Aussie needs help with assignment

[nellie75]

Ok everyone this is the last assignment and I am officially a qualified Optical Dispenser. I do not have access to a (corona) ring of dots target focimeter so from my text books it tells me squat about them.

The question is discuss the differences between a ring-of-dots (corona) target focimeter and a crossed-lines target focimeter. What advantages would either have over the other? the question is worth 20 marks and the only real difference that they tell me is their is no axis drum and one is a ring of dots the other a crossed line target. Which is better and why no one can seem to give me more than a sentance back as a reply? I am sure for a question worth 20 marks there is more to it right?

[David Wilson]

Hi Nellie,

Have you spoken to your teachers? I am sure that they would be happy to give you some help.
Some things to consider are, that a ring-of-dots focimeter has no axis drum and so it can be simpler to operate. With most ring-of-dots focimeters it can be difficult, though, to determine the centre of the ring and, hence, the optical centre (you need to 'frame' the entire ring in the graticule). With the crossed line target, the optical centre is easier to determine and, some users, find that small cyls are easier to read since the axis drum can be turned until the lines are unbroken. A small cyl in the ring-of-dots does not stretch the dots into very long lines and so it is simetimes hard to read the axis. Having said that, the dots are stretched along the principal meridians and are therefore accurate, whereas the crossed line target requires the judgement of the user as to when the lines are unbroken. It can also be more difficult to measure progressives on the dots since the target will be displaced by the prismatic effect at the checking circle and, so, the stretched dots will be away from the lie used to determine the axis.
I hope that this helps.
Regards
David