4_TSH Optical Defects and vision correction

Thermoregulation, Special Senses, Endocrine System, and Reproductive System

  • Date: 27th January 2025

Learning Outcomes

  • Differentiate between the terms:

    • Refractive Power: Measure of the degree to which a lens system converges/diverges light.

    • Power of Accommodation: The ability of the eye to change its focus from distant to near objects.

  • Calculate refractive power and accommodation under various conditions.

  • Define and differentiate optical defects:

    • Myopia: Short-sightedness

    • Hypermetropia: Long-sightedness

    • Presbyopia: Age-related vision loss

    • Astigmatism: Uneven curvature of the cornea

  • Explain methods of optical intervention for each defect.

  • Calculate the strength of corrective lenses for common optical defects.

Refractive Power and Power of Accommodation

Refractive Power

  • Formula: (P = \frac{1}{f}) where:

    • (P): refractive power in dioptres (D)

    • (f): focal length in metres

  • Shorter focal length = larger refractive power.

Power of the Eye

  • Lens Equation: (\frac{1}{f} = \frac{1}{u} + \frac{1}{v})

    • (u): object distance

    • (v): image distance (length of eyeball = 2.5 cm)

    • Normal eye (v): approximately 2.5 cm.

Maximum Refractive Power

  • Occurs when fully accommodated for near point (u = 25 cm)

    • (P_{max} = \frac{1}{0.25} + \frac{1}{0.025} = 44)

  • Convert distances to metres.

Minimum Refractive Power

  • Occurs when relaxed for far point (u = ∞)

    • (P_{min} = \frac{1}{\infty} + \frac{1}{0.025} = 40)

Power of Accommodation

  • Defined as the difference between maximum and minimum refractive powers:

    • (A = P_{max} - P_{min})

  • Changes in near or far points can affect the power of accommodation.

Corrective Glasses

Determining Lenses

  • Formula: (P_{corrective} = P_{normal} - P_{defective})

Lens Types

  • Converging lenses: Positive value of refractive power, e.g., +5 D lens.

  • Diverging lenses: Negative value of refractive power, e.g., -5 D lens.

Optical Defects

Myopia (Short-sightedness)

  • Parallel light from distant objects is focused in front of the retina.

  • Causes:

    • Eyeball too long or cornea too curved.

  • Corrective Measures: Diverging lens to diverge light before it enters the eye to focus on the retina.

  • Lens Strength Calculation Example: If a person with a far point of 1 m:

    • (P = \frac{1}{1} + \frac{1}{0.025} = 41 \Rightarrow P_{corrective} = -1 D)

Hypermetropia (Long-sightedness)

  • Parallel light is focused behind the retina; difficulty focusing on nearby objects.

  • Causes:

    • Eyeball too short or cornea too flat.

  • Corrective Measures: Converging lens to provide additional focusing power.

  • Lens Strength Calculation Example: Near point at 1 m:

    • (P = \frac{1}{1} + \frac{1}{0.025} = 41 \Rightarrow P_{corrective} = +3 D)

Presbyopia

  • Age-related reduction in accommodation; near point moves further away.

  • Corrective Measures: Converging lens for close activities (reading glasses).

Astigmatism

  • Distorted vision due to uneven cornea curvature, resulting in multiple focal points.

  • Assessment: Testing vision clarity with radial line patterns.

  • Corrective Measures: Asymmetric cylindrical lenses that differ in strength across vertical and horizontal axes.

Other Eye Problems

  • Additional problems may include:

    • Trauma, glaucoma, cataracts, etc. requiring direct medical intervention.

Conclusion

  • For more information, contact Ingmar Schoen via email: ingmarschoen@rcsi.ie