LENS POWER AND LENS THICKNESS Flashcards
Power of a thick lens depends on the following:
- Index of refraction
- Surface powers
- Thickness of the Lens
the power of a lens that is derived from the combination of its front and back curves
Nominal Power
Nominal power is measured with a ______
Lens Clock
is the power that the lens have if it was reduced to a thin lens
Equivalent Power/ True Power
is the power specified for ophthalmic lenses as the eye is usually positioned behind the back vertex of the lens
Effective Power / Back Vertex power
the reciprocal of the reduced distance from the primary focal point to the front vertex of the lens
Neutralizing power / Front Vertex power
is the change in vergence of light that occurs at different points along its path
This is related to vertex distance
Lens effectivity / Effective power of lens
Lenses in general should be placed before the eyes at a distance of ___mm to ___mm from the corneal vertex to coincide with the eyes anterior nodal point
12mm to 14mm
The vertex distance for contact lens is?
Zero
used to measure the vertex distance from the back surface of the spectacle to the cornea through a close eyelid
Vertexometer or Distometer
Effective power of PLUS lens when Moved Towards the eye:
Decreased Effective power
LESS PLUS
Effective power of PLUS lens when moved AWAY from the eye:
Increase Effective power
MORE PLUS
Effective power of MINUS lens when moved TOWARDS the eye:
Increased effective power
MORE MINUS
Effective power of MINUS lens when moved AWAY from the eye:
Decreased effective power
LESS MINUS
results in shifting the optical axis away from the center of rotation
it means that the line of sight is at an angle to the lens in its primary position of gaze
Lens Tilt Induced Power
When we tilt the lens either horizontally or vertically, we are potentially inducing ______ in the lens
astigmatism
The induced astigmatism when tilting the lens is called _______ or sometimes called as ______.
Marginal astigmatism
Oblique Astigmatism
- a lens tilt about the horizontal axis with respect to primary gaze
- tilt ranges from 8-12 degrees
Pantoscopic Tilt
achieved by angling the bottom rim of the frame/bottom of the lens closer to the face than the top of the lens
Pantoscopic Tilt
Spherical and Cylindrical power changes caused by Pantoscopic tilting can be corrected by changing the position of the _______
Optical Center
For every 1 degree of Pantoscopic tilt, lower the optical center by _______.
0.5mm (½)
is achieved by angling the bottom rim of the frame front away from the face
Retroscopic tilt
The purpose of Retroscopic tilt is to remove the lower rim of the from from a ______
Protruding Cheek
tilt in which the frame follows the natural form of the face
Positive Face form tilt
tilt used when the patient’s PD is narrow than the frame PD
Positive face form tilt
a tilt usually employed when the patient’s PD is actually wider than the frame PD
Negative face form tilt
- this tilt should be avoided
- instead find a suitable frame size
Negative face form tilt
Pantoscopic/Retroscopic tilt will increase a lens spherical power and cylinder power along the _____ degree axis
180 degrees
Face Form tilt will increase a lens spherical and cylindrical power along the ______ degree axis
90 degrees
Formula for lens tilt:
Martin’s Formula for tilt
