1.10 Trial case lenses

what is the overall diameter of a trial case lens

38mm

(zero tolerance for oversize, -0.2mm for undersize)

describe full and reduced aperture lenses

full aperture lenses have a rim width of about 1mm/ these lenses give a wide fov

reduced aperture lenses - the free lens aperture is usually around 20-25mm. they have a narrower fov.

maximum thickness of trial case lenses

maximum thickness should not exceed 2.8mm (full aperture lenses of greater than 5D and prism greater than 3 prism diopres may exceed this limit)

reduced aperture lenses provide significantly tinner lenses for higher powers

describe these lenses

symmertical - lenses are equi convex/concave

theyre usually found in full aperture trial cases

doesnt matter which way you put the lens

describe these lenses

they are reduced aperture plano- convex/concave

these are very common

these lenses should be placed so the curved surface is closest to the eye

the power marking on these lenses should be on the flat side and facing the practitioner

describe these lenses

reduced aperture curved form lenses

these lenses can match the form of a finished spectacle lens

describe these lenses

reduced aperture plano convex/concave sets with the front surface powered

these lenses are rarer

what do these symbols mean

what are additive vertex power lenses

when you combine 2 lenses they add up to the correct power if the lenses are placed in specific slots in the trial frame and are the correct way round (spheres placed further from the eye and cylinders placed closer to the eye)

the powers marked on the lenses are not the individual powers so they shouldnt be used for neutralisation purposes. the powers refer to the contribution that the lens will make to the combinatin of lenses when placed at a specific position in the trial frame

phoropters tend to use this type of lens system

what is this

retinoscopy rack

a series of lenses (usually in 0.50 DS) steps mounted in a handheld frame

what is this

refracto rack

what is a simultan test - slide 25

device slots into a trial frame which allows the pt to see 2 targets at the same time which makes it easier to determine which lens is clearer

it splits the ray path using a beam splitter so the target can be doubled. each target is seen through a different lens