An optical assistant (OA) may be required to help patients with frame selection, and to take certain frame and facial measurements. Many practices nowadays use digital measuring devices to take certain measurements, but it is important to understand the reason for these measurements, and their significance.
The measurements we are going to discuss in this article are the optical centration distance (OCD), or interpupillary distance (PD), near centration distance (NCD), vertex distance and pantoscopic tilt.
Strictly, the OCD is where the optical centres for distance will be placed on the lenses, and the distance PD is the distance between the pupil centres when looking at a distant object. In reality, these two measurements are the same.
The NCD is measured in the plane of the chosen spectacle frame, when the patient’s eyes are converging, and the working distance must be taken into account. For higher powers, progressive and aspheric lenses, it is advised to measure the PD or NCD monocularly, along with a vertical height measurement for accuracy.
It is important that all spectacle lenses are correctly centered as this provides the patient with the best possible vision. If the lenses are not correctly centered, this can induce unwanted prismatic effects, which can cause eyestrain, headaches, blurred or even double vision.
The vertex distance is the distance between the front surface of the cornea and the back surface of the lens. This measurement is needed on all prescriptions over plus or minus 5.00D. The optometrist will measure the vertex distance of the trial frame, and when the patient chooses a spectacle frame, the vertex distance of the spectacle frame needs to be measured and checked against the vertex distance of the trial frame.
If there is a difference, a calculation will need to be performed to see if the prescription needs to be adjusted. This is because as the lenses are moved closer or further away from the eye, the effective power of the lenses, and therefore the prescription received at the eye, alters. This means that if the vertex distance is not accounted for, the patient may not have the correct prescription.
If the vertex distance of the frame is different to that of the test then the prescription may need to be adjusted. there is a calculation that you can do to account for the change, but it is advisable and in the best interests of the patient to have this undertaken or checked by either an optometrist or a qualified dispensing optician.
The final measurement, pantoscopic tilt, is a measurement that needs to be taken whilst the patient is wearing their chosen frame. It is the angle of the frame front inclined towards the face. For every two degrees of pantoscopic tilt, the vertical position of the optical centre should be lowered by 1mm to ensure the optical axis of the lens passes through the centre of rotation of the eye. This results in an improved lens performance and matches the trial frame position more accurately. It is especially important to consider the pantoscopic tilt with progressive power lenses, higher prescriptions and aspheric lenses.
It is helpful to understand why these measurements are taken, the importance of accurate measuring, and why it is advisable to ask a qualified dispensing optician, or optometrist to check any measurements taken.
And remember: it is a legal requirement that under 16s, registered sight impaired, or severely sight impaired patients are be dispensed by a qualified dispensing optician or optometrist, or under the supervision of a qualified dispensing optician or optometrist.
Sue Deal FBDO R is a practising dispensing optician, ABDO College examiner, senior tutor and supervisor for dispensing opticians. She is also a practice visitor and external moderator for ABDO. She was recently awarded the ABDO Medal of Excellence for her outstanding services to the profession.