Vertex Distance Conversion Calculators

Hirst Contact Lens Limited, Manufacturer


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Details


Maker and Role
Hirst Contact Lens Limited, Manufacturer
Description
Vertex distance conversion calculators. Four circular discs made from cream coloured plastic. Each disc is composed of two circular pieces connected by a metal fastening in the centre. In the middle of the calculator printed in black text is “WITH THE / COMPLIMENTS / OF / HIRST CONTACT LENS / Ltd / VERTEX / DISTANCE/ CONVERSION / CALCUALTOR/ CON - O - COID / HARD and SOFT / LENSES.”
In this inner circle is a scale on either side for Plus and Minus lenses. The plus scale on the right starts from 24 and descends in increments of 2 down to 0. The arrow on the right reads “+ / Plus / Lenses / mm. / Scale”. The arrow on the left reads “- / Minus / Lenses / mm. / Scale”. The text for this is written upside-down and the scale starts from 0 and ascends in increments of 2 up to 24. This scale refers to the distance in millimetres the lens sits from the cornea. The outer circle has a scale starting from 5.25 divided into increments of .25 up to 25.00 – This refers to the power of the patient’s prescription (Dioptre scale).
The function of the calculator is to calculate the distance between the back of a contact lens and the patient’s cornea (back-vertex power) when a trial spectacle frame is used. It means that the eye care professional can determine the correct strength for the contact lens without the patient trialling the lens. For example, for myopia - the strength of the prescription is stronger for glasses wearers because the lens is further away from the eye than the contact lens.
On the back of the disc are the directions for use for spherical and compound lenses. “VERTEX DISTANCE CONVERSION CALCULATOR / DIRECTIONS FOR USE / SPHERICAL LENSES […] COMPOUND LENSES […]”.
Media/Materials
Processed Material/Synthetic/Plastic
Processed Material/Metal
Signature/Marks and Type
WITH THE / COMPLIMENTS / OF / HIRST CONTACT LENS / Ltd / VERTEX / DISTANCE/ CONVERSION / CALCUALTOR/ CON - O - COID / HARD and SOFT / LENSES Printed
+ / Plus / Lenses / mm. / Scale Printed
- / Minus / Lenses / mm. / Scale Printed
VERTEX DISTANCE CONVERSION CALCULATOR / DIRECTIONS FOR USE / SPHERICAL LENSES […] COMPOUND LENSES […] Printed
Accession No
2018.1.9
Credit Line
Hirst Contact Lens Limited. Vertex Distance Conversion Calculators, 2018.1.9. The Museum of Transport and Technology (MOTAT).
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