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LEA Symbols

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. (For more discussion on general principles of calibration click here.)

LEA Numbers Blur

Notice how the bottom row of objects all look similar? This is the blurring equally of the LEA Symbols.

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 Symbols

Historical Timeline for L.H. Optotypes

(a.k.a. LEA SYMBOLS® in the United States and LH Symbols in Europe)

1976

Lea Hyvärinen, MD, develops 4 symbols circle, square, apple, house calibrated against Snellen E (then current international reference optotype).

1980

Initial article in English about LEA Symbols (then called L.H. optotypes) published. Hyvärinen L,Näsänen R, Laurinen P. New visual acuity test for pre school children. Acta Ophthalmol (Copenh). 1980 Aug;58(4):507 511.

1988

L.H. optotypes calibrated against Landolt C (new international reference optotype).

Early 1990s

Lighthouse International contracts with Precision Vision to print the Lighthouse Low Vision LH Symbol Chart, using L.H. optotypes.

1993

L.H. optotypes renamed LEA SYMBOLS® in the United States and known as LH Symbols in Europe.

2002

Good-Lite receives exclusive rights to manufacture and promote LEA SYMBOLS® (LH Symbols in Europe).

2003

Precision Vision introduces Patti Pics.

Additional Resources

Pediatric Vision Consulting

Questions about screening children, ask us about helping you set up a program with our consulting services.

Find out more >>

LEA SYMBOLS® and HOTV Comparison Study

A study from the Illinois College of Optometry compares LEA SYMBOLS® and HOTV to screen children.

Read about the results >>

LEA SYMBOLS® References

LEA SYMBOLS®, the international standard for pediatric vision testing and screening. Find sited published studies using the LEA SYMBOLS®.

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