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Friday, January 23, 2015

Optician - Eye Glasses

An eye glasses is to measure the required strength glasses. On the basis of the measurement results can be made spectacle lenses. If the data of the old glasses are known, these can be used as a starting point, so that the measurement can be much faster.

The necessary correction in the refractive measurement is defined: the strength (in diopter), and the axial direction (in degrees) for any cylindrical correction for astigmatism and any additional strength of the reading area (called read addition). (Astigmatism is also called cylinder deviation called.)
Furthermore, measured the pupil distance (in millimeters).

There are in principle two measurement methods, which with a foropter or (hand-held) refractor, and those with a car refractor.

The measurement results

A glasses prescription consists of two data series, usually in two lines or in two columns. One set is for the right eye and one set for the left eye. Glasses rules are always listed from the viewing direction of the optician (or the one who carried out the measurement). Hence, the data of the right eye of the patient left and the data of the left eye to the right. An example of a glasses prescription is shown. Up to a quarter diopter strengths are accurately measured.

The right eye in this example, only need a spherical correction of + 1 dpt distance and a reading addition of 2,5 dpt. the read strength is always specified as a "addition" relative to the verafsterkte. In this example, the total strength for the reading part right so 1.0 + 2.5 = 3.5 dpt. see Lens (optics), under Lens strength, for an explanation of this addition of lens strengths.

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The left eye has also some degree of astigmatism, for which a cylindrical correction is given. It too is specified as "addition". Rays that in a plane through the axis of the cylinder raids, stay after refraction in the same direction continue. In the axial direction is the strength of the cylinder component so 0 dpt. perpendicular to the axis is the cylinder strength as indicated on the prescription glasses, here so – 0.25 dpt. the axial direction of the cylinder is 175 °, from the optician seen measured counterclockwise. The total verafsterkte for the left eye is + 1.25 dpt in the plane of the cylinder axle, and + 1.00 dpt perpendicular to it.

In Addition, the read addition mentioned. This is always spherical. After all if the lens of the eye shall no longer have any form annually handles, abnormalities of the lens affects all distances and they will be so in the verafgedeelte of the glasses prescription be corrected.

At Prisma is the any prismatic correction (for strabismus) mentioned, and based on the direction of the base of the Prism.

Furthermore, each eye is determined vision. Which is a measure for the visual acuity, which is defined usually in the Netherlands by the method of Snellen. The average visual acuity of the eye is set to 1.0 (or 100%). With a visual acuity of 0.5, details only on half of the 'normal' distance distinguished.

Often shown at right with OD (from the Latin oculus dexter, right eye), and left with OS (from the Latin oculus sinister, left). ODS means oculi dexter et sinister, so both eyes.

For fitting the glasses optician will always measure your pupillary distance (PD, pupillary distance).

Influence of diabetes

Diabetes can not only irreversible retina damage, but can also affect the effective lens strength. The lens strength change pretty quickly with changes in blood sugar levels; not long after a meal can already be measured a different strength. Opticians know this. Diabetics with glasses do therefore wise to, their optician informing that they have diabetes. It is advised to be able to buy a new pair of glasses to make multiple eye measurement, on different days. The best this can be done at a time that is fairly stable blood sugar levels, such as in the second half of the afternoon.

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