The correct position

Carl Zeiss developed the focimeter in order to measure the power of an unknown lens, however it was C.J. Troppman who produced the model that we use now. The power of a lens is said to be the ability of the surface to alter the curvature of light, i.e. altering the vergence of the incident light. The need for the focimeter in optical practices is that it enables us to identify the patients prescription from their spectacles, which in turn allows technicians to check whether the final spectacles are of the write prescription.

The focimeter can also be modified slightly to measure the vertex power of hard and soft contact lenses.There are several methods of determining the back vertex power of a lens, some of which are more accurate than others. The vertex focimeter is not the only way of measuring the power of an unknown lens, there is a technique of neutralization which involves placing lenses of known power and opposite sign in front of the unknown lens. This is done until the with or against movements of a distant image disappear at reversal. Therefore the unknown lens will have the opposite power to the lens that neutralized it.

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This method however takes a lot of time and is not as accurate as one cannot be certain that the power of the neutralizing lens and the power of the unknown lens are exactly the same.Another technique involves using the Abbe Refractometer, it is used initially to find the refractive index of a lens. This is then followed by finding the radius of curvature using a Spherometer. Both techniques are accurate but consume much time and are therefore unpractical. The advantage of using the vertex focimeter is not only is it quick and simple but the fact it can measure both positive and negative lenses over wide range.Using a double clamp the standard lens (+7.00D) was placed near the centre of the bench, the emphasis of the double clamp was important as movement of the lens during the experiment would affect the accuracy of the results. This was then followed by the calibration of the telescope using a standard collimator.

Thus allowing the telescope to receive collimated light.In order to do this, we looked through the eye piece of the telescope and focused on the cross wires. A plain piece of paper was then placed in front of the telescope so that a clearer image could be viewed of the cross wires. The eye piece was then inserted into the eye piece holder and the paper removed the collimator was then switched on. The draw-tube of the telescope was then pushed in and out until the cross wires could be viewed sharply. The accuracy of the calibration is vitally important because if it is done incorrectly the rest of the experiment could well be effected. To avoid any errors the telescopes eye piece, standard lens, positioning stop and the telescope objective all had to be in the same axis and at the same height. Once the calibration was done the telescope was placed between the light source and the standard lens.

The positioning stop was placed on the optical bench. In order to find the location of (F’) of the standard lens the positioning stop was moved back and forth until a length of thread came into sharp focus. This adjustment was carried out five times and the giving five readings from which the mean was taken as the correct position. When the equipment had been setup the optical centre of all the elements was aligned using a distance bar. It was particularly important to ensure that light passed through the optical centre of the test lens as when using thicker lenses the power is not the same as the periphery.With the tungsten lamp switched on the observer looked through the telescope and the positioning stop, and located the illuminated object at the focus “F” of the lens. The mean number of readings was used for accuracy, this result being the scale point from which all movements of the objects were measured.

 Then to find focal point (F) a similar procedure to locating the position stop was used. This involved viewing the target and moving it very finely both forwards and backwards until it came into fine focus. Again this was repeated five times and the mean taken as the correct position.Using a test lens of power +1.50D with the positioning stop caused the target to come out of focus. The target was brought back into focus by moving it back towards the standard lens. This measurement was repeated three times for the +1.

50D lens and the mean was taken as the correct position and recorded. This process was repeated with two more positive lenses of powers +2.00D and +3.00D and it was repeated with three negative lenses of powers -2.50D, -6.00D and -8.00D.