Several visual diseases not only reduce visual acuity, they also may impair a client’s ability to detect lower contrast objects that are well above acuity threshold. A letter contrast threshold function describes the lowest contrast letters that a client can recognize, over a range of sizes. Contrast sensitivity is the reciprocal of contrast threshold; higher sensitivity is better. Poor overall contrast sensitivity would increase a client’s problems with glare, and decrease reading performance with low contrast materials like newspapers, or currency. Very poor overall contrast sensitivity would slow reading of even the highest contrast magnified print and indicate the need for a CCTV or non-visual devices.
We have reviewed 4 experiments that related contrast sensitivity to reading performance in normally sighted and low vision observers. Our conclusions are that 1) letter contrast sensitivity charts are easier to administer and more predictive of reading performance than grating charts. Letter size may be varied by varying chart distance. 2) Letter contrast less than 10 times contrast threshold will reduce reading rate: Letter contrast less than 4 times contrast threshold will significantly impair reading rate and accuracy.
Thus, letter contrast thresholds higher than 9 % are considered poor and indicate a need for lighting assessment, extra magnification or a CCTV. Contrast threshold higher than 25 % are very poor and indicate the need for CCTV or non-visual interventions for fluent reading. These numbers are based on controlled experiments not clinical studies and should be used as guidelines not firm criteria.
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