The full moon is 30 arcminutes across, whereas Venus is barely resolvable as an extended object at around 1 arcminute across. (This angular measure stays the same regardless of whether an object is nearby or far away distant objects must be much larger to subtend the same angle as near objects). Under ideal conditions, an object must subtend an angle of at least 1 arcminute, or one sixtieth of a degree, in order to excite adjacent cones. Are you genetically more similar to your mom or your dad?īut how far away can we perceive that an object is more than just a twinkle of light? For something to appear spatially extended rather than point-like, light from it must stimulate at least two adjacent cone cells - the elements in our eyes that produce color vision. Thus, the absorption of 5 to 14 photons, or, equivalently, the activation of just 5 to 14 rod cells, tells your brain you're seeing something. Based on measurements of retinal absorption, the scientists calculated that a factor of 10 fewer photons were actually being absorbed by the participant's rod cells. The scientists found that for study participants to perceive such a flash of light more than half the time, the subjects required between 54 and 148 photons to hit their eyeballs. The experiment probed the threshold under ideal conditions: study participants' eyes were given time to adapt to total darkness, the flash of light acting as a stimulus had a (blue-green) wavelength of 510 nanometers, to which our eyes are most sensitive, and this light was aimed at the periphery of the retina, which is richest in light-detecting rod cells. Back in 1941, the vision scientist Selig Hecht and his colleagues at Columbia University made what is still considered a reliable measurement of the "absolute threshold" of vision - the minimum number of photons that must strike our retinas in order to elicit an awareness of visual perception.
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