An engraving (cut) in the contact lens determines if the lens is inside-out.
The engraving can be in the form of numbers or letters. It makes the handling of
contact lenses easy and prevents the application of a contact lens the wrong
way. For example, when you place the contact lens on your finger and look from
the side, you should see numbers “1–2–3” in the correct way. If the lens
is inverted, you will see the numbers “1–2–3” the wrong way around.
Nowadays most contact lenses are made of the following materials:
- More modern silicone hydrogel, which has a higher oxygen permeability and is
a combination of swelling silicone hydrogels, that provide the properties of
both soft and hard lenses. The result of which is form-stable contacts.
The development of existing contact lens' materials is an ongoing process. In
addition to production practicality, it is necessary to maintain the quality of
the lens' mechanical and chemical properties, biological inertness, tolerance,
resistance against storage of sediment, easy maintenance and transparency
(material should not be influenced by pH of tears, light, hydration and
dehydration between each blink, etc.).
The basic material for the production of soft contact lenses is HEMA. This
methacrylate allows increasing the water content and therefore contact lenses permeability to oxygen
original value of 40% to about 80%. Currently, there is a new generation of
soft contact lenses which uses silicone
. This material is used to produce contact lenses for continuous wear
,because of the high oxygen permeability.
The disadvantage of soft contact lenses is the predisposition to produce contact
. Modern soft contact lenses were invented by a group of Czech
chemists led by the scientist Otto Wichterle.
Silicone Hydrogel is the latest material for the production of contact lenses,
which gives much more oxygen permeability
compared to regular hydrogel. It's a combination of hydrogel and silicone,
fluorosilicone, which manages to increase the oxygen permeability, even with a
low volume of water. This compound combines the qualities of soft and hard
contact lenses. The disadvantage of these lenses is the lower level of
wettability and tendency to contact lens deposits.
Wettability is the tendency of one fluid to spread on, or adhere to,
a solid surface in the presence of other immiscible fluids. Contact lenses made
from silicone rubber (hydrophobic material) rarely achieve sufficient
wettability of the material, which is necessary to slip the contact lens on the
Water content in contact lenses
The water content of a lens is expressed by the percentage of water volume in a
particular material. The optimal amount is about 60%. For hydrogel soft lenses,
transparency of media is dependent on the water content. Material ageing
decreases the water content and affects contact lens permeability to light (in
which case the lens has a yellow or grey tint). Such a lens is unusable. If a
soft contact lens dries out, it is damaged and needs to be replaced.
The permeability of oxygen through the lens to the cornea is abbreviated by
using “Dk” (diffusion coefficient), where “D” is the ability of gas
molecules to move within the material and “k” is the amount of gas that will
fit in a specific volume. The oxygen transmissibility of lenses is defined as
- Low: less than 12 Barrer/cm
- Moderate: 12 to 34 Barrer/cm
- High: 35 to 100 Barrer/cm
Low Dk/t lenses are mainly older designs or high-powered lenses. High oxygen
permeability is achieved by silicone hydrogel
The Dk value determines the properties of contact lens materials.
A contactologist has to take the thickness of the lens into consideration, as
well as the permeability. Together they form the Dk/t value, called
transmissibility, which characterises the lens.
Read next: Are contact lenses with higher water content
The UV filter in contact lenses protects the cornea from damaging ultraviolet
rays. However, the protection is not guaranteed, as it is by wearing sunglasses,
since the eyelids and the conjunctiva are not protected. Therefore contact
lenses with UV protection are NOT to be treated as substitutes for protective
eyewear such as UV-absorbing sunglasses because they do not completely cover the
eye and surrounding area. Read next: All you need to know about UV protection sunglasses (UV400 Filter)
Many contact lenses usually have a light blue or green tint added, in order to
help you see it better when the lens is in the case, and during
insertion/removal, or even if you drop it (if you accidentally drop it on the
floor, throw it away). Visibility tints are hardly detectable and do not affect
your eye colour.
The diameter (DIA) is one of the most important parameters in the selection of
a suitable type of contact lens, for a specific user. Read next: How to read your contact lens prescription
Related to the term – contact lens
(Dk/t) The unit is defined in Barrer/cm. This value is
usually listed by manufacturers only for contact lenses with –3.00 D and the
“t” value is defined as the center thickness, that is the thinnest part of
the lens, which can be slightly misleading for contactologists.