Contact lenses are a growing industry, with currently over 3 million contact lens wearers just in the UK for example. There are a number of different types of lens, with traditional hard lenses being replaced by soft, oxygen-permeable lenses. Soft lenses are formed partly from water and are hydrated during the manufacturing process. This means that they move from a dehydrated to a hydrated state, and some properties of the lens are affected by this change. It is important during manufacture to be able to measure certain properties for classification and quality assurance purposes. There are a number of measurements which might be taken of a particular lens. The most commonly used are diameter, centre thickness, back curve optical radius (BCOR) and saggital height. While these measurements can easily be taken for a lens in dehydrated form, it is more important to measure the lens when it is in its final, fully hydrated state, as this is the state which it will be in while in place on an eye. However the need for the lens to be hydrated complicates the measurement process. Moreover, the need for accuracy is enhanced by the trend towards using digital rather than analogue measurement techniques.

In one known arrangement the lens to be measured is placed in a saline bath. To prevent movement which would affect the measurements, particularly the tendency for the lens to rise in the solution and flip over from the preferred configuration with its convex side facing upwards, the lens is held in place by placing it on an 8° inclined surface such that it is gently urged into a wedge formed by two walls which form a 'V shape. Whilst it is important to hold the lens in place during measurement, it is also important not to apply too great a retaining force in order not to distort the lens.

However the Applicant has appreciated a shortcoming of the above arrangement. It has appreciated that a result of the angled surface on which the lens is placed is that any change in the level of saline solution affects the measurements made

significantly. Thus careful control of the level of saline is required to ensure that it is kept at a consistent level between measurements. Whilst the need for a constant level of saline solution may be achievable in a laboratory setting, it is highly impractical for instruments intended for large scale commercial use such as for testing lenses as part of a manufacturing process.

The present invention aims to address the problems set out above. When viewed from a first aspect it provides an apparatus arranged so as in use to provide a flow of liquid over the convex surface of a contact lens to retain said lens in a fixed position in a bath of said liquid, such that the plane of the rim of the lens is parallel to the surface of the liquid.

Thus it will be seen by those skilled in the art that by retaining the lens using a liquid flow rather than an inclined surface, the lens may be positioned and retained on a flat surface - i.e. so that the plane of its rim is parallel to the surface of the liquid. This means that the optical axis of the measurement system may be normal to the surface of the liquid so that variations in the level of the liquid do not have a significant impact on the measurements due to refraction.

The apparatus is preferably configured to provide one or more retaining surfaces against which the lens is urged by the liquid flow. A single continuous surface could be provided, although this is considered less preferred since it is more likely to disrupt the flow of liquid over the lens. A hole in the surface could be provided to mitigate this issue but in a set of embodiments a plurality of discrete surfaces is provided. A number of different forms are possible. For example relatively small projections such as pins or raised platforms could be provided. In a set of embodiments however the retaining surfaces comprise a pair of converging lens retention shoulders.

The retaining surface(s) could protrude vertically downward or extend horizontally across the apparatus - e.g. as a horizontal bar, which is at a height such that it holds the lens part way up its body to allow the liquid to flow over the lens to hold it down, but also flow to around the rim of the lens and under the bar to ensure a smooth flow pattern. Conveniently however the retaining surface(s) protrude(s) from the base of the liquid bath. The surface(s) could be substantially vertical but equally could be either sloping away from the lens or undercut. The surface(s) may not be a distinct section of the base, but may instead be provided on a continuous curve which the lens rests against. A transparent window is preferably provided beneath the area arranged to retain the lens in order to allow measurement signals e.g. illumination to pass through from beneath to hit the lens or signals from above that have passed through the lens to be received.

The apparatus preferably comprises an internal conduit through which the liquid is pumped in use to provided the desired flow. The apparatus may be arranged to be connected to an external pump but in a set of embodiments a pump is provided as part of the apparatus. The liquid is preferably pumped at a low flow rate so that the movement does not affect the surface of the saline solution if images need to be taken. In a set of embodiments the liquid flow rate is less than 50 ml/min, e.g. less than 30 ml/min, e.g. less than 25 ml/min. The bath specified in accordance with the invention may comprise an integral part of a larger instrument. In a set of preferred embodiments however it is provided as a removable cuvette. The cuvette may include means for attachment to the instrument. This could comprise one or more clips, catches, magnets, screws or any other suitable attachment means.

When viewed from a further aspect, this invention provides an apparatus for the measurement of contact lenses, which contains an apparatus for retaining the contact lens in a fixed position in a bath of liquid as described above in accordance with the first aspect of the invention.

When viewed from a yet further aspect, this invention provides a method of taking one or more measurements of a contact lens, comprising using a flow of liquid over the convex surface of said lens to retain the lens in a fixed position in a bath of said liquid such that the plane of the rim of the lens is parallel to the surface of the liquid while the measurement is being taken.

The measurements could be taken using a number of different types of signal, such as ultrasound or visible light, but in a set of embodiments they are carried out using a laser. The contact lens could be measured using more than one different type of signal, and in a set of embodiments two are used to measure all the necessary features. The apparatus preferably contains a means for taking measurements of the contact lens, for example a laser source and sensor or a digital camera.

The measurements may be taken using any suitable type of signal, for example ultrasound, but preferably in a set of embodiments, light is used. This may come from a laser or other light source, or in some embodiments a combination of different sources could be used.

Any suitable liquid could be used which is able to maintain the desired degree of hydration of the contact lens and which has suitable transmissive properties to allow the measurements of the lens to be conducted. Conventionally a saline solution is used but it is envisaged that other liquids could be employed instead.

The liquid is preferably filtered and maintained at a constant temperature.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 shows the main parts of an instrument for taking measurements of a contact lens;

Fig. 2 is a plan view of a cuvette in accordance with the invention;

Fig. 3 is a vertical cross section along the line A-A of Fig. 1 ; and

Fig. 4 is a vertical cross section along the line B-B of Fig. 1. Fig. 1 shows some parts of an instrument which can be used to take measurements of a contact lens 2. In this embodiment, a laser 4 and optical camera 6 are used to take measurements of the contact lens 2, with the laser 4 taking measurements from below the contact lens 2 and the camera 6 from above. The contact lens 2 sits on a flat platform 8 in a bath of saline solution 10 within a removable cuvette 12, shown in more detail in Figs. 2 to 4.

Figs. 2 to 4 show an embodiment of the contact lens cuvette 12 for use in the instrument of Fig. 1 or indeed any optical measurement system where the contact lens must be maintained in a fixed position in a bath of liquid.

A soft contact lens 2 is placed centrally on the cuvette platform 8 against a pair of lens retention shoulders 14. In this embodiment, these shoulders are provided by raised flat platforms, for example made of glass, and have an angle of 120° between them, but the construction and/or angle may vary. The angled surfaces 14 are separated by a gap 16. The lens and shoulders are within a bath of saline solution 10, which is connected to an external pump (not shown) through a series of pipes 20.

As can be seen from Fig. 3, the cuvette 12 can be attached to a corresponding platform 20 on the measuring instrument by way of a number of magnets 22. A number of spacing shims 24 are included between the cuvette 12 and the measuring instrument platform 20 to ensure a fixed air gap is provided. They could be formed from any suitable material, such as glass or metal.

In use, the contact lens 2 is placed into the cuvette 12. The cuvette 12 is then placed onto the instrument platform 20, so as to be secured by the magnets 22 and the pipes 18 are connected to corresponding pipes in the instrument (not shown). The lens is fully submerged in saline solution, up to a fill level that is much greater than the saggital height of the contact lens 2, as can be seen from Figs. 3 and 4. Saline solution is then pumped through the internal saline gallery 18 across the surface of the contact lens 2 at a slow rate, for example 20ml/min. The rate is sufficient that it holds the contact lens 2 in position against the retaining shoulders 14, but it is also low enough that it does not distort the shape of the lens and so affect images or measurements taken of the lens 2 during the measurement process. After crossing the surface of the lens 2, the saline solution flows through the gap 16 between retaining shoulders 14 and out of the cuvette 12 back to the pump (not shown). The presence of the saline solution ensures the contact lens 2 remains hydrated throughout the measurement process.

The combination of the retaining shoulders 14 and flow of saline solution allow the laser 4 and camera 6 to be used to perform measurements of the lens such as diameter, thickness, back curve optical radius (BCOR) and saggital height to a high degree of accuracy even if the level of saline in the bath 10 changes slightly because the lens is held fixed and there is no mutual angle between the rim of the lens against the platform 8 and the surface of the saline bath 10.