achromatic threshold

(redirected from visual threshold)
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achromatic threshold

[¦a·krə¦mad·ik ′thresh‚hōld]
(physiology)
The smallest light stimulus that can be detected by a dark-adapted eye, so called because all colors lose their hue at this illumination.
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In a previous study, we also demonstrated that the scotopic visual threshold for optomotor reactions is at least 40-fold higher in juvenile bluefin tuna than in four marine teleosts (Ishibashi et al.
By starting with a diffraction-limited, color-corrected lens and a very clear vitreous humor (the gelatinous fluid that fills and pressurizes the eyeball), we can raise ocular transmittance to 80 percent, increasing the visual threshold by 2 1/4 magnitudes.
But if the contrast it makes with the surrounding planetary disk is below the visual threshold (less than about 0.