log chart va .pptx LogMAR and Snellen Difference

what is logMar chart, Vision Calculation on it, Snellen and LogMAR difference

1. Rani khan Log chart reading

2. LOGMAR CHART

3. LOGMAR CHART It was given by Bailey and lovie in 1916 and so it also called as Bailey lovie chart. The production was done by light house . The measurements are based on logarithm principle . Bailey Lovie also gave another logMAR chart which had decreasing contrast as we move a head to measure the contrast level.

4. Advantage of logMAR chart over snellen’s chart It is used to find minimum visual acuity . It can be used for low vision patients. The rate of progression is slow .Each line consists of 5 letters . If the patient cant read the side letters ,this will give the motivation level to the patient . It is measured at a distance of 3 meter the chart work up can also be done at a distance 1/2 meter .

5. The spacing between two lines is equal to the width of the letter presented in earlier line and the spacing between two letter symbol present in the same line ,it gives accurate readings.

6. LogMAR Notation  LogMAR is an abbreviation for "Logarithmic Minimum angle of resolution”.  The logic behind the LogMAR form of measurement is essentially that if a patient has 20/20 vision, or 6/6, what it means in terms of LogMAR , is that they got none wrong and hence would have a score of "0". 

7. Where each incorrect letter represents an additional score of 0.02 A Visual Acuity score of 6/7.5 on the regular chart (meaning not seeing any on the 20/20 or 6/6 line) would result in a true LogMAR value of 0.1, (5 letters wrong * 0.02 per letter) The LogMAR score of 0.1 in simple terms would mean "1 lines worse than "0".  The LogMAR scale scores what the patient "GETS WRONG", rather than what the patient gets right as is the case with most other notations . If the VA scored is better than 6/6 vision, e.g. 6/4.8, they would score -0.1 on the LogMAR scale. This essentially equates to 1 line better than "0". 

8. The notation gets more involved, and a bit confusing when the patient gets a few letters right on the next line down. We have to start thinking rather than "gets how many letters right", but rather "gets how many letters wrong ". For example; we no longer score a patient who has 6/9+2 in this way, but rather as 0.16. This is essentially calculated as 1 line worse than "0", giving us the 0.1, and then for every letter MISSED on next line UP, (in this case it would be 3 letters on the 6/7.5 line), we add 0.02 for every letter they get wrong , hence the 0.06 added to the 0.1. . If this patient had got 4 letters correct on the 6/7.5 line it would have been scored as 0.12, (as they only missed 1),and similarly if they got all the letters on the 6/9 and 1 extra letter on the 6/7.5 then the LogMAR score would be 0.18.

9. A score of 0.2 would indicate "no letters correct on the 6/7.5 line. (This would equate to 6/9 on the Snellen scale ) For LogMAR , think in terms of letters "MISSED" rather than letters "CORRECT". Work from the bottom up, rather than the top down.

10. Practical exercise 1 Calculate the LogMAR score for a patient who achieves a VA of 6/12-2 Calculation 1   They got 2 of the 5 letters wrong on the 6/12 line ( LogMAR 0.3) So his score is going to be somewhere between 6/12 ( LogMAR 0.3), and 6/15 ( LogMAR 0.4) The 2 wrong is 2/5ths = 0.04 which we add to the LogMAR 0.3 So our final score becomes:  0.34

11. Practical exercise 2   Calculate the LogMAR score for a patient who achieves a VA of 6/24+2 Calculation 2   They got 3 of the 5 letters wrong on the 6/19 line ( LogMAR 0.5) So his score is going to be somewhere between 6/19, ( LogMAR 0.5) and 6/24 ( LogMAR 0.6) The wrong is 3/5ths = 0.06 which we add to the LogMAR 0.5 So our final score becomes:  0.56  

12. Disadvantages It is very costly. Care should be taken .