Introduction to the oxygen transmittance scheme for contact lenses

Since contact lenses are attached directly to the surface of the cornea, the wearer pays special attention to the choice of lens material when choosing
contact lenses.

Affected by a lot of advertising, many wearers will choose lenses with high water content when buying contact lenses, believing that the higher the water content, the better
the quality of the lens.

As we all know, the oxygen permeability of the lens is an important quality indicator of contact lenses, the various uncomfortable reactions of wearing contact lenses and the length of wearing time depend on the oxygen transmission of the lens, and the high water content and ultra-thin of the lenses now advertised are also to improve the oxygen permeability
of the lenses.

With regard to the detection of oxygen permeability of contact lenses, ISO has developed testing standards specifically for the detection of oxygen permeability of contact lenses: ISO 9913-2 applies to rigidnon-hydrogel flexible contact lens materials (rigidnon-hydrogel flexible contact lens materials), and the test amount has oxygen flux - j, oxygen permeability (oxygen).
Permeability) - Dk, oxygen transmissibility - Dk / t, and the thickness of the specimen 0.
degree (thickness) - t, generally use the Dk value to evaluate the oxygen permeability index
of contact lenses.

The detection principle is shown in Figure 1: after the specimen is loaded into the test chamber, the test chamber is divided into two parts, the upper cavity is filled with a certain oxygen content of the test gas, the carrier gas flows through the lower cavity and flows into the Coulomb sensor, the pressure on both sides of the specimen is equal, but the oxygen partial pressure is different, oxygen enters the carrier gas flow through the film under the action of concentration difference, and is sent to the Coulomb sensor, and the amount of oxygen carried in the nitrogen gas stream is measured by the coulomb sensor, so as to calculate the oxygen transmittance of the material
。
Oxygen permeability testing equipment needs to use a reference film for device calibration before formal testing, determine the correction factor of the equipment, and use it in the
calculation of formal testing.

Standard ISO 9913-2 clearly states that the specimen for calibration is SRM 1470, which is the same as the reference film used in the
thin film testing standard ASTM D 3985.

Detection instruments: OTR-D3 oxygen transmittance tester test principle: the specimen is installed between the test chamber sample gas and the carrier gas chamber, the temperature of the test chamber is controlled, and the oxygen in the sample chamber penetrates into the carrier gas chamber, and is carried by the carrier gas to the trace oxygen sensor; Accurately measure the current generated by the trace oxygen sensor, calculate the oxygen transmission of the sample and other parameters
.

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