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Purpose Pseudoisochromatic plates use a pseudo-chaotic pattern of image dots to conceal the contour of colour optotypes. By our study we wantto improve the understanding of functional principles of pseudoisochromatic plates. Methods The circular elements of red/green pseudoisochromatic plates were varied in size and degree of pseudo-chaotic arrangement. Illuminated by standard white D 65, colour optotypes were presented to daltonians and to normal observers, and the rates of failure and success in recognition were recorded. Results Recognition of pseudoisochromatic optotypes is related to the size of elements in a non- linear way. Towards small visual angles, colour optotype recognition may improve due to fusion of the perceived colour of adjacent small elements. Towards large visual angles, colour optotype recognition may again improve due to detection of the individual colours of elements. Weakening of contours by defocus may improve the recognition of colour optotypes due to loss of camouflage. Loss of camouflage also occurs with a more regular, less pseudo-chaotic arrangement of image dots, thus facilitating the recognition of colour optotypes. Conclusions Besides the colours themselves, arrangement and size of coloured elements need careful consideration in designing pseudoisochromatic plates. In application of the plates defocusing has to be avoided. |
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