Trial lens set is an ophthalmology calculating apparatus used in the department of ophthalmology in hospitals and glasses shops to exam a patient’s refractive error including hyperopia, myopia, presbyopia, astigmatism, strabismus or color blindness. It usually contains sphere lens, cylinder lens, prism lens and other accessory lens, etc. Following is the details of all the major components.
The curved surface focuses all paraxial parallel rays to one point (usually called the virtual focus) and the dioptric power on all axes positions is the same. Spherical lens includes concave lens (-) and convex lens (+). The former is used to correct myopia and the latter hyperopia and presbyopia.
The curved surface forms a part of cylindrical lens and the dioptric power on all axes positions is not the same. After passing through the lens, the light beam focuses on a straight line (or a broken line). Cylindrical lens consists of concave (-) and convex (+) cylindrical lens. They are used to correct astigmatism.
The tangent plane of a prismatic lens is a cuneiform. After passing through the lens, the light beam bends to the bottom and the object shifts towards edges. This kind of lens is used to exam and correct strabismus or heterophoria as well as to train eye-muscle.
This is an opaque lens used to cover the uninspected eye.
Cross lens (CL)
There are two mutual vertical lines on the plane lens, used for locating pupil's center and measuring intraocular distance.
Frosted lens (FL)
This is a semi-transparent cover lens and mainly used for babies or used outdoors to replace occluder.
Slit Lens (SS)
There is a slit in the center, which is the only way light beams can pass through. By rotating this lens in front of the patient’s eye, astigmatism can be examined as your vision changes in better or in worse at a certain axis position. On the contrary, it proves no astigmatism if the patient’s vision has not changed during the rotation.
Plano lens (PL)
Plano lens are transparent lens with no dioptric power. It is usually used to determine malingering blindness.
Color Lens (RF, GF)
This kind of lens has different colors, e.g. red, green, blue, yellow and dark brown and is used to examine color sensitivity.
Cross cylinder (Jacson cross cylinder, JCC)
This is a kind of lens with contrary dioptric power in two axis positions and can be used to examine the degree and axis position of the cylindrical lens when determining astigmatism.
When using, put the cross cylinder lens before cylindrical lens and make one of its axis positions overlap that of the cylindrical lens. Then overturn the cross cylinder lens, the axis will turn 90° and observe the change of vision. If the patient’s vision has no change, the degree of cylindrical lens used can be considered as suitable.
To get the correct axis position, place the cross cylinder lens before the cylindrical lens and let the hand shank of cross cylinder lens point to primary axis of cylindrical lens, then overturn the cross cylinder lens and observe the differences of vision at the two axis positions. If vision on one position is better than on another one, the axis of cylindrical lens can be slightly turned to the direction of the “better position”. Then test again by the above method until difference of vision at two positions can hardly be noticed by the patient.
Maddox lens (MR)
Maddox lens is made of glass rods that convert a light spot into a streak 90o from the axis of the rods. It is used to measure the strength of eye muscle and to examine latent and real strabismus.
Put the Maddox lens in front of one eye and have the patient watch with both eyes. If a light spot he/she observes with his/her bare eye is on the line formed by the Maddox lens, the patient has neither strabismus nor latent strabismus. Otherwise he/she has one of them. If the light spot is on either side of the vertical line, he has horizontal strabismus. If the light spot is above or below the horizontal line, he has vertical strabismus. Then the strabismus can be corrected by using a prism lens, which makes the light spot coincides with the line. The strength of the prism indicates the degree of latent or real strabismus in prism diopters.
Pin hole (PH)
In its center, there is a small hole, through which light beam penetrates to form an artificial pupil and it is used to improve diopter especially the astigmatism.
135°is used to inspect visional function, balance both eyes, and mix function.
45° is used to inspect latent strabismus, stereoscopic vision, unequal vision and so on.