Contact Lens Optics and Design Flashcards
(24 cards)
What is the primary function of contact lenses?
Contact lenses are primarily used for refractive correction, but also for cosmesis, protection, and myopia control.
advantages of contacts over specs:
- more natural vision w/o the obstruction of frames
- wider field of view compared to specs
- no fogging during cold weather or physical activity
- better for sports esp contact sport and side vision
- UV protection
- improved comfort for individuals with anisometropia or irregular astigmatism
- cosmetic benefits and easier to wear w/ eye protection like goggles/ snorkels
what is the difference in the optical power bet specs and contact lenses??
the effective power of specs differ from the power at the cornea due to the vertex distance ( distance bet the spec lens and the cornea) in high prescriptions, the contact lens power must match the ocular refraction for full correction, which differs from the spec power
What formula is used to calculate the Back Vertex Power (BVP) of a contact lens from the spectacle prescription?
BVP (Contact Lens Power) = Spectacle Power / [1 - (Vertex Distance × Spectacle Power)]
How does accommodation differ when wearing contact lenses vs. spectacles for myopes and hypermetropes?
Myopes: Need more accommodation when switching from spectacles to contact lenses.
Hypermetropes: Need less accommodation when switching from spectacles to contact lenses.
What is the prismatic effect of spectacles and how does it compare to contact lenses?
Spectacles create a prismatic effect during off-axis viewing (e.g., for near vision).
Myopes experience a base-in prismatic effect with spectacles, while hypermetropes experience a base-out effect.
Contact lenses eliminate prismatic effects because they are aligned directly with the eye, so myopes need more convergence and hypermetropes need less convergence compared to spectacles.
How do spectacle lenses and contact lenses affect magnification?
Spectacles: Positive lenses magnify objects, and negative lenses minify. Magnification increases with vertex distance.
Contact lenses:
Myopes: See objects larger with contact lenses.
Hypermetropes: See objects smaller with contact lenses compared to spectacles.
What are the key parameters involved in contact lens design?
Base Optical Zone Radius (BOZR) – the curvature of the lens.
Base Optical Zone Diameter (BOZD) – the central optical zone’s diameter.
Total Diameter – the overall diameter of the lens, including peripheral areas.
What does the BOZR (Base Optical Zone Radius) represent in contact lens design?
The BOZR represents the curvature of the lens, and it should be around 0.6-1.0mm flatter than the average keratometry curvature of the cornea.
How do you calculate the Total Diameter of a soft contact lens?
Total Diameter = HVID (Horizontal Visible Iris Diameter) + 2.0mm. For example, if the HVID is 11.5mm, the total diameter will be 13.5mm.
What is keratometry and how is it used in contact lens fitting?
Keratometry measures the radius of curvature of the cornea. It is used to determine the steep and flat meridians of the cornea, which are essential for selecting the correct base curve of the contact lens.
What are the two types of corneal astigmatism that keratometry can measure?
WTR (With-The-Rule) Astigmatism: The steepest meridian is vertical.
ATR (Against-The-Rule) Astigmatism: The steepest meridian is horizontal.
What is the formula used to calculate the Back Vertex Power (BVP) of a contact lens?
BVP (Contact Lens Power) = Spectacle Power / [1 - (Vertex Distance × Spectacle Power)]
How do you calculate the ideal soft contact lens BOZR and Diameter from keratometry?
Base Curve (BOZR):
0.6-1.0mm flatter than the average keratometry curvature (K value).
Total Diameter:
2.0mm greater than the Horizontal Visible Iris Diameter (HVID).
What is the Back Vertex Power (BVP) calculation used for in contact lens fitting?
The BVP calculation is used to adjust the contact lens power according to the spectacle prescription, taking into account the vertex distance.
What is the Horizontal Visible Iris Diameter (HVID) and how is it used in contact lens fitting?
HVID is the diameter of the visible iris, typically ranging from 10-14mm (average 11.7mm). It is used to calculate the total diameter of the contact lens.
How does keratometry help in determining contact lens parameters?
Keratometry provides measurements of the corneal curvature, which helps in selecting the appropriate base curve and lens fitting for the contact lenses.
What is corneal topography, and how is it useful for contact lens fitting?
Corneal topography maps the shape of the cornea, helping to assess conditions like astigmatism or irregular corneal surfaces, which are crucial for proper contact lens fitting, especially for rigid lenses or scleral lenses.
How does the HVID affect the Total Diameter of the contact lens?
The Total Diameter of the contact lens is typically 2.0mm greater than the HVID. For example, if the HVID is 11.5mm, the total diameter would be 13.5mm.
What are the main factors influencing soft contact lens fitting?
The main factors include keratometry readings, HVID, and the back vertex power (BVP) calculation based on the spectacle prescription.
optical doubling technique measures:
curvature of cornea
For a healthy cornea, keratometry is generally drawn to-
two principal meridians
During near vision a spectacle wearing hypermetropic patient experiences:
base-out effect
What is denoted as BPZD in contact lens design?
Back peripheral zone diameter
