wk2: CL/M - Nomenclature and Verification of CLs Flashcards
Why is CL verification necessary? (4)
To confirm lens received is as ordered
To identify parameters of lens prescribed elsewhere
Confirm parameters of trial lenses
Ensure that the lens meets ISO standard
Define BOZR
Back Optic Zone Radius
Define BOZD
Back Optic Zone Diameter
Define BPR
Back Peripheral Radius
Define BPZD
Back Peripheral Zone Diameter
Define TD
Total Diameter
Define FOZR
Front Optic Zone Radius
Define BVP
Back Vertex Power
Define CT
Centre Thickness
Define ET
Edge Thickness
Define Dk
Oxygen permeability
Define Dk/t
Oxygen transmissability
What tool is used to verify the BOZR/BC in RGPs?
Radiuscope (optical microspherometer)
What techniques can be used to verify the BOZR/BC in SCLs? (3)
Template method
Radiuscope
Spherometer (optimec device)
What is the “Drysdale’s principle”?
For a curved mirror, an image is formed in the same plane as the objectwhen the object is at the centre of curvature of the mirror
Briefly explain how a radiuscope (optical microspherometer) works (3)
The curved surface of the contact lens acts as a mirror. Two images of the projected target are formed: a surface image and cleaner aerial image. The distance between focusing these two images gives the radius of curvature of the contact lens
Describe the procedure to use a radiuscope to measure radius of curvature (6)
Centre the beam on the CL
Focus target image on lens surface (surface image)
Set internal scale to zero
Obtain aerial focus (aerial image) by moving microscope up
Read radius of curvature off the internal scale
Use an average of 3 readings
When using a radiuscope to measure radius of curvature, in what size steps should you work in?
work in 0.5mm steps
How accurate is the internal scale at measuring radius of curvature in radiuscopy?
within 0.01-0.02mm
How does radiuscopy differ for toric lenses? Explain
Toric lenses have two radii of curvature, meaning that not all spokes of the aerial image will come into focus together. This means that 2 radings willn eed to be taken
How does radiuscopy differ for a warped lens?
aerial image is not focusable
What is the main problem with doing radiuscopy on SCLs?
Only a small amount of incident light is reflected back, therefore you need a very high intensity illumination system
Describe how the template method for measuring radius of curvature is carried out (4)
Lens placed concave down on a template of known radius
If the two surfaces are not aligned, a bubble forms
The position of the bubble indicates if too flat or too steep
Can then place lens into an adjacent radius template until no bubble
How accurate is the template method for measuring radius of curvature?
Very approximate (accuracy +/- 0.30mm)
In what type of CLs is BPR measured?
RGPs
In what type of CLs is FOZR measured?
RGPs
Describe how a spherometer works (5)
Lens mounted and centred with convex surface upward in saline
Viewed at 15x magnification
Probe advanced until contact is observed
Amount by which it needs to be advanced gives measure of sagittal depth (or “sag”)
BOZR is then calculated from the “sag”
How does the radiuscope procedure differ when verifying BPR instead of BOZR?
Mount is tilted so the light falls on teh peripheral curve
When would verification of BPR with a radiuscope be difficult? (2)
if narrow band width (<0.7mm) or blended peripherally
Why is verification of BPR with a radiuscope not performed for SCLs? (1)
there is no visible junction for SCLs
How does the radiuscopy procedure differ when verifying FOZR instead of BOZR? (2)
A special FOZR mount is used (i.e. convex surface UP)
The aerial image is obtained by moving the microscope DOWN (i.e. the image is below the lens rather than above it)
How can we verify BOZD and BPZD in RGPs? (2)
Measuring magnifier
Projection magnifier
What is a measuring magnifier?
magnifying lens with adjustable eyepiece focussed onto a clear platform with a graticule scale
What is a projection magnifier? (2)
lens placed on stage in path of light beam.
shadow projected onto screen which has a mm grid scale.
What 3 tools can we use to verify TD in RGPs?
Measuring magnifier
Projection magnifier
V-gauge
What 3 tools can we use to verify TD in SCLs?
Measuring magnifier
Projection magnifier
Optimec device
How does a V-gauge work? (3)
CL placed in wide end of gauge
Slips down by gravity until it stops (do not push)
Total diameter (TD) reads directly off the scale
What is the tolerance for the V-gauge?
+/- 0.05mm
What tool can we use to verify BVP? How easy is this in SCLs?
Projection vertometer. Difficult for SCLs
What tools can we use to verify CT (centre thickness)? (4)0
Thickness gauge
Pacymetry, A scan
Electrical micrometer
Radiuscope
(also OCT, which is being developed for verification)
How commonly do we verify water content?
uncommonly
What tool would we use to verify water content?
Atago soft contact lens hand refractometer
Outline the procedure for verifying water content (in SCLs)? (6)
Place lens on prism face
Close plate and apply gentle pressure to obtain firm contact between lens and prism
Point in direction of bright light source
Observe scale through eyepiece
Division of field into light and dark fields
%H2O reads directly off the scale
How can you verify surface quality of CLs? (4)
Slit lamp
Radiuscope or keratometer
Projection magnifier
Hand magnifier
How can you verify edge quality of CLs? (5)
Feel
Slit lamp
Simple inspection
Projection magnifier
Hand magnifier
How do you calculate 1st BPR?
1st BPR = BOZR + 1st PCR
How do you calculate 1st BPZD?
1st BPZD = BOZD + (1st PCW)x2
How do you calculate 2nd BPR?
2nd BPR = 1st BPR + 2nd PCR
How do you calculate 2nd BPZD?
2nd BPZD = 1st BPZD + (2nd PCW)x2
Outline the 5 steps in contact lens practice
Full eye test
CL fitting
CL delivery/teach
CL aftercare
CL refitting?
