COT - Certified Ophthalmic Technician Optics and Lensometry Questions and Answers — Questions and Answers
Question 1: A technician uses a manual lensometer to neutralize an unknown spherical lens. The target mires come into sharp focus when the power wheel is set to -3.25 D. What does this measurement represent?
- The back vertex power of the lens. (Correct answer)
- The front vertex power of the lens.
- The equivalent spherical power.
- The prismatic power of the lens.
Correct answer: The back vertex power of the lens.
Standard manual lensometers are designed to measure the back vertex power, which is the effective power of the lens relative to the eye's position. This measurement is crucial for ensuring the patient receives the correct prescribed power when wearing the glasses.
Question 2: A patient's prescription is +3.00 D sphere in the right eye. If the patient looks 5 mm down from the optical center of the lens, what is the magnitude and direction of the induced prism according to Prentice's Rule?
- 3.0 Δ Base Down
- 1.5 Δ Base Up (Correct answer)
- 1.5 Δ Base Down
- 3.0 Δ Base Up
Correct answer: 1.5 Δ Base Up
Prentice's Rule states that Prism (Δ) = Power (D) x Decentration (cm). Here, D = +3.00 and Decentration = 5 mm = 0.5 cm. So, Prism = 3.00 x 0.5 = 1.5 Δ. For a plus lens, the base of the prism is in the direction of decentration (looking down), so the induced prism is Base Up.
Question 3: An optical lab requires a prescription to be in minus cylinder form. Which of the following is the correct transposition of the prescription -1.00 +2.50 x 110?
- +1.50 -2.50 x 020 (Correct answer)
- -3.50 -2.50 x 020
- +1.50 +2.50 x 020
- -1.00 -2.50 x 020
Correct answer: +1.50 -2.50 x 020
To transpose from plus to minus cylinder form: 1) Calculate the new sphere by algebraically adding the old sphere and cylinder (-1.00 + 2.50 = +1.50). 2) Change the sign of the cylinder (+2.50 becomes -2.50). 3) Change the axis by 90 degrees (110 - 90 = 020). The transposed prescription is +1.50 -2.50 x 020.
Question 4: A patient wearing high-power spectacle lenses complains of seeing color fringes, especially around lights and high-contrast objects. This optical phenomenon is known as:
- Spherical aberration
- Coma
- Chromatic aberration (Correct answer)
- Pincushion distortion
Correct answer: Chromatic aberration
Chromatic aberration occurs because a lens material refracts different wavelengths (colors) of light at slightly different angles, causing them to focus at different points. This dispersion of light results in visible color fringes, an effect that is more pronounced in lenses with a low Abbe value (higher dispersion).
Question 5: When neutralizing a pair of progressive addition lenses (PALs) in a manual lensometer, what is the correct procedure to determine the near add power?
- Measure the power at the fitting cross and subtract it from the power in the near viewing zone.
- Focus the mires on the distance verification circle, then move to the near verification circle and read the new spherical power directly.
- Measure the sphere power in the distance portion, then measure the total sphere power in the near portion; the difference between these two values is the add power. (Correct answer)
- Add +2.50 D to the distance power, as this is the standard add power for all progressive lenses.
Correct answer: Measure the sphere power in the distance portion, then measure the total sphere power in the near portion; the difference between these two values is the add power.
To find the add power, one must first measure the spherical power in the designated distance portion of the lens. Then, the lens is moved upward to the near viewing zone, and the new, stronger spherical power is measured. The add power is the algebraic difference between the near spherical power and the distance spherical power.
Question 6: Which of the following best defines the 'base curve' of a spectacle lens?
- The dioptric power of the cylinder component of the prescription.
- The thickest edge of a prism-containing lens.
- The curvature of the lens surface closest to the eye.
- The standard, foundational curve on a lens surface from which all other curves are calculated. (Correct answer)
Correct answer: The standard, foundational curve on a lens surface from which all other curves are calculated.
The base curve is the starting point for lens design. It is the primary curve on one of the lens surfaces (traditionally the front surface) that establishes the general shape of the lens. The other curves are ground relative to the base curve to create the final required prescription power.
COT - Certified Ophthalmic Technician Optics and Lensometry Questions and Answers
A technician uses a manual lensometer to neutralize an unknown spherical lens.
The target mires come into sharp focus when the power wheel is set to -3.25 D.
What does this measurement represent?