The present invention relates to improvements in fluorescent materials and, in particular, to a luminescent sunlight collector or solar concentrator that includes a fluorescent dyed polymer composition having improved half-length properties that creates enhanced light emission from a long length of dyed polymer composition in the luminescent sunlight collector. BACKGROUND ART It is known to incorporate certain fluorescent dyes in polymer sheets for the purpose of making luminescent sunlight collectors. For instance, the red fluorescent dye, known by the trade name Lumogen F Red 300 and having the formula perylene-1 , 6,7,12-tetraphenoxy-3, 4, 9,10- tetracarboxylic acid -bis (2',6'-diisopropylanilide), has been incorporated into a poly(methyl methacrylate) (PMMA) polymer to provide sunlight collector sheets for use in solar based illumination of buildings and as fluorescent concentrators for solar cells.

Fluorescent dyes of this type have, however, suffered from the shortcoming that there are light emission losses through various mechanisms, such as from absorption and from scattering in dyed polymer.

Losses through absorption include dye based self-absorption due to overlap between the dye's emission and absorption spectra (i.e. insufficient Stokes Shift in the dye molecules), non-radiative absorption by the dye, and absorption by the polymer. Scattering losses are primarily due to inhomogeneities in the polymer from which the collector is fabricated. Sources of scattering include complexes of dye molecules, colloidal particles, crystal boundaries, regional variations in refractive index, bubbles, voids, foreign inclusions (such as dust particles) and surface defects.

Any type of absorption or scattering (or any other light emission loss mechanism) will decrease the dyed polymer's half-length.

The half-length of a material is the distance over which light generated by fluorescent emission falls by a factor of two. The value of this parameter is a key factor in the suitability of a dyed material for making a luminescent sunlight collector. This is because any material in a luminescent sunlight collector located more than one half-length from the point at which fluorescent light is taken out of the system (usually through the sheet's edges) makes only a small contribution to the collector's output. Thus the material's half-length limits the maximum useful size of a luminescent sunlight collector and hence the maximum luminous output. (Beyond a certain point, doubling the area of a collector gives only a small increase in output.) The small half-lengths of dyed polymer made by previous methods have prevented the construction of luminescent sunlight collectors which have a useful luminous output and good colour balance. DISCLOSURE OF INVENTION It is an object of the present invention to provide a means for increasing the half-length of a fluorescent dyed polymer so as to increase the distance over which the fluorescently emitted light can travel within any length of a luminescent sunlight collector and be emitted from the collector with maximum intensity or brightness. According to the invention, there is provided a process for preparing a fluorescent dyed polymer composition useful in a luminescent sunlight collector, comprising dissolving by means of ultrasonic agitation a fluorescent dye in a solution of mono(methyl methacrylate) or partially polymerised methyl methacrylate, so that in the presence of a sufficient quantity of an organic peroxide initiator added either before, during or after the dissolution of the fluorescent dye in the solution, and at a sufficient temperature, polymerisation is initiated so as to produce the fluorescent dyed polymer composition without the formation of voids or bubbles. It is preferred that the fluorescent dye is a perylene dye or a napthalimide dye of a type that is useful for luminescent sunlight collectors

or solar concentrators. Preferred forms of these dyes are the red fluorescent dye, known by the trade name Lumogen F Red 300 and having the formula perylene-1 ,6,7, 12-tetraphenoxy-3, 4,9, 10-tetracarboxylic acid - bis (2',6'-diisopropylanilide), the green fluorescent dye (sometimes called a "yellow" dye), known by the trade name Lumogen F Yellow 083, and having the formula isobutyl 4,10-dicyanoperylene-3,9-dicarboxylate, the violet/blue fluorescent dye, known by the trade name Lumogen F Violet 570, and having the formula 4,5-dimethyloxy-N-(2-ethyl hexyl) napthalimide, and the orange fluorescent dye, known by the trade name Lumogen F 240, and having the formula perylene-3, 4,9, 1 1 -tetracarboxylic acid bis-(2'6'-diisopropylanilide).

It is preferred that the dye is dissolved in a solution of mono(methyl methacrylate) or partially polymerised methyl methacrylate at a temperature between about 30°C and about 100°C by means of ultrasonic agitation for between about 1 and 5 minutes combined with stirring.

In the context of the present invention, it is to be understood that reference to the term "a fluorescent dye" in the aforementioned process may refer to a mixture of fluorescent dyes considered suitable by persons skilled in the art. Partially polymerised methyl methacrylate solutions may comprise solutions of varying mixtures of mono(methyl methacrylate), di(methylmethacrylate) and possibly small amounts of higher molecular weight methyl methacrylates that a person skilled in the art would appreciate as providing a sufficient starting solution for casting poly- (methyl methacrylate). As is well known in the art, solutions of partially polymerised methyl methacrylate may be prepared by terminating polymerisation of mono(methyl methacrylate) prior to completion. Advantageous features of utilizing a starting solution of partially polymerised methyl methacrylate in the process of the present invention are that shrinkage of the resulting solidified polymer is reduced and heat

production during the polymerisation step of the process of the present invention is reduced.

In carrying out the process of the invention with a starting solution of partially polymerised methyl methacrylate, it will be apparent to the skilled person in the art that such a starting solution must not contain any residual initiator left over from the partial polymerisation process that would interfere with the process of the present invention and compromise the efficacy of the fluorescent dyed polymer composition so produced.

It is preferred that the organic peroxide initiator is an alkyl peroxy ester. A particularly preferred alkyl peroxy ester is t-Butyl Per-(2-Ethyl)

Hexanoate, also known as TBPEH.

It is theorised that an especially advantageous characteristic of the organic peroxide initiator selected for use in the process of the present invention is that it has low cross-linking tendency, so that polymerisation results in a preponderance of linear chain dyed polymer molecules over cross linked chains of dyed polymer molecules.

Polymerisation should not result in the formation of voids or bubbles in the resultant fluorescent dyed polymer composition as these may cause unwanted light scattering effects and other diminutions of the half length of the fluorescently emitted light.

It has been found by the inventors that the organic peroxide initiator may be added to the solution either before, during or after the dissolution of the fluorescent dye in the solution, without adversely affecting the reaction kinetics of the process or the fluorescent dyed polymer composition so produced.

The process may further include the step of adding diethylenegiycolbis(allylcarbonate) (DEGBAC).

According to another aspect of the present invention, there is provided a fluorescent dyed polymer composition prepared by any of the above mentioned processes.

According to yet another aspect of the invention, there is provided a luminescent sunlight collector comprising a fluorescent dyed polymer composition prepared by any of the abovementioned processes. METHODS FOR CARRYING OUT THE INVENTION In order that the invention may be more readily understood and put into practical effect, reference will be made to the following Examples of preferred processes for preparing fluorescent dyed polymer compositions that can be used as luminescent sunlight collectors. Example 1 A small quantity of Lumogen F Red 300 dye (powder form) was dissolved in mono(methyl methacrylate) (referred to hereinafter as MMA) at 50°C by ultrasonic agitation for about 5 minutes. No solvents were used for dissolution purposes. A quantity of this solution was then added to pure MMA to give a dye concentration in the final solution of about 0.005% W/W. The solution was then stirred and ultrasonic agitation was applied for about two minutes.

A small amount of initiator in the form of t-Butyl Per-(2-Ethyl) Hexanoate (TBPEH) was then added so that it comprised about 0.050% W/W of the solution. The solution was then stirred and ultrasonic agitation was applied for about one minute.

The solution was cast in a verticle Teflon (trade mark) tube and then heat treated by immersion in a hot water bath to enable polymerisation to proceed to completion, thereby forming a fluorescent dyed polymer composition that can be used as a luminescent sunlight collector. The half-length of the fluorescent emission of the Lumogen F Red

300 dye incorporated in a luminescent sunlight collector produced as in this Example was found to be about 1 07cm. This compares favourably with the half-length of 31 cm for Lumogen F Red 300 dye incorporated in a PMMA based polymer made with standard techniques.

Example 2

A fluorescent dyed polymer composition was prepared in accordance with the process conditions described in Example 1 but using the green dye

Lumogen F Yellow 083 at a concentration of 0.007% W/W instead of the aforementioned Lumogen F Red 300 dye. (This dye gives green fluorescent emission at the aforementioned concentration.) The half-length of the fluorescent emission of the green Lumogen F Yellow 083 dye incorporated in the luminescent sunlight collector produced as in this

Example was found to be about 98cm. This compares favourably with the half-length of 56cm for Lumogen F Yellow 083 dye incorporated in a

PMMA based polymer made with standard techniques.

Example 3

A small quantity of Lumogen F Red 300 dye (powder form) was dissolved in mono(methylmethacrylate) (MMA) at 50°C by ultrasonic agitation for about 5 minutes. No solvents were used for dissolution purposes.

Diethyleneglycolbis(allylcarbonate) (DEGBAC) was added in liquid form by stirring for about 1 minute so that the solution comprised about

84% W/W MMA and about 1 6% W/W DEGBAC. A small amount of TBPEH initiator was added by ultrasonic agitation for about 1 minute so that it comprised about 0.05% W/W of the total solution. The Lumogen F Red 300 dye had a final concentration in the solution of about 0.005% W/W.

The solution was cast in a verticle Teflon (trade mark) tube and then heat treated by immersion in a hot water bath to cause polymerisation to completion. The half-length of the fluorescent emission of the Lumogen F

Red 300 dye incorporated in a luminescent sunlight collector produced as in this Example was found to be 1 09 cm.

Example 4

A fluorescent composition was prepared in accordance with the process conditions described in Example 3 except that it had final concentrations of about 4% W/W DEGBAC and about 96% W/W MMA. The half-length of fluorescent emission of the Lumogen F Red 300 dye incorporated in such a collector was found to be 93cm. Example 5

A fluorescent dyed polymer composition was prepared in accordance with the process conditions described in Example 1 but using the green dye Lumogen F Yellow 083 at a concentration of 0.007% W/W instead of the Lumogen F Red 300 dye, and adding DEGBAC in liquid form to the solution by stirring for about 1 minute so that the solution comprised about 96% W/W MMA and 4% W/W DEGBAC. The half length of the fluorescent emission of the Lumogen F Yellow

083 dye incorporated in a luminescent sunlight collector produced as in this Example was found to be about 95cm.

Various modifications may be made in details of concentration ranges and other process parameters without departing from the scope or ambit of the invention.