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
Principle of the geneva lens
The total power of a thin lens equals the sum of its surface powers
However, the instrument is calibrated for lenses made of crown glass (refractive index 1.523) and a correction factor must be applied in the case of lenses made of materials of different refractive indices
What is a focimeter used for
used to measure the vertex power of a lens, the axes and major powers of an astigmatic lens and the power of a prism
What is a collimating lens used for
one which renders light parallel
What does a tinted lens do
modifies the spectral profile of the radiation passing through it. Tints are either absorptive or reflective
What are absorptive tints
absorb light passing through them
(Cr, NiCr, MgF2, SiO)
What are reflective tints
reflects unwanted wavelengths
(Cr, NiCr)
What wavelength of light do CR-39 lenses absorb
shorter than 370nm
How do photochromatic lenses work
changes its transmission characteristics depending upon the intensity of incident radiation. The lens becomes darker in brighter light. The process of darkening is more rapid than that of lightening. The reactions in glass and plastic are different
Reaction within photochromatic lenses to enable them to darken and lighten
colourless silver halide crystals suspended in borosilicate. Electromagnetic energy dissociates the silver and halogen to cause darkening. Each type of glass has an optimum activating (usually UV or blue) and bleaching wavelength; the tint will depend upon which wavelength predominates. Heat opposes the effect of light. Thus, glass darkens more easily when cold and lightens more easily when warm. A glass lens becomes gradually darker over time if it is used repeatedly.
Principle of how anti reflective coatings work
The reflection of light from the surface of a lens may be reduced by coating it with a material the thickness of which is a quarter of the wavelength of the incident light- leads to destructive interference of incident light.
Constructive interference and anti reflective coatings
a coating which has a thickness half the wavelength of the incident light produces a mirror coating because of constructive interference. Any wavelength may be selectively reflected by a coating which is half a wavelength thick. Mirror coatings are usually combined with an absorptive tint.
