CAT. NO. 2425 Illuminator Cabinet is necessary with these translucent low contrast charts.
Louise Sloan developed Sloan optotypes on a principle of equality. Each Sloan letter features many of the same characteristics. They are all calibrated to a 5×5 instead of a 5×4 grid size, giving them a uniform and equal size. Each optotype takes up similar space inside the grid. Also, each letter has very similar difficulty rating. Each line in a Sloan chart balances the 5 optotypes to make the chart the same difficulty from beginning to end. Because Sloan optotypes are so calibrated, vision testing using them is accurate and very reliable.