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Your question is referring to display polarity. A positive polar display consists of dark letters on a light background, a negative polar display consists of light letters on a dark background. Polarity by itself is independent of text-to-background contrast, as you rightly state.

Generally, positive polarity facilitates performance (e.g. Buchner & Baumgartner, 2007). This effect is due to the higher overall display luminance (Buchner, Mayr and Brandt, 2009) possibly leading to a greater constriction of the pupil (Taptagaporn and Saito, 1990, 1993) and thus increasing the depth of field and decreasing spherical abberation.

However, this advantage of positive polar displays does not generalize to readers with visual impairment. People with low vision due to cataract (ocular clouding) perform better with negative polar displays (Legge et al., 1985, Sandberg & Gaudio, 2006). This effect has been discussed to be due to a scattering of light leading to a resulting veiling luminance and reduction in text-to-background contrast (Rubin & Legge, 1989).

In short: At the level of the display, a change of polarity has no effect on the text-to-background contrast. If the reader's eyes are cloudy, however, the contrasts at the retinal level is decreased for positive compared to negative polarity. This is why offering negative polarity is considered an accessibility feature.

EDIT: As to why design folks claim negative polarity to be higher in contrast, I can only speculate on the basis of these findings. I suspect that it has to do with the adaptation of the eye to the lower overall luminance of the display. The intensity of the white letters may be perceived as more intense. Also the wider pupil dilatation would lead to a decreased depth of field. This in turn would require wider spacing and lighter weights of letters to achieve readability comparable to positive polarity. Unfortunately, I don't know any research to substantiate these speculations (which doesn't mean that it does not exist).

Your question is referring to display polarity. A positive polar display consists of dark letters on a light background, a negative polar display consists of light letters on a dark background. Polarity by itself is independent of text-to-background contrast, as you rightly state.

Generally, positive polarity facilitates performance (e.g. Buchner & Baumgartner, 2007). This effect is due to the higher overall display luminance (Buchner, Mayr & Brandt, 2009) possibly leading to a greater constriction of the pupil (Taptagaporn & Saito, 1990, 1993) and thus increasing the depth of field and decreasing spherical abberation.

However, this advantage of positive polar displays does not generalize to readers with visual impairment. People with low vision due to cataract (ocular clouding) perform better with negative polar displays (Legge et al., 1985, Sandberg & Gaudio, 2006). This effect has been discussed to be due to a scattering of light leading to a resulting veiling luminance and reduction in text-to-background contrast (Rubin & Legge, 1989).

In short: At the level of the display, a change of polarity has no effect on the text-to-background contrast. If the reader's eyes are cloudy, however, the contrasts at the retinal level is decreased for positive compared to negative polarity. This is why offering negative polarity is considered an accessibility feature.

As to why design folks claim negative polarity to be higher in contrast, I can only speculate on the basis of these findings. I suspect that it has to do with the adaptation of the eye to the lower overall luminance of the display. The intensity of the white letters may be perceived as more intense. Also the wider pupil dilatation would lead to a decreased depth of field. This in turn would require wider spacing and lighter weights of letters to achieve readability comparable to positive polarity. Unfortunately, I don't know any research to substantiate these speculations (which doesn't mean that it does not exist).

Your question is referring to display polarity. A positive polar display consists of dark letters on a light background, a negative polar display consists of light letters on a dark background. Polarity by itself is independent of text-to-background contrast, as you rightly state.

Generally, positive polarity facilitates performance (e.g. Buchner & Baumgartner, 2007). This effect is due to the higher overall display luminance (Buchner, Mayr and Brandt, 2009) possibly leading to a greater constriction of the pupil (Taptagaporn and Saito, 1990, 1993) and thus increasing the depth of field and decreasing spherical abberation.

However, this advantage of positive polar displays does not generalize to readers with visual impairment. People with low vision due to cataract (ocular clouding) perform better with negative polar displays (Legge et al., 1985, Sandberg & Gaudio, 2006). This effect has been discussed to be due to a scattering of light leading to a resulting veiling luminance and reduction in text-to-background contrast (Rubin & Legge, 1989).

In short: At the level of the display, a change of polarity has no effect on the text-to-background contrast. If the reader's eyes are cloudy, however, the contrasts at the retinal level is decreased for positive compared to negative polarity.

Your question is referring to display polarity. A positive polar display consists of dark letters on a light background, a negative polar display consists of light letters on a dark background. Polarity by itself is independent of text-to-background contrast, as you rightly state.

Generally, positive polarity facilitates performance (e.g. Buchner & Baumgartner, 2007). This effect is due to the higher overall display luminance (Buchner, Mayr and Brandt, 2009) possibly leading to a greater constriction of the pupil (Taptagaporn and Saito, 1990, 1993) and thus increasing the depth of field and decreasing spherical abberation.

However, this advantage of positive polar displays does not generalize to readers with visual impairment. People with low vision due to cataract (ocular clouding) perform better with negative polar displays (Legge et al., 1985, Sandberg & Gaudio, 2006). This effect has been discussed to be due to a scattering of light leading to a resulting veiling luminance and reduction in text-to-background contrast (Rubin & Legge, 1989).

In short: At the level of the display, a change of polarity has no effect on the text-to-background contrast. If the reader's eyes are cloudy, however, the contrasts at the retinal level is decreased for positive compared to negative polarity. This is why offering negative polarity is considered an accessibility feature.

EDIT: As to why design folks claim negative polarity to be higher in contrast, I can only speculate on the basis of these findings. I suspect that it has to do with the adaptation of the eye to the lower overall luminance of the display. The intensity of the white letters may be perceived as more intense. Also the wider pupil dilatation would lead to a decreased depth of field. This in turn would require wider spacing and lighter weights of letters to achieve readability comparable to positive polarity. Unfortunately, I don't know any research to substantiate these speculations (which doesn't mean that it does not exist).

Your question is referring to display polarity. A positive polar display consists of dark letters on a light background, a negative polar display consists of light letters on a dark background. Polarity by itself is independent of text-to-background contrast, as you rightly state.

Generally, positive polarity facilitates performance (e.g. Buchner & Baumgartner, 2007). This effect is due to the higher overall display luminance (Buchner, Mayr and Brandt, 2009) possibly leading to a greater constriction of the pupil (Taptagaporn and Saito, 1990, 1993) and thus increasing the depth of field and decreasing spherical abberation.

However, this advantage of positive polar displays does not generalize to readers with visual impairment. People with low vision due to cataract (ocular clouding) perform better with negative polar displays (Legge et al., 1985, Sandberg & Gaudio, 2006). This effect has been discussed to be due to a scattering of light leading to a resulting veiling luminance and reduction in text-to-background contrast (Rubin & Legge, 1989).

In short: At the level of the display, a change of polarity has no effect on the text-to-background contrast. If the reader's eyes are clouding, however, the contrasts at the retinal level is decreased for positive compared to negative polarity.

Your question is referring to display polarity. A positive polar display consists of dark letters on a light background, a negative polar display consists of light letters on a dark background. Polarity by itself is independent of text-to-background contrast, as you rightly state.

Generally, positive polarity facilitates performance (e.g. Buchner & Baumgartner, 2007). This effect is due to the higher overall display luminance (Buchner, Mayr and Brandt, 2009) possibly leading to a greater constriction of the pupil (Taptagaporn and Saito, 1990, 1993) and thus increasing the depth of field and decreasing spherical abberation.

However, this advantage of positive polar displays does not generalize to readers with visual impairment. People with low vision due to cataract (ocular clouding) perform better with negative polar displays (Legge et al., 1985, Sandberg & Gaudio, 2006). This effect has been discussed to be due to a scattering of light leading to a resulting veiling luminance and reduction in text-to-background contrast (Rubin & Legge, 1989).

In short: At the level of the display, a change of polarity has no effect on the text-to-background contrast. If the reader's eyes are cloudy, however, the contrasts at the retinal level is decreased for positive compared to negative polarity.