Some people have hypersensitive photoreceptors that react inappropriately to some wavelengths of light. Thus, when they read black type on white paper, their eyes react to the white glare of the paper, creating havoc with the smooth reading process. They may experience eyestrain, fatigue, or headaches while reading, because of these visual perceptual distortions. When the child or adult puts a darker colored transparency over the reading page, such as blue, green or blue/green, they really “even the playing field.” Now there is less of a discrepancy between the black words and the white background, taking stress out of their visual system. They often become visibly relaxed while reading.