The ring of dots or “corona” target is sometimes referred to as the European target. It takes the form of a ring of very small dots superimposed over a graticule target indicating a standard axis notation protractor reading from 0 degrees to 180 degrees anticlockwise. When an astigmatic lens is under rest and the target is in focus for either of the principal powers, each dot appears to be drawn out into a focal line which is at right angles to the meridian of power which create them. When checking a spherical lens, the target remains a ring of dots. A ring of dots focimeter has no need for an axis drum since the focal lines automatically line up along the axis or the power meridian, and the axis is measured by using the graticule (D Wilson, 1999).
The focimeter also gives the user the possibility to mark the optical centre of the lens as is usually fitted with a marking device.
However, as the focimeter is an instrument of measure, it is liable to errors. To correct the errors, a calibration process must be used to ensure that the focimeter is working correctly. The focimeter can be calibrated using the following steps: first, is important to read the manufacturer’s instructions. Then, the user should correct the adjustment of the eyepiece by setting the dioptre scale at any position other than zero, withdrawing the eyepiece to its greatest extent, moving the eyepiece slowly inward and observe the graticule, ceasing the movement of the eyepiece when the graticule is seen distinctly. Finally, the dioptre scale should be adjusted by rotating the power barrel until the target is seen distinctly.
During the steps mentioned above is important to keep both of the eyes open to ensure that the target is viewed with the accommodation relaxed. (Jalie and Wray, 2002).
In the process of focimetry, the focimeter used is a telescopic one with the ring of dots target. During the process of focimetry three steps were followed to read the spectacles powers and the Optical Centres:
- First, it was adjusted the angle of focimeter on the workbench so the eyepiece is in a comfortable position.
- Then it was checked if the eyepiece is focused on the eye
- Finally, both eyes were open when looking through the eyepiece.
It should be used the dominant eye to look through the eyepiece with the dominant eye as it is much easier to ignore the image in the non dominant eye. According to David Wilson (1999, p140), to find the dominant eye the subject needs to focus at a distant object such as a door frame with both eyes open. A finger is hold at an arm’s length and lined up with the distant object as though it were the sight of a gun. The one eye is closed. If the finger remains lined up with the object, it means that the dominant eye was open, if the finger moves to the side, it means that the non dominant eye was open.
The process of checking any lenses involves calibrating the focimeter according to the steps mentioned above. Therefore, this is the case when checking the powers of spherical lenses. “The fundamental characteristic of a spherical lens is to bring parallel rays of light top a single point focus” (D. Wilson, 1999). In order to read the power of a spherical lens the back surface of the lens is put against the lens rest table. The lenses in the process were fitted into frames, therefore the frame table was raised until it supported the frame. The lens holder arm is released and the lens is placed securely against the lens stop. The power drum is rotated towards the plus readings with the lens in position. If the target becomes clear and crispy focused, hence the ring of dots are sharp, then the power read from the focimeter is recorded as the power of the lens. But if the ring of dots become less visible, then the power drum is rotated towards the minus readings until the ring of dots are sharp. After the reading of the first lens is done, the marking tool is lowered and is imprinted a dot in the centre of the lens to mark the optical centre. After the same procedure is done for both lenses of the spectacles, the Optical Centre is measured using a PD Ruler.
In order to take accurate Optical Centre readings is important to move the lens on the rest table so that the target (in our case, the ring of dots) is at the centre of the graticule.
One of the common issues that it was observed in measuring the Optical Centre was that the measurements were not done all the time accurately. The optical centre distance is, according to British Standard Institute (2003, p.9) “ the actual horizontal distance between the optical centres of a pair of mounted lenses, with any prescribed prisms being neutralised”. And the centration distance is “ the specified horizontal distance between the right and the left centration point”( British Standard Institute, 2003, p9).
To measure a spherical-cylindrical lens it was noticed that the dots become a set of lines pointing towards a certain axis direction. The first reading is recorded by rotating the power drum until the set of lines are sharp. Is recorded the axis direction and the sphere reading as well. Then the same procedure applies again on the same lens and it will be noticed that the set of lines point towards a different power and axis position. The higher sphere reading is recorded as the sphere power and the smaller reading minus the higher reading will give the cylinder power.
When reading prescription of lenses with sphere and cylinder incorporated, an error of +/- 0.25D was made. The reasons why can be that the lens was not aligned flat on the table of the focimeter or the eyes are accommodating with the focimeter and the reading will be wrong. These errors can be overcome by having both of the eyes opened and making sure that the lens sits flat on the table of the focimeter.
The focimeter is an important tool to read the power of the spectacles. In my practice I found that by using the focimeter many problems can be avoided, problems like giving the wrong prescription spectacles to the customer or giving the right prescription but with the optical centres wrong. At the beginning of the process of focimetry I found that the power of the spectacle lenses were not exactly as the power on the dispense record. But later I realised that if one of the steps in reading a prescription is missed, like calibrating the focimeter, or making sure that the eyepiece is sitting straight and aligned to me, the readings can be wrong.
References:
British Standard Institute Extracts (2003), BS 3521, part 2, 1991
Jalie, M. and Wray L. (2002) Practical Ophthalmic Lenses.3rd edn. Guilford, Surrey: Butterworth &Co Ltd, 1983
Pears, R.& Shields G.( 2007) Cite them right: a guide to referencing and plagiarism. Newcastle upon Tyne: Pear Tree Books, 2005.
Wilson, D. (1999) Practical Optical Dispensing Stratified: Open Training and Education Network
