Perret Opticians
 
We have been opticians for three generations in our family, and our activity is targeted on three areas, optometry, contact lenses and optical instruments.
 
 

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OPTICAL CORRECTION

Press-On Prisms Fresnel

 

Press-On Prisms are Fresnel optics that can be applied to either part of or the entire carrier lens inner surface. Ultra-thin Fresnel prisms, measuring only 1mm thick, can be cut to fit most frame sizes and shapes and adhere to existing lenses with only water.

Press-On Prisms provide a simple therapeutic and inexpensive way to correct several visual disorders. Press On Prism are useful for bed-ridden patients, Diabetes mellitus, post cataract surgery, symptomatic phorias, decompensated phorias and low vision. Also used as a fusion stimulus in other muscle deficiencies.
 


Fresnel Press-On Prisms offer an immediate correction. They are more comfortable and cosmetically appealing treatment for strabismus than conventional prisms. Offering an optic that adds no noticeable weight or thickness to the spectacle.

They are useful in treating temporary and/or variable visual problems without damage to the patients' spectacles.

These ultra-thin Fresnel lenses are designed for pre- and post-operative trial fittings, inexpensive vision therapy, during power changes, and for instant corrections on sports goggles and sunglasses. The lenses, made of polyvinyl chloride, are available in 1.00D to 40.00D prisms, plus powers to 16.00D, minus powers to 14.00D and D-25 segs to +6.00D. 3M Press-On Optics, all only 1mm thick, are comfortably lightweight for patients to wear and can be easily applied to any eyewear, whether prescription or plano carrier lenses.

The observation that prisms reduce visual acuity is brought here to attention: the visual acuity of normal young individuals is reduced in all cases proportional to the strength of the prism. The reduction in VA was found to be 2% per prism dioptre when using conventional opthalmic or wafer-prisms and 3% in the case of Fresnel-prisms.

The least reduction of VA is found when using conventional ophthalmic prisms; this reduction however is not significantly different when using Wafer-prisms of lower strengths. A significant difference is ascertained between conventional opthalmic and Wafer-prisms of higher strength and between conventional opthalmic and Wafer-prisms as opposed to Fresnel-prisms within the range of 5 to 30 prism diopters.

The advantage of using Fresnel-prisms as opposed to Wafer- and conventional ophthalmic prisms is their low weight, their simple adjustment and being able to change them without ordering a new correction.

The disadvantage of Fresnel-prisms as opposed to conventional ophthalmic prisms and Wafer-prisms of low strength is their poorer optical quality. The visual acuity is reduced to 50% by using conventional ophthalmic wedge prisms of 30 prism dioptres and to between 30% and 40% when using Fresnel-prisms. Fresnel-prisms are according to our brief experience a useful optical aid especially in the application of strabismustherapy.

 

Augustin Fresnel

 

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