GB2211985A - Improvements in plant growth lamps - Google Patents

Abstract

An arc tube (12) for a metal halide discharge lamp (10) suitable for promoting plant growth has a fill which comprises mercury (Hg), inert gas and not less than 1.1 mg in total of metal halides per cm<3> of arc tube volume. The metal halides consist of the iodides of sodium (NaI), Scandium (ScI3) and lithium (LiI), and the iodide of at least one of rubidium (RbI) and potassium (KI). Such arc tubes may be incorporated in sealed beam reflector lamps 10, as shown, which in operation have been found to be particularly useful in the promotion of growth of plants from cuttings and are also believed to be advantageous to the promotion of growth of plants from seed. A number of specific examples of suitable fill compositions are given. It is preferred that RbI alone is added to the iodides of Na, Sc and Li. <IMAGE>

Description

IMPROVEMENTS IN PLANT GROWTH LAMPS This invention relates to discharge lamps, primarily but not exclusively for providing artificial lighting for plants.

There are several physiological processes in plants which are dependent on electromagnetic radiation. One such process is photosynthesis, i.e. the synthesis of organic compounds from water and carbon dioxide in plants containing chloroplasts, which utilises radiation of wavelengths in the general range about 400-700nm (ie the visible spectrum). The wavelength range of radiation that may be utilised in a particular plant physiological process depends on the energy required to effect the chemical changes of the process and hence on the pigments utilised in the process.

Another such process is photomorphogenesis, the control of the development of a plant - which, in certain plants, utilises radiation of wavelengths in the range about 600nm - 700nm. If a plant is grown from seed, the seed being planted in a suitable medium, eg soil or compost, both roots and a stem develop. When the growing tip of the stem emerges out of the medium in which the seed is planted, absorption by the plant of radiation of certain wavelengths in the range about 700nm - 800nm triggers this process of photomorphogenesis.

It is known, eg as disclosed in the article 'Shining Examples for the Gardener' by David Raeside, Electrical Review 16, 29 September 1987, Vol. 220, No. 22, to provide artificial lighting for plants, particularly to overcome the effects of limited daylight during winter months. Growing rooms, which are totally artificially lit, allow light to be controlled completely and create ideal conditions for propagation of seedlings and cuttings.

One type of lamp presently used for horticultural purposes is the high pressure sodium lamp which is an efficient and economical light source with a long useful life. A typical spectral output for a high pressure sodium lamp is shown in Figure 1. As can be seen, such lamps emit radiation of wavelengths primarily in the visible range.

It is also known to use metal halide lamps in horticulture. Metal halide lamps are more expensive than other mercury lamps but also more efficient. The halides can also be chosen so that the characteristics of the radiation produced are close to that of daylight.

One metal halide lamp which has been tested as an artificial light source for plants has a discharge arc tube of internal volume about 10cm3 provided with a fill containing sodium iodide and scandium iodide (NaI/ScI3), lithium iodide (liy), mercury (Hg) and argon (Ar).

Its spectral output, as measured on a computer-controlled spectroradiometer, is shown in Figure 2, from which it can be seen that the majority of the radiation emitted is in the wavelength ranges about 400- 450nm (produced mainly by Hg), about 540-630nm (produced mainly by NaI/ScI3) and about 660-690nm (produced by LiI).

Known lamps have been useful in assisting the growth of seedlings and cuttings. However, it is believed to be advantageous to the promotion of growth of plants from seed to provide radiation of certain wavelengths in the range about 700nm-800nm.

It is accordingly an object of the present invention to provide an improved lamp for promoting plant growth from seed or cuttings.

According to a first aspect of the present invention, there is provided an arc tube for a metal halide discharge arc lamp suitable for promoting plant growth wherein the fill comprises mercury, inert gas and not less than l.lmg in total of metal 3 halides per cm of arc tube volume, the metal halides consisting of the iodides of sodium (NaI), scandium (ScI3) and lithium (LiI) and the iodide of at least one of rubidium (RbI) and potassium (KI).

Such arc tubes in operation produce resonance peaks of radiation of wavelengths 780nm and 795nm if rubidium is used or 766nm and 770nm if potassium is used and have been found to be particularly useful in the promotion of growth of plants from cuttings. Such arc tubes are also believed to be advantageous to the promotion of growth of plants from seed.

Preferably the metal halides consist only of NaI, ScI3, LiI and RbI. Though the intensity of the resonance peak in the 700nm-800nm range is higher when the potassium halide is used than when the rubidium halide is used without potassium halide, it has surprisingly been found that the plants grow better when the arc tube containing rubidium halide without potassium halide is used.

Preferably the fill includes from 0.4mg to 1.8mg in total of Nay and ScI3 per cm of arc tube volume and from 0.4mg to 3 1.8mg per cm of arc tube volume of LiI, and at least one of RbI and KI.

Preferably the fill includes l.lmg in total of NaI and ScI3 per cm of arc tube volume, and 1.1mg per cm of arc tube volume of LiI, and at least one of RbI and KI.

The invention also provides a metal halide discharge arc lamp, preferably a sealed beam reflector lamp, suitable for promoting plant growth comprising an arc tube according to the first aspect of the present invention.

A particular advantage of using a sealed beam reflector lamp having a prismatic lens is the wide distribution of radiation produced for the compact size of the lamp, particularly when the artificial light source is used to supplement rather than replace sunlight. In such a case, sunlight radiation should be sufficient to promote plant growth in the summer months and the artifical light sources would then block out some sunlight radiation. A compact lamp is therefore desirable.

The invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a graph of the spectral output against wavelength of a known high pressure sodium lamp.

Figure 2 is a graph of the spectral power against wavelength of a discharge arc tube having a fill not in accordance with the invention.

Figure 3 shows the combination of a discharge arc tube and a sealed beam reflector according to the present invention.

Figure 4 shows the discharge arc tube of Figure 3 to alarger scale.

Figure 5 is a graph of spectral power against wavelength of a first embodiment of a discharge arc tube having a fill in accordance with the present invention.

Figure 6 is a graph of spectral power against wavelength of a second embodiment of a discharge arc tube having a fill in accordance with the present invention.

Figure 7 is a graph of spectral power against wavelength of a third embodiment of a discharge arc tube having a fill in accordance with the present invention.

In Figure 3, the reference numeral 10 denotes generally a 400W sealed beam reflector lamp combination comprising a reflector 11 containing a metal halide quartz discharge arc tube 12. A prismatic lens 13 is sealed to the reflector providing a front window and an aluminised reflector layer 14 is provided on the rear inner surface of the reflector 11. The discharge arc tube 12 is positioned adjacent the focus of the reflector 11 and is carried on conductive metal inner lead-in members 15, 16 joined to external push on blade terminals 17, 18.

As shown in greater detail in Figure 4, the discharge arc tube 12 comprises a quartz envelope 22 of approximately 10cm3 internal volume. Tungsten electrode coils 24, 25 are carried on tungsten shanks 26, 27 joined respectively to inner molybdenum foils 28, 29. Outer molybdenum lead-in members 30, 31 are also joined to the foils 28, 29 and hermetically sealed in the pinches 32, 33.

The discharge arc tube 12 is provided with a fill asset out in the accompanying examples. The spectral output for each example was measured using a computer-controlled spectroradiometer.

Example 1 The discharge arc tube 12 was provided with a fill having the following ingredients: 11 mg sodium iodide and scandium iodide (NaI/ScI3) 11 mg lithium iodide (LiI) 11 mg rubidium iodide (RbI) 90 mg mercury (Hg) 60 Torr argon (Ar) Its spectral output is shown in Figure 5, from which it can be seen that there is a peak in the range about 780nm - 800nm.

Example 2 The discharge arc tube 12 was provided with a fill having the following ingredients: 11 mg NaI/ScI3 11 mg LiI 11 mg potassium iodide (KI) 90 mg Hg 60 Torr Ar Its spectral output is shown in Figure 6 from which it can be seen that there is a peak in the range about 760nm - 780nm.

Example 3 The discharge arc tube 12 was provided with a fill having the following ingredients: 11 mg NaI/ScI3 11 mg LiI 11 mg KI 11 mg RbI 90 mg Hg 60 Torr Ar Its spectral output is shown in Figure 7, from which it can be seen that there is a peak in the range about 760nm - 800nm.

Example 4 The discharge arc tube 12 was provided with a fill having the following ingredients: 11 mg NaI/ScI3 11 mg LiI 11 mg RbI 90 mg Hg 0.7 mg scandium (Sc) 60 Torr Ar.

In this example extra scandium metal was provided to combine with free iodide atoms which might otherwise be present in the arc tube.

The spectral output of this example was similar to that for Example 1, the anticipated effect of adding surplus scandium being to improve the through life radiation maintenance.

Example 5 The discharge arc tube 12 was provided with a fill having the following ingredients: 11 mg NaI/ScI3 11 mg LiI 11 mg RbI 11 mg KI 90 mg Hg 0.7 mg Sc 60 Torr Ar The spectral output of this example was similar to that for Example 3.

As artificial light sources are used to replace or supplement sunlight, a guide to their suitability is the comparison of the ratio of the power emitted by the light source in each of the wavelength ranges 400-500nm, 600-700nm, and 700-800nm to the same ratio for sunlight. For sunlight and the examples given, the ratio was as set out in Table 1 below.

Table 1 Source Range Range Range 400-500nm 600-700nm 700-800nm Sunlight 1 0.70 0.60 Example 1 1 1.08 0.66 Example 2 1 0.94 0.66 Example 3 1 0.85 1.06 Example 4 1 1.26 0.83 Example 5 1 c 1.0 > 1.0 In tests with plants grown from cuttings, the preferred artificial light source was Example 1 which contained rubidium iodide. This was rather surprising as the spectral output for this example neither had the highest resonance peak in the wavelength range 700-800nm nor had the power ratio closest to that of sunlight as set out in Table 1.

It is to be noted that the addition of rubidium and potassium iodides did not affect the starting characteristics of the lamp.

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.

Claims (8)

1. An arc tube for a metal halide discharge lamp suitable for promoting plant growth wherein the fill comprises mercury (Hg), inert gas and not less than 1.1 mg in total of metal halides per cm3 of arc tube volume, the metal halides consisting of the iodides of sodium (NaI), scandium (ScI3), and lithium (LiI), and the iodide of at least one of rubidium (RbI) and potassium (KI).

2. An arc tube according to Claim 1 wherein the metal halides consist only of NaI, ScI3, Lii and RbI.

3. An arc tube according to Claims 1 or 2 wherein the fill includes from 0.4 mg to 1.8 mg in total of NaI and ScI3 per 3 3 cm of arc tube volume, and from 0.4 mg to 1.8mg per cm of arc tube volume of LiI, and at least one of RbI and KI.

4. An arc tube according to Claim 3 wherein the fill includes 1.1 mg in total of NaI and ScI3 per cm of arc tube volume, 3 and 1.1 mg per cm of arc tube volume of LiI, and at least one of RbI and KI.

5. An arc tube for a metal halide discharge arc lamp suitable for promoting plant growth substantially as herein described with reference to Figures 3 to 7 of the accompanying drawings.

6. A metal halide discharge arc lamp suitable for promoting plant growth comprising an arc tube in accordance with any one of Claims 1 to 5.

7. A sealed beam reflector lamp suitable for promoting plant growth comprising an arc tube in accordance with any one of Claims 1 to 5.

8. A sealed beam reflector lamp suitable for promoting plant growth substantially as herein described with reference to Figures 3 to 7 of the accompanying drawings.