Dispensing

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Basic Lens Dispensing Part 4

mivision | 1 July 2009
The ODMA board is committed to the ongoing education and training of the optical dispenser. To that end, in this issue we continue our series of articles from the ODMA Optical Retailers Guide with the fourth part of our series on the Basics of Lens Dispensing: Prescription Interpretation.

With prescription interpretation:

  • All prescriptions require an inter Pupillary Distance (PD) - or preferably a measurement of the distance between the visual axes - for this measurement a pupillometer is essential.
  • All lenses other than Single Vision require Segment Heights or Pupil Heights.
  • All Single Vision lenses need to be identified as either:
    1. Distance
    2. Intermediate or,
    3. Near


The Five Essentials

These are the minimum requirements for every prescription order:

  • The Rx (sphere, cyl, axis)
  • The Add (reading addition)
  • Pupillary Distance (PD)
  • Pupil Heights or Segment Heights must be included if dispensing multifocals (progressives, bifocals etc.)
  • Lens Type (distance, near, bifocal etc.)

Other requirements may include such things as surface coatings or lens colour tints.

The choice of lenses including types, materials, refractive indices, asphericity and abbe values should be discussed with the client to suit the prescription.

For example, a person with a strong prescription (+/- 4.00 Ds) would probably be best served by a hi-index plastic (1.67 o greater), aspheric, multicoated lens in a smaller frame.

This would result in a thin, light weight, non ref lective, comfortable pair of spectacles.


Features of Lens Materials

  • Refractive Index - Ability to bend light.
  • Weight - Relates to wearer comfort.
  • UV Protection - Amount of UV light stopped.
  • Durability - Ability to survive daily wear and tear.
  • Impact Resistance - Mechanical strength.
  • Transmission Value - How much light passes through.
  • Abbe Number - Measure of chromatic aberration.


Refractive Index

  • The speed of light through material is slower than the speed of light in air.
  • Refraction creates a longer path for light to travel through.
  • As the material's refractive index increases, the amount of curvature required to produce any given power decreases. This allows lenses to be thinner.
  • Some high refractive index materials usually create higher ref lections unlessthe lenses are coated with anti-ref lection coatings.
  • If anti-reflection coatings are not used the lens's optical qualities will be reduced.


Visible Light

  • Light is generated by the sun and artificial light sources.
  • We assume that light travels in a wave motion.
  • The direction of light is depicted in the form of a light ray.
  • Visible white light results from the combination of all of the various wavelengths of the electromagnetic spectrum including: 1. Visible - comprising those colours seen in a rainbow
    2. Invisible - long wavelengths such as infra-red and invisible short wavelengths such as ultra-violet, x-ray and gamma radiation
  • Wavelength is measured in nanometres.


                         

UV Light

Filters in spectacle lenses and sunglasses can protect the eyes from even the most extreme UV light exposure.

 

UV C 10nm to 286nm
UV B 287nm to 320nm
UV A 321nm to 400nm


Durability or Impact Resistance

  • Ability to withstand a strong impact

  • Hard resin breaks into large dull edge piece.


Transmission

  • This refers to the transparency of a material - better transmission enables more light to pass through the lens.
  • Most lenses have a transmission value of 85-92 per cent, with the rest of the light being ref lected - transmission value is improved dramatically by utilising anti- ref lection and multi-coatings.


Lens Material Comparison Table


 


ABBE Number

The ability ofa material to split light into its different constituent colours is signified by an ABBE number. High numbers are good, low numbers are bad



Aspheric Design vs. High Index

  • Aspheric design provides:

- Improved optical performance
- Allows flatter front curves
- Reduces thickness

  • High Index materials will reduce thickness but may sacrifice optical performance in high power lenses.
  • Combining aspheric design and a balanced high index material provides the best combination of cosmetic and optical performance.


Other terms

  • Add Power - the amount of power added to a distance prescription to enable the patient to read or view objects at a near distance. Also the amount of additional plus power in the segment of bifocals or multifocals.


Balance of Properties

  • Lens material provided should suit the needs of the prescription and the patient's requirements.
  • Refractive Index is not the only measure of lens material performance.
  • A good lens material should have a balance of all material properties.

Part 4 of the Basic Lens Dispensing series has been reproduced with kind permission from ODMA.

This article appeared in the ODMA Optical Retail Guide, in the Education and Training section, under the heading of ‘Basic Lens Dispensing Part 4'.

The first edition of the Optical Retail Guide was distributed to independent practitioners across Australia in early December 2008.

Independent practitioners, who would like to receive a copy of the ODMA Optical Retail Guide, should contact Finola Carey on: E: marketing@odma.com.au