Contact lens verification Flashcards
give 5 reasons why you will bother with CL verification
- Check quality of manufacturing
- Ensure matches the CL spec you have ordered
- If the px mixes up the right and left lenses
- To confirm current pair of lenses worn
- Verify trial lens (fitting) set
We have a obligation to check that lens we are giving to the px is correct
list 9 things about a lens you should check
- Base curve: BOZR, (central and peripheral
- Diameter (TD,BOZD)
- Power (BVP, FVP)
- Centre thickness
- Edge thickness
- Edge form
- Surface quality
- Colour
- Special features
what can be used to check the power of a CL
focimeter
which power of the lens are you using a focimeter to check
what is a disadvantage to using this and what can you do that will help
what can be done to the focimeter to make it better to use with CLs and why
- Back vertex power
- Image quality may be poor due to lens surface e.g. scratches or deposits - so make sure lens is clean and dry before use
- Better to use a small aperture:
usually the aperture stop is too large for CLs when it is set up for glasses
Because:
as the CL is very steep it cannot align with the aperture stop as the aperture stop of the focimeter is not in the centre of the lens
so it does not measure the correct area of the power of the lens which will be in the middle
so you can attach a small aperture stop on the focimeter to see through the power of the centre of the CL and measure it correctly
instruct how you will check the BVP of a RGP lens using a focimeter
- Make sure lens is clean and dry
- Concave side down on the focimeter stop
- Focus mires and take reading
instruct how you will check the BVP of a SCL using a focimeter and what you need to be careful of
- Set the focimeter scale at the expected reading (if known)
- Rinse lens and dry with lint-free tissue (no kleenex)
- But SCLs have a tendency to dry out, so take the reading quickly
instruct how you will check the FVP using a focimeter
and what can you do with the FVP measurements
- Procedure is same as for BVP except the convex side is placed downwards
- Can convert FVP measurements to BVP if centre thickness and BOZR is known
how can you measure the peripheral curve width PCW
the difference between TD and BOZD
what 3 instruments can be used to verify TD of a CL
- V-guage
- Magnifier
- Projection magnification with scale
instruct how you will use a V-guage to verify a TD of a lens
- It is a ruler with a slot in it
- Ensure lens is dry
- Pop lens in and push down until it doesn’t go down any further
- Don’t force lens-it will break or edges will be damaged
what 2 parameters of a CL can a magnifier measure
what power does it use
which types of CL can be used
- BOZD and TD
- Approx. x10 mag
- RGP or SCL
describe how a magnifier is used to check the BOZD and TD of a CL
what must you ensure for a RGP
what must you ensure for a SCL
- Magnifier provides light
- For BOZD: look for the markings of the optic zone seen inside the concentric line and line up that marking where the BOZD begins with the 0 on the scale and measure
- For TD: exactly the same way but from edge to edge of the lens
- Look through eyepiece and measure
- RGP: ensure lens is dry
- SCL: lens is placed in a wet cell = a thin CL case where can put saline in
what 2 things must you be careful of when using a magnifier to measure the TD and BOZD a SCL
- be careful of cross contamination when using the case to hold the SCL in the wet cell
and - take into account of refractive index of saline
what can you also measure off in a multifocal lens using a magnifier
can line up the seg of the bifocal part and measure the near zone
what 2 parameters of a CL can you use to measure using a projection magnification with scale which 2 types of CL can you measure describe how you will carry this out what must you ensure with a RGP what will a SCL require
- BOZD and TD
- RGP or SCL
- Light up the lens with a strong light and shadow of lens will be shown
- Line up lens with scale
- Ensure it’s in focus
- RGP: ensure lens is dry
- For a SCL, would require lens to be held in wet cell with saline
solution
as well as BOZD and TD which are the main uses of a projection magnification with scale, what else can you check about a CL
can use it to check the lens quality = good way to show px if theres lots of deposits etc
which 3 instruments can be used to measure base curve
- Radiuscope
- Keratometer
- Photokeratoscope (topographer)
describe how you will measure a CL BC with a radiuscope and which principle does it use
- uses the drysdale principle
- Clean and dry lens
- Use saline in holder
- Focus aerial image
- You get th first image into focus and carry on until you see the bulb/filament and keep going until you see the 2nd image of the spoke
- Ensure it reads zero on dial
- Focus on front lens surface
- BC = difference between the first and the second image
- Read dial
- Repeat 2-3 times
- Tilt lens to measure peripheral curves
how is a radiuscope used to measure the BC os a toric lens
- either no spokes get into focus
- or one of them might be bit clearer
- what you aiming for is to get one of them very sharp
- you may have to rotate the lens to get it into focus
- keep going until the perpendicular one get into focus
how is a keratometer used to measure BC of a CL
what does it require
what can be included for a SCL
what will you do to measure the peripheral BC
- Special lens mount (can’t just hold CL in front) - lens is put on top of a prism
- Requires modified scale
- Clean and dry lens
- Add saline to holder
- Can include wet cell for
soft lenses to stop it from drying out - For periphery BC-tilt lens
how does a photokeratoscope (topographer) measure the BC of a CL
- lens is put on a special holder
- take image of the CL
- the machine uses special software to work out the power of the lens
- it produces a power profile map of the lens
when placing a SCL in a saline immersion/wet cell to measure BC on all instruments, what needs to be done to get the true BC/BOZR value
- readings are multiplied by the
refractive index of saline to calculate the BOZR - can use a radiuscope or ultrasonic measurement
what 2 instruments can be used to verify the thickness of a CL
- dial thickness gauge
- electronic thickness gauge
down to which value can a electronic thickness gauge measure a CL
100th of a mm
it doesn’t mean this machine is more accurate, it just measures down to that
describe how you will use either a dial thickness gauge or electronic thickness gauge to measure the thickness of a RGP CL and what must you be careful of whilst doing this
- for central thickness tc - you want to measure along the optic/geometric axis
- central thickness is also referred to as the axial thickness
- to measure the edge thickness te - rotate the lens and move it along the edge
- edge thickness is also referred to as radial thickness
be careful not to damage the lens
what is central thickness of a lens also referred to as
what is edge thickness of a lens also referred to as
axial thickness
radial thickness
what 4 different thickness gauges can you use to measure the thickness of a SCL
Variety of gauges you can use:
- Electronic thickness gauge
- Pressure controlled gauge
- Electrical thickness gauge
- Radiuscope (modified)
what 2 instruments can be used to check edge profile
why will you want to check this
- microscope
- slit lamp
to examine the CL and make sure of no defects which can cause inflammation or discomfort
how will you use a slit lamp or microscope to check the edge profile of a lens
- Dark (lights off)
- Rotate lens to inspect edges
- Increase magnification
what 3 different types of edge shapes are there
- knifepoint
- chisel
- round
other than looking fr the edge shape, what else may you want to find out about a edge profile of a CL and what can’t cause this
- edge quality
- can be a torn lens
- damaged by case or nails
what 2 instruments are used to measure surface quality of a CL
- microscope
- slit lamp
what 5 things will you be look out for when checking surface quality of a CL
how will you carry this out
You are looking out for
- Manufacturing errors
- Mottling
- Lathe marks
- Scratches
- Pitting
- Focus eyepieces
- Use magnification dials
- Rotate and examine lens
list 6 special features you can look for when verifying a CL and what 4 things you can use to do this
- Engravings
- Lab codes
- Fenestrations: how many? Where? Size?
- Carrier design
- Prism ballast?
- Truncation?
Magnifying 10x loupe
Naked eye
Microscope
Slit lamp
which 3 colours are used to verify tints/colour of a CL and what does each colour stand for
why is this used
what 3 instruments can be used to check this
what is the best way to do it
- Grey = right eye
- Green = right eye
- Blue = left eye
Rule: second letter of colour - Naked eye
- Slit lamp
- Band magnifier
- View against white paper
what is the reality about SCL checking
In reality, most lenses are in sterile pre-packed blister packs and are verified by the manufacturer
We usually just check the packet to confirm lens power, diameter, base curve, date of expiry, brand, material etc
list all the things about a CL the a slit lamp and microscope can be used for to measure/check
- edge profile
- surface quality
- special features
- CL colour/tint
what is the BS code for an RGP
the parameters it includes and their tolerances
- BS 7208: part 1: 1992
- ISO 8321-1:1991
BOZR - ± 0.05
BOZD - ± 0.20
TD - ± 0.10
Centre thickness - ± 0.02
what is the BS code for a SCL
the parameters it includes and their tolerances
- BSEN ISO 8321-2:2000
- BS 7208-24:2000
BOZR - ± 0.05
sag (at specified diameter) - ± 0.05
TD - ± 0.20
Centre thickness (unto 0.1mm) - ± 0.01 +10%
which instrument used to verify BOZD and TD is mainly used in lab
projection magnification with scale
which is a more practical instrument to use in clinic to measure BC/BOZR
keratometer (more practical than radiuscope)
