Measurement of contact lenses

One or more optical parameters of a contact lens can be measured with the lens contained in aqueous fluid sealed in a sachet of optically acceptable plastics film.

This invention relates to the measurement of contact lenses. 
There are several important parameters of a contact lens in which a 
practitioner is particularly interested, including back vertex power 
(BVP), back central optic radius (BCOR), and lens diameter (LD). The need 
for measurement of these parameters arises at the manufacturing stage, and 
also after periods of use. Ideally with soft lenses, the parameters have 
to be measured using a wet cell system so that the lens remains fully 
hydrated, and to this end there are several modifications available for 
keratometers. 
A keratometer is ordinarily constructed for use in measuring the curves of 
the cornea, but by inclusion of a 45.degree. reflecting prism or mirror 
system, and an optically ground base or plastics wet cell with a flat 
parallel base, it becomes possible to take measurements of a contact 
lenses submerged in saline in the wet cell. 
This modification is widely adopted, and is described for example at page 
459, Chapter 30, "The Verification of the Optical Dimensions of the Soft 
Lens", by J. Chaston in the book "Soft Contact Lens: "Clinical and Applied 
Technology", published by Cassells in 1978; also in the Feb. 26, 1983, 
issue of "The Ophthalmic Optician" at page 128; and also at page 100, 
volume 3, no 3 (July 1980) of the British Contact Lens Association 
Journal. In general, measurements of contact lenses can thus be made on 
keratometers or other forms of ophthalmometer. 
Care is needed to obtain useful results, and much thought has gone in to 
the design of suitable wet cells. 
Some designs are relatively complicated,as clearly implied in an article by 
J Chaston and I Fatt published in Issue 1, Volume 9 (1982) of 
"International Contact Lens Clinic" on page 12. The article states that 
the only require to fulfill the same function as a wet cell is to have a 
storage vial which has a reasonably flat base. However, in the ordinary 
course of events, the glass vials used to store and transport contact 
lenses do not have a sufficiently flat and optically perfect base for 
accurate measurements, and so the authors therefore employed a piece of 
polymethylmethacrylate sheet cemented to a tube of the same material. 
In accordance with the present invention, there is provided a method for 
measurement of one or more optical parameters of a contact lens, wherein 
the lens under measurement is contained in aqueous fluid sealed in a 
sachet of optically acceptable plastics film. 
The present invention also provides a sachet of optically acceptable 
plastics film containing an aqueous fluid and a contact lens.

Referring to the drawings there is shown a sachet 10 containing a lens 12 
immersed in an aqueous fluid 14. The sachet is folded along edge 16 and 
heat sealed in order to completely encase lens 12. 
In FIG. 2 a Keratometer 26, in cooperation with a reflecting prism or 
mirror 28 is shown positioned for obtaining optical measurements of lens 
12. For these purposes a flat object 30 is positioned overlying the sachet 
10. 
Sealing of the lens 12 in a sachet 10 not ony permits optical measurements 
to be made, but leads to other advantages. Thus, the lens 12 can be 
transported and stored in the sachet 10, and the need to transfer from a 
glass vial to a wet cell is avoided. Sterility within the sachet can be 
obtained. Presentation of stock sets of lens can be improved, and it is 
readily possible to reseal the lens in the sachet following withdrawal for 
a trial use. Any tonicity and pH variations of the storage solution will 
be minimised and changing environmental conditions will have little 
affect, particularly as the sachet can withstand a wide range of 
temperatures. 
In order further to protect the lens 12 and also to help maintain the 
orientation of the lens within the sachet 10, the lens 12 can be retained 
in a support or holder 24. Such a support can be made of plastics material 
and secure the lens to facilitate the taking of the optical measurements. 
The preferred material for the sachet 10 is a polyester film such as that 
available from du Pont under the trade mark Mylar Type M-30. However, 
investigation will show that other materials can be used, such as pvc 
laminates, cellophane, polyolefines, and polyesters. The aqueous fluid 14 
will usually be sterile saline, and the sachet is best sealed by heat 
sealing. 
It is ordinarily essential for the preferred material that it can be 
sterilized and withstand the pressures and temperatures for heating in an 
autoclave (British Pharmacopoeia 1978). The preferred material has a low 
water vapour property below 20 gms per square meter in 24 hours (A S T M 
E96 38.degree. C.-90% RH). The preferred material is also capable of 
achieving a durable and hermetic seal using a suitable process for 
example, heat sealing. The material thickness is suitably within a range 
on 5 micron to 100 micron. Furthermore the material used for the sachet 10 
is preferably capable of accepting print matter using a suitable process. 
In one example in accordance with this invention, a soft lens was heat 
sealed together with about 1 ml of saline in a rectangular sachet of about 
10 cm by 5 cm. Optical measurements were then taken a Zeiss (trade mark) 
Keratometer 26 modified in the usual way to include a prism 28 and a stage 
for a wet cell. 
In this instance, the sachet 10 was placed on the stage instead of the wet 
cell, and a flat object 30 lodged on top to flatten out the sachet and to 
wedge it in position. The lens 12 could readily be maneuvered into 
position before taking optical readings, and there was no tendency to 
adhere, as occurs with normal storage vials. Using the available optical 
formula, there was no difficulty in determining the back vertex power, 
BVP, from measurements of the back and front optic radius taken on the 
keratometer. Furthermore, the sachet provided an ideal way of packaging 
the lens for delivery by conventional methods.