Optical Prescriptions, Spectacle Lenses Flashcards
What is lens transposition
When a lens prescription is changed from one lens form to another optically equivalent form, the process is called transposition of the lens.
How is simple transposition of spherical lenses achieved
The lens power is given by the algebraic sum of the surface powers
How is simple transposition of cylinders achieved
This is a change in the description of a toric astigmatic lens so that the cylinder is expressed in the opposite power.
1) Let the cylindrical element be at axis 90°
2) Let the cylindrical element be of opposite power and at axis 180°
How is toric transposition achieved
1) Transpose the prescription so that the cylinder and the base curve are of the same sign
2) Calculate the required power of the spherical surface (the numerator of the final formula). This is obtained by subtracting the base curve power from the spherical power given in in step 1
3) Specify the axis of the base curve. As this is the weaker principal meridian of the toric surface, its axis is at 90° to the axis of the required cylinder found in (b) in step 1
4) Add the required cylinder to the base curve power with its axis as in (b) in step 1
How to ascertain if a given lens is spherical, astigmatic or a prism
by studying the image formed when two lines, crossed at 90°, are viewed through the lens
How to ascertain spherical lens by looking at it
Spherical lenses cause no distortion of the cross. However, when the lens is moved from side to side and up and down along the arms of the cross, the cross also appears to move
How to ascertain convex lens when looking at it
the cross appears to move in the opposite direction to the lens, termed as ‘against movement’
How to ascertain concave lens when looking at it
a movement in the same direction as the lens, a ‘with movement’, is observed
How to ascertain an astigmatic lens when looking at it
cause distortion of the cross unless their axes coincide with the cross lines. Rotation of the lens thus causes a ‘scissors’ movement as the crossed lines are progressively displaced
How to ascertain prismatic lenses
A prism has no optical centre and thus displaces one line of the cross regardless of its position with respect to the cross. Furthermore, the direction of displacement is constant
How can the power of a lens be found by the technique above
Once the nature of the unknown lens is determined, lenses of opposite type and known power are superimposed upon the unknown lens until a combination is found which gives no movement of the image of the cross lines when the test is performed
In the case of astigmatic lenses, each meridian must be neutralised separately
How are spectacle lenses named
By their back vertex power
How to measure back vertex power accurately
the neutralising lens must be placed in contact with the back surface of the spectacle lens. However, with many highly curved lenses this is not possible and an air space intervenes. It is then better to place the neutralising lens against the front surface of the spectacle lens
When is neutralization somewhat inaccurate
for curved lenses of more than about 2 dioptres power and an error of up to 0.50 dioptre may be incurred with powerful lenses
What is the geneva lens used for
used to find the surface powers of a lens by measuring the surface curvature
