IMPROVEMENTS IN PHOTOPRINTING LAMPS

Abstract: The invention relates to discharge lamps and arc tubes therefor for the photoprinting industry. The lamp has an arc tube (12) enclosed in a sealed beam unit (11, 13) with a dichroic or aluminised layer selected to pass UV as the surface of the reflector thereof. The arc tube has a fill including Gallium and iron iodides to give a radiation spectrum especially suited to photoprinting.

Description: IMPROVEMENTS IN PHOTOPRINTING LAMPS This invention relates to discharge lamps, primarily but not exlusively for the photoprinting industry. In the photoprinting industry there is a requirement to expose diazo and photopolymer printing plates, which are then used to transfer ink to the printing paper. The former (diazo) requires a lamp having a spectral energy distribution which peaks at approximately 420nm while the latter material requires a lamp having a spectral energy distribution peaking at approximately 380nm. At the present time these different requirements are met by two different lamps each with a fill designed to produce radiation of the desired wavelength. For example a lamp with a fill of 1.3 mg of gallium tri iodide would be would be used for diazo prints while a lamp with a fill of 2.5mg of iron iodide would be used for the polymer material, each lamp of course would have the necessary fill of mercury. Metal halide lamps having in addition to mercury, a mixture of gallium and ferric iodide are disclosed in the article "A metal halide lamp using both gallium and ferric iodides" by Shunji Kondoh, Lighting Research Laboratory, Lamp and Lighting Division, Storage Battery Co. Kyoto, Japan. The lamps disclosed therein are restricted to linear tubular lamps of 1000-5000 watts and arc lengths of between 98 to 200mm. These lamps require some kind of cooling system because of the heat generated. In its broadest aspect the present invention embraces a discharge arc tube having a basic fill of mercury plus between 0.025 to 7.0mug GaI3 per cc of the arc tube volume and between 0.1 to 1.Omg of Fel2 per cc of the arc tube volume. According to one aspect of the present invention there is provided a sealed beam reflector lamp for photoprinting comprising in combination a reflector and a discharge lamp arc tube of quartz material, the reflector including a dichroic or aluminised layer selected to reflect ultra violet radiation and the discharge lamp arc tube including spaced electrodes to sustain a discharge therebetween and a fill selected from: 0.025 to 1.0 mg Gal3 per cc of the arc tube volume, 0.1 to 1,0 mg FeI2 per cc of the arc tube volume, 2.5 to 6.5 mg Hg per cc of the arc tube volume. According to another aspect of the present invention there is provided an arc tube of quartz material for a sealed beam reflector photoprinting lamp, said arc tube including spaced electrodes to sustain a discharge therebetween, the arc tube, in addition to mercury, having a fill including between 0.025 to 1.0 mg GaI3 per cc of the arc tube volume plus between 0.1 to 1 mg of Fel2 per cc of the arc tube volume. By providing the outer jacket with a dichroic layer on the inner rear surface thereof and positioning the discharge arc tube near the focus of the reflector, radiation can be diffused through the prismatic window at the front of the reflector and a substantially uniform directionally controlled beam of light is produced. In addition the dichroic layer reflects W radiation and provides for heat dispersal. Also, the glass outer jacket affords a high degree of protection from short wave UV radiation. The geometry of the discharge arc tube with an electrode length of approximately 25mm or less and a diameter of 26mm allows a high wall loading and a hot restart facility. If a hot re-start facility is not required then an aluminised reflector may be substituted for the dichroic layer. This feature combined with a fast run up facility avoids the need to simmer at a reduced power thus achieving reduced running costs when compared with conventional linear sources for photoprinting. The invention will now be described by way of example only and with reference to the accompanying drawings wherein: Figure 1 shows the combination of a discharge arc tube and sealed beam reflector according to one aspect of the present invention, Figure 2 shows in more detail a discharge arc tube according to a further aspect of the present invention, and Figure 3 shows a graph of irradiance against total iodide dose in accordance with the present invention. In Figure 1, the reference numeral 10 denotes generally a 800 watt sealed beam reflector lamp combination according to the present invention comprising a reflector 11 containing a metal halide quartz discharge arc tube 12. A prismatic lens 13 is sealed to the reflector 11 providing a front window and a dichroic layer 14 is formed on the rear inner surface of the reflector 11. The dichroic layer is selected to reflect W rather than visible light. The discharge arc tube 12 is shown in more detail in Figure 2. The discharge arc tube 12 is disposed adjacent the focus of the reflector 11 and is carried on nickel alloy inner lead in members 15 and 16 joined to outer lead in members 17 and 18 formed of copper braid. A ceramic bridge member 19 maintains the electrically conducting pins 20 and 21 at the correct spacing. This is a standard G38 bi-pin construction providing a hot restart facility. In Figure 2 the discharge arc tube 12 is shown in more detail. It is seen to comprise a quartz envelope 22 of approximately 10cc internal volume having a fill 23 selected from a mixture of iron iodide (FeI2), Gallium Tri iodide (Gal3) and mercury (Hg). Tungsten electrode coils 24, 25 are carried on tungsten shanks 26, 27 which are joined respectively to inner molybdenum foils 28 and 29. Outer molybdenum lead-in members 30, 31 are also joined to the foils 28, 29 and hermetically sealed in the pinches 32, 33. A typical electrode length between the electrodes 24 and 25 would be 25 mm with an overall length over the pinch seals of 78 mm while the maximum envelope nominal diameter would be 26 mm. Such an arc tube 12 would then be suitable for a metal halide discharge lamp. In Figure 3 there is shown a graph of a Fel2 - GaI3 Hg system according to the invention which indicates a peak irradiance at the particular total iodide dose of 2.5mg Fel2 + 0.5mg Gal3 + 50mg Hg in an approximately 10cc volume arc tube. Other values of relative irradiance at different dosage rates are given in the following tables 1 and 2 where all values refer-to arc tubes of approximately lOcc volume. TABLE 1 Relative Ave Example Fe12 GaI3 Hg Irr (No Units) 1 2.5 0.25 50 22.8 2 2.5 0.5 50 24.9 3 2.5 1.0 50 22.1 4 2.5 2.0 - 50 20.8 5 2.5 5.0 50 14.1 6 2.5 10 50 11.7 TABLE 2 Example FeI2 GaI3 Hg Irr 1 1.0 1.2 45 19.99 2 1.5 1.2 43 18.02 3 2.5 1.2 40 19.63 4 4.0 1.2 37 18.12 5 5.0 1.2 35 16.02 6 7.5 1.2 30 12.53 7 10 1.2 25 10.33 In Table 2 the mercury content had to be varied in order to achieve the correct volt drop. In all the tests the UV.A and effective response was measured using a "Macam" W radiometer on a screen positioned 1.0 metre from the source. Readings were taken on a screen 30" x 20" and converted to average W/M2 values. In further tests lamps having arc tubes of approximately 10cc volume with a fill of 2.5mg Fel2 plus 0.5mg GaI3 and 65mg Hg instead of 50mg were tested and found to have an average irradiance of between 46.2 and 48.5 WIM2. Further ranges of fills suitable for approximately 1 0cc arc tubes comprise a) 0.5 to 0.55mg GaI3 plus 2.45 to 2.65mg FeI2 and 60 to 65mg Hg and b) 0.5mg GaI3 plus 2.Smg FeI2 and a quantity of mercury selected from between 50 to 65mg. The examples set out hereinbefore are examples of lamps with arc tubes having specific fills, however, it will be clear to the man skilled in the art that variations in the fills and arc tube size could be made. In general the invention can be utilised in a range of fills comprising 0.025 to 1.Omg Gal3 per cc of arc tube volume 0.1 to 1. Omg FeI2 per cc of arc tube volume 2.5 to 6.5mg per cc of arc tube volume.

Claims: CLAIMS 1. A sealed beam reflector lamp for photo printing comprising in combination a reflector and a discharge lamp arc tube of quartz material, the reflector including a dichroic or aluminised layer selected to reflect ultra violet radiation and the discharge lamp arc tube including spaced electrodes to sustain a discharge therebetween and a fill selected from: 0.025 to 1.0 mg GaI3 per cc of the arc tube volume, 0.1 to 1,0 mg FeI2 per cc of the arc tube volume, 2.5 to 6.5 mg Hg per cc of the arc tube volume. 2. A lamp according to Claim 1 wherein the fill comprises 0.5 to 0.55 mg GaI3 plus 2.45 to 2.65 mg FeI2 plus 60 to 65 mg Hg in an arc tube having an internal volume of substantially 10 cc. 3. A lamp according to Claim 1 wherein the fill comprises 0.5 mg GaI3 plus 2.5 mg FeI2 and mercury between 50 to 65 mg in an arc tube having an internal volume of substantially 10 cc. 4. an are A lamp according to Claim 3 wherein the mercury content is 50 mg. 5. A lamp according to any preceding Claim wherein the distance between the electrodes is substantially 25 mmt 6. A lamp according to any preceding Claim wherein the arc tube is disposed adjacent the primary focus of the reflector. 7. An arc tube of quartz material for a sealed beam reflector photoprinting lamp, said arc tube including spaced electrodes to sustain a discharge therebetween, the arc tube, in addition to mercury, having a fill including between 0.025 to 1.0 mg GaI3 per cc of the arc tube volume plus between 0.1 to 1 mg of FeI2 per cc of the arc tube volume. 8. An arc tube according to Claim 7 including mercury selected from 2.5 to 6.5 mg per cc of the arc tube volume. 9. An arc tube according to Claim 7 wherein the distance between the electrodes is substantially 25 mm or less.

Inventors: PAGE, ROBERT BRIAN
Applicants: THORN EMI PLC
Application year: 1983
Main classification: H01J61
IPC codes: H01J61