitor. No external ignitor is required.
 
Wattage
Standard mercury lamps are available in 50W, 80W, 125W, 250W, 400W, 700W and 1000W wattages. Blended mercury lamps tend to be 160W, 250W and 500W. All lamp types are suitable for Single phase supply with the 700W and 1000W also available for use on a Cross-Phase supply.
 
Life
The life of HPMV ranges from 12,000 hours for the lower wattage 50W and 80W with the remaining wattages being 15,000. Blended lamps are only 8000 hours.
 
Lumen Maintenance
HPMV lamps have inherently poor lumen maintenance; 50% is normal with some lamps dropping to 40%.
 
Type
HPMV lamps are always coated and therefore will be elliptical in shape. There is also a range of reflector lamps.
 
Cap
HPMV lamps utilise standard E27 and E40 bases although a tri-pin bayonet cap is also used on lower wattages. This B22d-3 lamp cap is similar to the standard household incandescent lamp cap but has three pins equally spaced at 120 degrees apart, preventing its use directly into a mains circuit.
 
Starters
By the nature of their design HMPV lamps do not require any starting aids or ignitors and successfully start up on their own.
 
Ballasts
Ballasts (often called Chokes) are part of the operating “gear” for Mercury "Discharge Lamps". There is no one ballast solution for metal halide lamps - they either run on Sodium gear or Mercury gear so it is imperative to match the ballast with the corresponding  lamp.
 
Restrike Time
A hot lamp will take between 4 and 7 minutes depending upon the thermal mass of the luminaire.
 
Advantages
HPMV lamps are cheap, readily available and do not require a starting device. They are normally very tolerant of mains supply variations with regards to life. Although they do have an average life rating, some models can in some circumstances appear to operate for many years.
 
Disadvantages
Very poor light output and poor lumen maintenance.
 
Key Properties
 
Standards
BS EN 62035: Discharge lamps (excluding fluorescent lamps). Safety specifications
BS EN 60188: High-pressure mercury vapour lamps. Performance specifications
 
Metal Halide Lamps
How it works



 
Metal Halide lamps are the most advanced high pressure discharge lamps. They have quartz or sintered alumina (ceramic) arc-tubes. Most lamps have an outer glass bulb. Like other gas-discharge lamps, metal halide lamps produce light by passing an electric arc through a mixture of gases. In a metal halide lamp, the compact arc tube contains a high-pressure mixture of argon, mercury, and a variety of metal halides. The mixture of halides will affect the nature of light produced, influencing the correlated colour temperature and intensity (making the light bluer, or redder, for example).
 
The argon gas in the lamp is easily ionized, and facilitates striking the arc across the two electrodes when voltage is first applied to the lamp. The heat generated by the arc then vaporizes the mercury and metal halides, which produce light as the temperature and pressure increases. Common operating conditions inside the arc tube are 70-90 PSI (480-620 kPa) and 1090 °C.
 
Lamps with very low ultra violet output have now been introduced which incorporate UV absorbing quartz. These do not require external UV filters on the luminaires. Metal halide lamps of the ‘protected’ type are now available for operation in luminaires without safety screens. Fragments from a shattered lamp are prevented from leaving theluminaire, either by suppressing the violence of the exploding arc-tube by the inclusion of an open-ended quartz tube surrounding the arc-tube, or by using a PTFE coating on the outer bulb to maintain the integrity of the lamp in the event of a shattered arc-tube.
 
Compact Metal Halide lamps invariably utilize ceramic technology. Ceramic arc tubes are more stable than quartz and offer improved life, efficacy and colour rendering.
 
Like all other gas discharge lamps, metal halide lamps require auxiliary equipment to provide proper starting and operating voltages and regulate the current flow in the lamp.
 
About 24% of the energy used by metal halide lamps produces light (65-115 lm/W), making them generally more efficient than fluorescent lamps, and substantially more efficient than incandescent bulbs.
 
Colour Rendering
Depending on the mix of elements, there is a wide range of efficacy and/or colour appearance.
  • Elliptical or Tubular Quartz lamps are generally good at 68-70 Ra
  • Compact Ceramic Metal Halide lamps can be as high as 95+ Ra
 
Colour Temperature
Typical colour temperatures would be in the range 3,700 – 4,000K, although some lamps can be up to 10,000K. ‘Ceramic’ arc-tube metal halide lamps have improved colour stability throughout their life. Colour temperature variance is seen greatest in "probe start" technology lamps (±300 K). Newer metal halide technology, referred to as "pulse start," has improved colourrendering and a more controlled variance (±100 to 200 K).
 
Application
 
Metal halide lamps are mainly used in commercial interiors, industry and floodlighting, as well as for colour TV lighting in stadia and studios. Smaller ratings are used for retail lighting.
 
Metal halide lamps, which can be retrofitted into high pressure sodium lamps installations, are specifically manufactured to be dimensionally and electrically compatible with the replaced lamp. It is relatively easy to replace HPS lighting with Metal Halide lamps as most Metal Halide lamps can operate directly from existing HPS control gear. Even ‘difficult’ 70W HPS lamps with internal starters can be directly replaced, using a simple conversion kit.

Efficacy
About 24% of the energy used by metal halide lamps produces light (65-115 lm/W), making them generally more efficient than fluorescent lamps, and substantially more efficient than incandescent bulbs.
 
Wattage
Due to the broad range and types available, metal halide lamps are available in 20W up to 3,500W.
 
 
Life
Metal Halide lamps tend to average life of 10,000 – 15,000 hours
 
Lumen Maintenance
Dependant on the various technologies that are available to produce a metal Halide lamp; maintenance figures of between 60% and 90% are possible - the latter associated with the latest technology lamps.
 
Type
Metal Halide lamps have been developed to suit many applications both internal and external. Consequently, a variety of shapes are available:-
  • Elliptical
  • Tubular
  • Double Ended
  • Compact Pin ended and Reflector
Cap
Metal Halide Lamps have an E27 or E40 screw in Edison cap.
 
Finish/Coating
Metal Halide Lamps are available in clear or coated glass.
 
Starters
Old technology “standard” MH lamps - typically those originally produce to operate on CWA (Constant Wattage Autotransformer ballasts and lamps with penning mix and other similar technologies - are able to ignite with a relatively low ignition pulse of 600V.
 
High technology lamps (which normally assumes lamps with a higher arc tube pressures such as Ceramic Metal Halide and Quartz) require ignition voltages of 3.0KV although some low wattage lamps only require 1.8kV.
 
 
High wattage Double Ended lamps may require 5-6KV.
 
As with HPS lamps Metal Halide starters are available in different forms:
  • Two Wire low voltage; connecting directly across the lamp,
  • Three wire SIP (superimposed pulse) connecting in series between the ballast and lamp and Impulser,
  • or Semi-Parallel types that utilise tapping points on the ballast to generate the high voltages.
Two wire ignitors are simple and very cost effective but are limited to standard MH lamps.
 
SIP ignitors are universal and can operate with any lamp and ballast. Because the ignitor also conducts the lamp current and the higher the current the greater the component cost, they are normally designed in wattage ranges, say 35W to 100W, 100W to 400W etc.
 
Impulser and semi parallel ignitors do not pass lamp current and one small ignitor can suit many lamp wattages. However, the ignitor and ballast must be designed as a package, thus one manufacturer’s ignitor will only operate correctly with that manufacturer’s ballast.
 
Ignitors can also incorporate timers and lamp cycling counters to shut down the ignitor should a failed lamp remain unattended, thus reducing the stress of applying prolonged high voltage pulses to the remaining control gear.
 
Ballasts
As lamp manufacturer´s reference values regarding lamp current and voltage are generally identical for metal halide (HI) and high-pressure sodium lamps (HPS) of the same lamp wattage and the impedance values required for the ballast are also identical, the same ballasts can frequently be used for both lamp types.
 
HI lamps react sensitively to impedance deviations from the rated value with appreciable colour changes. Modern ballasts therefore comply with the lamp´s narrower tolerances. Moreover, ballasts remain below the maximum peak DC value for HI lamps. This value is not specified for HS lamps; instead, the maximum stated start-up current must not be exceeded.
 
Restrike Time
A "cold" (below operating temperature) metal halide lamp cannot immediately begin producing its full light capacity because the temperature and pressure in the inner arc chamber require time to reach full operating levels. Starting the initial argon arc sometimes takes a few seconds, and the warm up period can be as long as five minutes (depending upon lamp type). During this time the lamp exhibits different colours as the various metal halides vaporize in the arc chamber.
 
If power is interrupted, even briefly, the lamp's arc will extinguish, and the high pressure that exists in the hot arc tube will prevent re-striking the arc; a cool-down period of 5-10 minutes will be required before the lamp can be re-started. This is a major concern in some lighting applications where prolonged lighting interruption could create manufacturing shut-down or a safety issue. A few metal halide lamps are made with "instant restrike" capabilities where the lamp, ballast and socket are built to withstand the 30,000 volt re-ignition pulse supplied via a separate anode wire.
 
Performance
 
The colour temperature of a metal halide lamp can also be affected by the electrical characteristics of the electrical system powering the bulb and manufacturing variances in the bulb itself. If a metal halide bulb is underpowered it will have a lower physical temperature and its light output will be 'cooler' (more blue, or very similar to that of a mercury lamp).
 
This is because the lower arc temperature will not completely vaporize and ionize the halide salts which are primarily responsible for the warmer colours (reds, yellows), thus the more-readily ionized mercury will dominate the light output. This phenomenon is also seen during warm-up, when the arc tube has not yet reached full operating temperature and the halides have not fully vaporized.
 
The inverse is true for an overpowered bulb, but this condition can be hazardous, leading possibly to arc-tube rupture due to overheating and overpressure. Moreover, the colour properties of metal halide lamps often change over the lifetime of the bulb. Often, in large installations of MH lamps, particularly of the quartz arc-tube variety, it will be seen that no two are exactly alike in colour.
 
Advantages
MH lamps are an excellent white light source. They are readily available from numerous sources and manufactures, have an established history of reliability and offer reasonably long life and reasonable lumen maintenance. Primarily over other HID sources they have an excellent CRI (Colour Rendering Index) or Ra of more than 60 with some technologies reaching 90.
 
Latest design technologies are now offering increasing life and lumen maintenance figures approaching some early HPS lamps.
 
Disadvantages
Depending on the technology, some MH lamps can be expensive. Arc tubes can rupture at end of life making them only suitable for enclosed luminaires, although protected lamps are available.
 
Lamps can cycle at towards end of life.
 
 
Key Properties

 
Standards
BS EN 62035: Discharge lamps (excluding fluorescent lamps). Safety specifications
BS EN 61167: Specification for metal halide lamps
 
New Developments
New, smaller electronically controlled MH lamps are now being developed which can be a building block for EuP compliant systems. Rather than focusing on the light source alone, manufacturers are concentrating on a coupled system where lamp and ballast together can offer significant technological advantages over the existing lighting package.
 
The lamps are compact in design and offer a pure white light which brings benefits for the luminaire designers and the client, whilst their operation only via an electronic ballast offers superior performance, energy savings and life.
 
Manufacturers are currently working with both ceramic and quartz technology.
 
These next generation compact MH lamps enable highly effective white-light street illumination. By producing brighter, warmer white light, they improve the appearance of streets and other urban features and, in the process, provide a safer, more secure environment for residents to enjoy.
 
Not only do they transformed the ambience of municipal locations, they can reduce energy costs by more than 50% verses traditional high pressure mercury or sodium outdoor lighting.

 
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