NOCE Ophthalmic Products and Materials Questions and Answers — Questions and Answers
Question 1: A patient with a high-minus prescription who is sensitive to peripheral color fringing would benefit most from a lens material with which of the following characteristics?
- A low specific gravity and a low Abbe value
- A high refractive index and a low Abbe value
- A high specific gravity and a high Abbe value
- A low refractive index and a high Abbe value (Correct answer)
Correct answer: A low refractive index and a high Abbe value
The Abbe value is a measure of a lens material's chromatic aberration. A higher Abbe value indicates less chromatic aberration, meaning less color fringing, which is crucial for patient comfort in higher prescriptions. While a high refractive index creates a thinner lens, it is often associated with a lower Abbe value (more chromatic aberration). Therefore, prioritizing a high Abbe value is most critical for this patient's specific complaint. A low refractive index material like CR-39 often has a high Abbe value (58).
Question 2: A construction worker requires prescription safety glasses. According to ANSI Z87.1 standards, which lens marking indicates the eyewear has been tested for high-impact resistance?
- Z87
- Z87-2
- Z87+ (Correct answer)
- D3
Correct answer: Z87+
The ANSI Z87.1 standard specifies markings for safety eyewear. The '+' sign indicates that the lens has passed a high-velocity impact test, which involves a 1/4-inch steel ball traveling at 150 feet per second. The 'Z87' mark alone signifies basic impact resistance. 'Z87-2' indicates a prescription safety frame. 'D3' indicates protection from liquid splashes and droplets.
Question 3: An optician is designing a progressive addition lens for a patient with a high 'add' power who prefers a smaller, fashionable frame. To maximize the usable near vision area within the constraints of the small frame, which lens design characteristic should be prioritized?
- A longer corridor length
- A harder lens design
- A shorter corridor length (Correct answer)
- A wider distance zone
Correct answer: A shorter corridor length
A shorter corridor length in a progressive lens allows the full add power to be reached more quickly, requiring less downward eye rotation. This is advantageous for smaller frames because it ensures the near zone is fully contained within the bottom of the lens shape. A longer corridor would place the near zone too low, potentially cutting it off.
Question 4: A patient with anisometropia of 2.50 D experiences vertical imbalance and diplopia when reading through their bifocal. To correct this, a slab-off is ordered. On which lens is a conventional slab-off typically ground?
- The lens with the most plus power in the vertical meridian.
- The lens with the most minus or least plus power in the vertical meridian. (Correct answer)
- The lens with the higher cylinder power.
- The lens with the lower base curve.
Correct answer: The lens with the most minus or least plus power in the vertical meridian.
A conventional slab-off is a bicentric grind that adds base-up prism to the reading area of one lens to counteract vertical imbalance. It is always applied to the lens that has the most minus or least plus power in the 90-degree (vertical) meridian. This effectively neutralizes the excessive base-down prism induced in that eye at the reading level.
Question 5: When comparing polycarbonate and Trivex lens materials for a drill-mounted frame, Trivex is often preferred due to its superior:
- Refractive index
- Abbe value
- Tensile strength (Correct answer)
- Specific gravity
Correct answer: Tensile strength
Trivex has a higher tensile strength than polycarbonate, making it more resistant to cracking and splitting around drill holes. This makes it an ideal choice for rimless or drill-mounted frames where the lens material itself must bear structural stress. While polycarbonate has excellent impact resistance, its lower tensile strength makes it more prone to stress fractures in this type of mounting.
Question 6: An aspheric lens design is most beneficial for a patient with a high hyperopic prescription primarily because it:
- Increases the magnification effect for better reading.
- Reduces lens thickness, weight, and peripheral distortions. (Correct answer)
- Enhances chromatic aberration correction.
- Allows for a greater range of tinting options.
Correct answer: Reduces lens thickness, weight, and peripheral distortions.
Aspheric lenses have a flatter curvature profile compared to traditional spherical lenses. This design minimizes the lens profile, making it thinner and lighter, which is especially noticeable in high plus (hyperopic) prescriptions. This change in curvature also significantly reduces spherical aberration, which causes distortion and a 'bulging' look at the lens periphery, thereby improving both aesthetics and visual quality.
NOCE Ophthalmic Products and Materials Questions and Answers
A patient with a high-minus prescription who is sensitive to peripheral color fringing would benefit most from a lens material with which of the following characteristics?