COT - Certified Ophthalmic Technician Optics and Lensometry 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?

Accepted Answer

The back vertex power of the lens.

Answer

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?

Accepted Answer

1.5 Δ Base Up

Answer

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?

Accepted Answer

+1.50 -2.50 x 020

Answer

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:

Accepted Answer

Chromatic aberration

Answer

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?

Accepted 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.

Answer

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.

COT - Certified Ophthalmic Technician Practice Test

COT - Certified Ophthalmic Technician Practice Test

COT - Certified Ophthalmic Technician Optics and Lensometry Questions and Answers