So that tennis balls could be more clearly seen under various lighting conditions, during the 1970's the colour of balls was changed from the traditional white to a shade of yellow on the basis that the human eye is most sensitive to light in the yellow region of the spectrum. In this specification the relevant yellow in the spectrum is defined as being of a wavelength between 500 and 560nm, while the normally accepted range of wavelengths to which the human eye is sensitive ranges from 400 to 700nm. It should be noted that the wavelength of the yellow referred to is only a guide to the actual required colour of the balls and further specified parameters are referred to later.

The object of the present invention is to maximise the amount of light reflected at any wavelength between 500 and 560nm for the particular shade of yellow to be defined later. This is achieved by the use of certain special dyes in the cloth covering the ball which produce a significant fluorescent effect. While the mechanics of fluorescence are generally understood in that certain dyes can convert invisible incident radiation from an illuminating source (usually ultra-violet radiation) into visible reflected light, the basis of this invention is the greater-than-normal degree to which this can be achieved.

According to a first aspect of the present invention, there is provided a games ball which when submitted to spectrophotometer testing using

standardised conditions of a D65 illuminant and a 10° Observer which gives values for C*, L* and h within the ranges 90-120,90-100, and 100-110 respectively, produces a maximum reflectance value between the wavelengths of 500-560nm of greater than 135%.

Preferably, the games ball is a cloth-covered ball, and the above-mentioned reflectance values are imparted to the ball by suitably dyeing the cloth.

In order to obtain the required reflectance properties in a finished ball, the initial reflectance of the dyed cloth may need to be even greater; this is because some reflectance may be lost during manufacture of the ball.

The games ball will normally be a tennis ball, but the present invention is not necessarily restricted to this.

According to a second aspect of the invention, there is provided a method of manufacturing a games ball comprising the steps of: (a) providing a core for the games ball; (b) dyeing a material to be used in covering the ball; and (c) covering the core with the material; wherein in step (b), the material is dyed so as to obtain reflectance properties of the covered ball such that, under standardised conditions of a D65 illuminant and a 10° Observer giving values for C*, L* and h within the ranges 90-120,90-100 and 100-110 respectively, the maximum reflectance value between the wavelengths of 500 and 560nm is greater than 135%.

In this method, preferably, the games ball is a tennis ball and the material is tennis ball cloth.

According to a third aspect of the invention, there is provided a dye which, when applied to cloth covering a games ball, gives the ball a maximum reflectance exceeding 135% in the wavelength range 500-

560nm when illuminated using standardised conditions of a D65 illuminant and a 10° Observer such as to produce values for C*, L* and h in the range 90-120,90-100, and 100-110 respectively.

Reference is made, by way of example only, to the accompanying Figures in which: Figure 1 is a diagram for explaining an L C h colour space model; Figure 2 (a) is a table of reflectance values for various tennis balls A to H; Figure 2 (b) is a table of values L*, C* and h for the tennis balls A to H of Figure 2 (a); and Figure 3 shows graphs of wavelength against reflectance for the tennis balls A to H in Figures 2 (a) and (b).

Before explaining the experimental results obtained in devising the present invention, some background information will be given.

In order to quantify the shade of yellow applied to games balls in accordance with the present invention, some explanation of an accepted method of closely specifying colour is necessary. This is based on the established and accepted CIELAB colour space model established by the International Commission on Illumination. The particular format of the model referred to here is the L C h version where: L stands for Lightness C stands for Chroma (or Saturation) h stands for Hue The model is shown in Fig. 1 and consists of two mutually perpendicular horizontal axes and a vertical axis through their intersection, the latter forming the axis of a notional cylinder.

A notional disc inside the cylinder is used as a platform for a continuous transition in colours from red at 0 degrees through yellow and green at 90 and 180 degrees to blue at 270 degrees and back to red at 360 (0) degrees. The radius of the disc (C) represents Chroma (or Saturation, i. e. colour intensity), the lowest intensity being at the centre. The notional disc may rise and fall between values of 0 and +L in its notional circumscribing cylinder so that as it rises the colours on the disc get lighter and as it falls the colours get darker. At L= +100 the colours are completely white and at L= 0 they are completely black.

Using such a system, colours can be closely defined by specifying values of C, L and the third factor h (hue) which is the angle a radius on the disc makes with the 0-180 degree axis.

The simple model described above is more realistic if account is taken of the spectral power distribution curve of the illuminant, the spectral reflectance curve of the object and the spectral response curve for the retina of the eye. Taking these factors into account (which produce so called'Tristimulus'values for colours) the parameters C and L become C*, L* while h remains unchanged. These parameters are calculated automatically by modern spectrophotometers (such as the Minolta Model 3700 as used for data specified below) which incorporate microcomputers. The standard illuminant most commonly used is termed D65 which approximates natural daylight.'A more complete explanation is given by A. R. Robertson in'The CIE Colour Difference Formulae'Colour Res. Appl. 2,7-11 (1977).

It has become usual for tennis balls made by most manufacturers to be coloured within fairly close limits of C*, L* and h and an examination of 7 groups of such balls by a spectrophotometer have found them generally

to lie within the following bands: C* L* h 90-120 90-100 100-110 In addition, the above balls were examined for Reflectance which is the amount of light reflected from the surface of a ball as a percentage of the incident light. As noted before, this percentage can exceed 100% due to fluorescence. It should be noted that the Reflectance of a finished ball covered in dyed material may differ from that of the dyed material when in sheet form. In particular, some degree of Reflectance may be lost in heat processing during manufacture of the ball.

It was found that when at least 8 balls from each of 7 different manufacturers were examined and the mean calculated, Reflectance values lie within the range 118.40 to 128.10%. The data for Reflectance and for C*, L*, h for these balls together with balls of the invention are given in Figures 2 (a) and 2 (b) and the reflectance data is shown graphically in Figure 3. In these Figures, letters A to G denote the conventional balls from different manufacturers, and letter H denotes balls made in accordance with the present invention.

After development of an improved dye and dyeing system meeting the criteria of C*, L* and h referred to above, the reflectance was measured and was found to be 137. (mean of 28 samples of ball H). Statistical significance tests showed this value of reflectance to be significantly greater than those of the other groups of balls A to G tested at a level of 0.1%, i. e. highly significant.

To evaluate the balls of the invention further against conventional balls, the balls H and groups of conventional balls A to G referred to above were

offered for subjective assessment to tennis players in non-public facilities. The balls were unmarked and players were asked to rank the balls on a 1 to 5 scale for their visual assessment of: (1) Colour (2) Brightness (3) Visibility (4) Ease of sighting in play The balls were viewed under the following conditions: (a) Indoors under strip lights (b) Outdoors under natural light (c) Outdoors under floodlights The results were as follows: (a) Indoors under strip lights: 23 players took part and assessment categories 1,2 and 3 showed balls of the invention to be preferred at 95% significance level and category 4 at 80% level.

(b) Outdoors under natural light: 27 players took part and balls of the invention were preferred in all assessment categories at 99% significance level.

(c) Outdoors under floodlights: 16 players took part and balls of the invention preferred in categories 1,2 and 3 at 99% significance level, and category 4 at 90% level.

This subjective data shows conclusive superiority of the ball of the invention under natural light, marked superiority under floodlights and a good indication of superiority under strip lights.

Thus, the present invention provides a tennis ball of markedly increased visibility, a so-called"ultra- visibility"ball. Although primarily envisaged for use with tennis balls, the present invention may also find application to games balls used in other sports where high visibility is important.