NOBC 2025 SESSION A - UNDERSTANDING BASIC TECHNOLOGY IN BIOMETRY

1. Regarding the refractive index of a medium

• A. It is the ratio of the velocity of light in air to the velocity of light in the medium

• B. It is a measurement of optical density of the medium

• C. When light travels from a less dense to a denser medium, the ray is deviated away from normal

• D. It plays a crucial role in various optical phenomena including light reflection and refraction

Answer explanation

C. When light travels from a less dense to a denser medium, the ray is deviated towards normal

2. Regarding the refractive structures of the eye

• A. It made up of cornea, aqueous humour, lens and vitreous humour

• B. Lens has the strongest refractive power

• C. Refractive index of cornea is 1.376

• D. Crystalline lens contributes to 1/3 of total refractive eye power

Answer explanation

B. False:  The cornea has the strongest refractive power of 40D (2/3^rd of total eye power)

3. Regarding advantages of Immersion A-scan compared to Contact A-scan

• A. Immersion A-scan provides more accurate measurement

• B. Immersion A-scan produces less corneal irritation to the patient's eye

• C. Immersion A-scan measurements will yield shorter axial length than Contact A-scan

• D. Presence of macular oedema will not affect the measurement taken using Immersion A-scan

Answer explanation

C. False: Immersion A-scan will yield a longer AXL measurement due to elimination of corneal indentation (by 0.14 -0.36mm longer)

D. False: Macula oedema will affect the measurement using both Immersion & contact A-scan due to irregularity of the interface (weaker & shorter amplitude spike)

4. Regarding optical biometry

• A. It measures along the optical axis

• B. The results are reproducible and repeatable

• C. The measurement combines the axial length and keratometry reading

• D. It measure up to RPE at the fovea

Answer explanation

A. Optical biometer measures along the visual axis.

Ultrasound biometer measures along the optical axis

5. 5. Choose the correct statement

• A. Optical axis is an imaginary line perpendicular to the cornea that intersects the center of the pupil

• B. Visual axis is an imaginary line that connects the object in space, the center of the pupil, and the center of the fovea

• C. Ultrasound biometer measures the AXL from corneal vertex to inner limiting membrane of retina

• D. Optical biometer measures the AXL from corneal vertex to retinal pigment epithelium