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Polycarbonate mounting disc
Reference: BV20.D30

Polycarbonate mounting disc

  • Material: Polycarbonate
Public price (Tax incl.) 38.69 €

  • Working Distance
General conditions
Delivery: 2-3 working days
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Mounting disc: all the options:
  1. Plano lens.
  2. Recessed lens, for a better vertex distance.
  3. Converging lens for binocular vision at:
  • 200 mm
  • 250mm
  • 330mm
  • Inner Ø 27.7 mm
  • Outer Ø 76 mm






Galilean system - Principle:


  • The Galilean monocular is an afocal magnifier, meaning that the image is formed at infinity.
  • The point of this system is therefore simply to magnify in distance vision.
  • For near vision, just add an add-on lens to the front.
  • Note that its power determines the working distance, which equals its focal length.


Monocular:

  • Anodised aluminium body, very stable.
  • Very light.
  • Achromatic telescope.
  • Wide field of view: 24° for the 1.8x
  • Anti-reflective coated.


Telescopes focused at infinity ∞

1.8 x

2.1 x

2.5 x
Objective Ø 20mm 20mm 20mm
Field 24° 20° 16°
Weight 10 gr 12 gr 16 gr






  • You can:
    • Buy a trial case with 3 monoculars that fit the optician's standard trial frame
    • Rent a trial case, for a one-off sale.

Our add-on lens holder, used with its trial box, lets the optician find the wearer's optimal magnification.

  • It is universal: all you need is one telescope with a trial ring.
  • No more need for excessive stock.
  • Placed on the trial frame, the patient's prescription can be added to assess the benefit with correction.
  • The trial is done in real-life conditions.
    • With a reading chart or a newspaper.
    • On a computer screen.
    • At intermediate distance, a television screen.
  • To find the focal power of the add-on lens, divide 100 by the eye-to-object distance (in centimetres).
  • Choose a large "panto" style frame.
    • The optical centre must be the near-vision centre.
    • The distance from the optical centre to the frame must be at least 19mm.
    • Mark the optical centre (near vision) and check it.
  • For a monocular fitting, order:
  1. The chosen 1.8x, 2.1x or 2.5x telescope (Code BV20.Gxx ?).
  2. The add-on lens determined with its trial box. Code (BV20.Axx ?)
  • Option: pre-calibrated ophthalmic lens to be edged to 21mm.
  • The plain mounting ring (Code BV20.B24) or prescription mounting ring (Code BV20.B20), depending on the option above
  • A mounting disc to be edged. (Code BV20.D30)
  • A large case. (Code BV20.E01)
  • For a binocular fitting, order:
    1. 2 chosen 1.8x, 2.1x or 2.5x telescopes (Code BV20.Gxx ?).
    2. 2 add-on lenses determined with the trial box. Code (BV20.Axx ?)
    • Option: 2 pre-calibrated ophthalmic lenses to be edged to 21mm.
  • 2 plain mounting rings (Code BV20.B24) or prescription mounting rings (Code BV20.B20), depending on the option above
  • 2 mounting discs to be edged. (Code BV20.Dxx ?).
  • A large case. (Code BV20.E01)
  • A mounting kit. (Code BV20.D34)

    • Or simply mark the near-vision optical centre on the spectacles and LESA FRANCE takes care of everything..............Code BV20.MONT

      • Simple, isn't it?........

    How to proceed

    Conversion table for removable add-on lenses

     

    Telescope magnification

     

    1.8 x

    2.1 x

    2.5 x
    0.5 D 2 metres Watching television 2.00 2.40 2.80
    1 D 1 metre Watching television 2.25 2.60 3.10
    1.5 D 66 centimetres Looking at a screen 2.50 2.90 3.40
    2 D 50 centimetres Looking at a screen 2.70 3.10 3.75
    3 D 33 centimetres Looking at a screen, reading 3.10 3.60 4.30
    4 D 25 centimetres Looking at a screen, reading 3.60 4.20 5.00
    5 D 25 centimetres Looking at a screen, reading 4.00 4.70 5.60
    6 D 17 centimetres Looking at a screen, reading 4.50 5.20 6.20
    8 D 12 centimetres Looking at a screen, reading 5.40 6.30 7.50
    10 D 10 centimetres Looking at a screen, reading 6.30 7.30 8.70
    12 D 8 centimetres Looking at a screen, reading 7.20 8.40 10.00
    14 D 7 centimetres Looking at a screen, reading 8.10 9.40 11.20
    16 D 6 centimetres Looking at a screen, reading 9.00 10.50 12.50

    How do you choose the telescope magnification?


    • Look at the table above.
    1. Suppose the patient achieves useful acuity with a 1.8x Galilean and a 12D add-on lens: the working distance is 8cm.
    2. With a 2.1x telescope, the working distance increases from 8cm to 10cm for the same magnification (±≈).
    3. With a 2.5x telescope, the working distance increases from 8cm to 12cm for the same magnification (±≈).
  • You gain distance on one hand but lose field on the other: testing is the only way to find the best solution.


    • The Galilean telescope.
     
     
    Telescopic systems are multi-lens systems that allow both distance and near vision.
     
     

    How does it work?

    Its basic principle is that of binoculars.
     

    Binoculars are a Keplerian system.
     
    • It consists of a convex objective and an eyepiece that is concave..
    • When the 2 focal points coincide, the system is said to be afocal.
    • The magnification of such a system is the ratio of the objective focal length to the eyepiece focal length.
    • For example, with an 80mm objective and a 40mm focal-length eyepiece, the magnification is 80/40=2.
    • The total length of the system is then 120mm, and the image is also inverted.
    • Rather difficult to fit on spectacles.

    The Galilean telescope is the most convincing alternative.
    • It consists of a convex objective and a concave eyepiece.
    • When the 2 focal points coincide, the system is said to be afocal.
    • The 2 focal points coincide; since the eyepiece is negative, both lenses lie in front of the focal point.
    • The total length of the system is then 40mm, and the image is upright, so no erecting prisms are needed.
    • The drawback is that the eyepiece focal length is very long for a negative lens, which limits magnification to 3x at most to keep a good image.
    • That said, the system is compact and light, so it can be fitted on a spectacle frame.

    Near vision.
    • Adding a lens in front of a telescope makes the focal length of the system that of the add-on lens.
    • The magnification is the telescope power multiplied by that of the lens.
    • For binocular vision, angled mounts are used.
    • The limit for a binocular fitting is 5D.
    • To correct ametropia, a prescription lens is placed in front of the eyepiece.
    • We can carry out the complete fitting.

    In summary.
    • The system is light and can be fitted on a frame.
    • A Galilean telescope without a lens gives vision at infinity when ametropia is corrected
    • Near vision at the focal length of the add-on lens.
    • Without corrected ametropia the image is still good; only the focal distance varies slightly.
    • Strong astigmatism must be corrected.
    • To correct ametropia, a prescription lens is placed in front of the eyepiece.
    • By design, our telescopes have a pinhole effect.

    The big advantage.
    • Lets you work hands-free.
    • Lets you watch television with additions below 1.
    • Lets you read an entire screen just by moving your head.

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    Characteristics:

    Reference BV20.D30
    Net weight 11 g
    Working distance ∞ mm
    Dimensions 70 mm
    Material Polycarbonate
    Public price (Tax incl.) 38.69 €