On June 24th, 2015 we learned about

Testing the lower limits of our eyes’ light sensitivity

Tonight I tried to explain to my two-year-old why he saw his new band-aid as red. We “talked” about the idea that light from the light bulb in his room was bouncing off the truck picture on band-aid and going up to his eyes. Some parts (the cones) in his eye detected the red light, and told his brain about it. It was at this point that things really went south as I realized he wasn’t really aware of what a brain was.¬†Of course, expecting him to understand human sight in one shot wasn’t really to be expected, especially considering new aspects of it are still being discovered.

Most recently, scientists were looking into just how sensitive our “rod” cells were to light. While the aforementioned cones are give us color (and slightly higher resolution,) the rods detect light and dark values, and are especially useful in situations with low light. To determine exactly how low, test participants were asked to indicate when they saw a flash of light in an otherwise perfectly dark room. Thanks to advances in technology, the amount of light could be limited to exactly 30 photons and still get a response from participants.

This is even more impressive after factoring in how many of those photons were expected to actually be hitting anyone’s eyes. The light wasn’t coming from a focused source like a laser, and only ten percent were thought to actually hit the participants’ eyes. Assuming that’s close to accurate, we seem to be able to detect a minuscule three¬†photons. For reference, the 100w lightbulb in my son’s room was emitting somewhere along the lines of 3×1020 photons every second, making it fairly amazing that our visual system has such amazing sensitivity to low light. Individual rod cells have actually been proven to be sensitive to as little as one photon in lab settings, but it’s likely that single activations like that are ignored by the brain to avoid filling our visual field with noise from accidental cellular misfires.

Source: The human eye can see individual particles of light by Alexandra Ossola, Popular Science

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