ABO NOCE Basic Opticianry Optical Instrumentation Use Questions and Answers — Questions and Answers
Question 1: An optician uses a lens clock to measure the surface curves of a lens. The instrument provides a direct reading of the surface power in diopters, but it is calibrated for a specific refractive index. If a lens clock calibrated for crown glass (n=1.53) is used to measure the front surface of a polycarbonate lens (n=1.586), and the reading is +6.00 D, what is the actual surface power?
- Slightly lower than +6.00 D
- Exactly +6.00 D
- Slightly higher than +6.00 D (Correct answer)
- It cannot be determined without the radius of curvature
Correct answer: Slightly higher than +6.00 D
A lens clock measures sagittal depth and is calibrated to display dioptric power based on a specific refractive index (typically 1.523 or 1.53 for crown glass). When measuring a material with a higher index of refraction, like polycarbonate (1.586), the lens clock will underestimate the true surface power. Therefore, the actual power will be slightly higher than the reading. The formula to find the true power is: F(true) = F(clock) * (n_true - 1) / (n_clock - 1). In this case, F(true) = 6.00 * (1.586 - 1) / (1.53 - 1) ≈ +6.63 D.
Question 2: While verifying a pair of finished eyewear with a manual lensometer, the optician cannot center the target mires within the reticle for the right lens, even with the lens platform centered. The target is displaced 2 rings upward and 1 ring toward the nasal side. How should the induced prism be recorded?
- 2Δ Base In, 1Δ Base Up
- 2Δ Base Down, 1Δ Base Out
- 2Δ Base Up, 1Δ Base Out
- 2Δ Base Up, 1Δ Base In (Correct answer)
Correct answer: 2Δ Base Up, 1Δ Base In
In a manual lensometer, the displacement of the target mires from the center of the reticle indicates the presence of prism. The direction of displacement is the direction of the prism's base. Each ring on the reticle typically represents one diopter of prism. An upward displacement means Base Up prism, and a displacement toward the nasal (inward) side means Base In prism. Therefore, a displacement of 2 rings up and 1 ring nasally corresponds to 2Δ Base Up and 1Δ Base In.
Question 3: A patient presents with a high-power prescription (-11.50 D) that was refracted at a vertex distance of 12mm. The chosen frame will sit at a vertex distance of 15mm. Which instrument is essential for the optician to use to ensure the compensated lens power is calculated correctly?
- Pupilometer
- Lens Clock
- Distometer (Correct answer)
- Polariscope
Correct answer: Distometer
A distometer (or vertexometer) is specifically designed to measure the vertex distance, which is the distance from the back surface of the lens to the front of the cornea. For prescriptions generally greater than +/- 4.00 or 5.00 D, a change in vertex distance between the refraction and the as-worn position can significantly alter the effective power of the lens. Therefore, measuring this distance accurately with a distometer is crucial for calculating the necessary power compensation.
Question 4: An optician is using a polariscope to inspect a new pair of plastic lenses that have been edged and mounted into a metal frame. They observe bright, spiky patterns of light flaring from the edges of the lens. What is the most likely cause of this observation?
- The lens is made of polycarbonate material.
- The lenses have a high-quality anti-reflective coating.
- The lenses are undersized for the frame.
- The lenses are oversized and under stress from the frame. (Correct answer)
Correct answer: The lenses are oversized and under stress from the frame.
A polariscope is used to detect internal stress in lens materials. When a plastic lens is cut too large for a frame, the frame rim exerts pressure on the lens edge. This physical stress becomes visible in a polariscope as bright, jagged, or spiky patterns of light, indicating areas where the lens is more likely to chip or crack. While polycarbonate inherently shows some rainbow patterns due to birefringence, stress from mounting appears as distinct flares from the edge.
Question 5: When using a pupilometer to take a monocular pupillary distance (PD) measurement for progressive lenses, what is the correct procedure?
- Ask the patient to look at the optician's open right eye, then open left eye, while the optician measures with a ruler.
- Align the vertical hairline on the right pupil, then the left, and add the two numbers together for the total PD.
- Have the patient look at the illuminated internal target and align the hairlines with the corneal reflections of each eye independently. (Correct answer)
- Measure the distance from the outer canthus of one eye to the inner canthus of the other eye.
Correct answer: Have the patient look at the illuminated internal target and align the hairlines with the corneal reflections of each eye independently.
A pupilometer is an instrument used for accurately measuring pupillary distance. The patient is instructed to look at a lighted target inside the device. The optician then uses separate controls to align a vertical hairline, often over the purkinje image or corneal reflection, for the right and left eyes individually. This method provides precise monocular PDs (the distance from the center of the pupil to the center of the bridge), which are critical for accurately fitting progressive addition lenses.
Question 6: Which of the following instruments directly measures the sagittal depth of a lens curve to determine its surface power?
- Lensometer
- Pupilometer
- Lens Clock (Correct answer)
- Distometer
Correct answer: Lens Clock
A lens clock, also known as a lens gauge or spherometer, is a mechanical instrument with three pins. The central pin moves, and its displacement relative to the two fixed outer pins measures the sagittal depth (or sag) of a lens curve. The dial on the instrument is calibrated to convert this sagittal depth measurement into a dioptric surface power, typically based on the refractive index of crown glass.
ABO NOCE Basic Opticianry Optical Instrumentation Use Questions and Answers
An optician uses a lens clock to measure the surface curves of a lens.
The instrument provides a direct reading of the surface power in diopters, but it is calibrated for a specific refractive index.
If a lens clock calibrated for crown glass (n=1.53) is used to measure the front surface of a polycarbonate lens (n=1.586), and the reading is +6.00 D, what is the actual surface power?