In the LEA test series there are line tests, tests with more crowded symbols and single symbol tests to allow assessment of function of the visual system in these three functionally different situations. The number of tests has grown over the years because the needs in screening and assessment of children and adults vary at different ages and different functional levels.
Development of the LEA Optotypes
The LEA optotypes originally carried my initials LH. They were designed in 1976 and calibrated against the Snellen E, the international reference optotype of those days. The size of the 1.0 (10/10, 3/3) symbols was found to be 7.5′ of arc (Hyvärinen et al. 1980).
Designing these four optotypes required much more work than one would imagine. I drew them 15cm high and compared the visibility by measuring at which distance I could recognise the one from the other. Drawing the apple and the house took the whole summer of 1976 before the dips of the apple and the edges of the roof of the house got their present forms and became equally legible with the square and the circle.
When the Landolt C became the reference optotype in 1988 (Consilium Ophthalmologicum Universale), the symbols were tested against it and a reduction of the size of the symbol optotypes was found necessary. This set of optotypes became the Lighthouse Low Vision LH Symbols Test. The size of the 1.0 (10/10, 3/3) symbols was 6.84′ of arc.
The printing techniques changed in late 1993 so that the printing films are computer generated as opposed to the previous techniques that required camera work. The edge quality improved and changed visibility of the symbols. Therefore the test symbols had to be calibrated once more. During this calibration it was first found that the change in the edge quality affected the square more than the other symbols. Therefore each symbol was separately tested against Landolt C. This work was done with Juvi Mustonen, O.D. at the Vision Laboratory, Department of Physiology, in the following way: – 20 optometrist students, unfamiliar with the test paradigm, were the subjects for the testing. The optotypes were presented on a computer screen one at a time for 0.5 seconds at the distance of 7.7 meters. The threshold of 84% correct was determined using an ordinary force choice staircase with16 turning points. After four consecutive correct responses the size was reduced by a factor of 1.13, and after each incorrect response the size was increased by the same factor. The threshold was calculated individually for each symbol. Of the 20 subjects, 16 subjects had consistent results. Their visual acuity mean based on Landolt-C thresholds was 1.5 (20/13, 6/4).
The coefficients for the symbols were found to be: apple 1.31 times larger than the Landolt C, house 1.35, square 1.20 and ball 1.42 larger than the Landolt C. The average of the symbols was 1.32 times larger than the Landolt-C. It was used as the size of the space between the symbols on printing the charts. When Landolt-C is 5′ of visual angle in the 1.0 (20/20, 6/6) size, the average size of the LEA Symbols is 6.6′.
In order to keep the total amount of blackness closely equal, the line of the symbols was adjusted to be 13.9% of the heigth of the symbol ‘house’ in all symbols. The space within the lines was decided to be equal to the height of the ‘house’.
There is of course a possibility that there is a difference between the testing using a computer screen and testing with printed tests. Therefore the visibility of the symbols on printed charts may slightly differ from the visibility on a screen. However, the variation of results caused by variation of the subjects’ individual abilities to recognize forms is likely to be a much greater source of variation than the variation in the image quality on print compared with that on the computer screen.
The LEA Symbols now have the two important basic features of good optotypes: They blur equally and they are calibrated against the reference optotype of Landolt C.
LEA Numbers Tests were developed for international studies in occupational health and school health services. Letters like the Sloan letters are used in the Western world and in the old colonies of western empires but not in a large part of the rest of the world whereas numbers are the same world over. The four numbers 5,6,8 and 9 were chosen because they could be designed to blur equally.
The experiences that the previous manufacturer and I have had in designing the symbol optotypes helped us when the number optotypes for occupational health were created. However, numerous hours were spent refining the details of each number. Fortunately, we nowadays have the facilities to exchange information via fax, phone and computers thus reducing the time needed in this type of industrial designing. The LEA Numbers were designed to have equal legibility, to blur equally. This makes the threshold value much narrower than in older tests where such easy numbers as 2, 4 and 7 are used.
In clinical use of visual acuity tests letters and numbers are generally used instead of the reference optotype Landolt C or “Ring” although there are countries were Landolt C is used even in pediatric testing. Letter and number optotypes require recognition of the form.
The round numbers 5,6,8 and 9 are more difficult to recognise than it is to detect where the space is in the Landolt C. Calibration of the size of the number optotypes was done by Juvi Mustonen, O.D. and Risto Näsänen Ph.D. at the Vision Laboratory of the Department of Physiology, Helsinki University, Finland.
Assessment of visual acuity usually follows measurement of refractive error, first without, then with correcting lenses, monocularly and binocularly.