Rod cells function in twilight and normally become inhibited at higher luminance levels when the function of the cone cells increases. If the number of cone cells decreases below a certain level, the rod cells are not inhibited and become overstimulated by day light. In the filter lenses there is nearly no transmission in blue-green, the wavelengths that activate rod cells, and thus rods "believe" that it is twilight and function normally, while the cone cells "know" that it is day light and also function normally. This way there are millions of rod cells functioning simultaneously with the cone cells in full day light. Since there are more cells activated we experience an increases in subjective brightness when we look through these lenses, despite the fact that there is less light transmission through them than there is without the lens. Filter lenses are specific for this type of retinal problems and are not suitable for other types of photophobia, for example when caused by corneal irritation or allergic reactions.

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