How the Eye sees in the dark

One recurring question that is asked about our VizLite DT phosphorescent product is how best to view the afterglow after the initial charging period, and how to view the long-lasting afterglow.

To best answer this question requires an explanation of how the eye functions in the dark. Humans while not having the same level of night vision as animals, can see as in the dark if the eye is allowed to adapt to the dark conditions. There are three distinct types of vision that allows humans to view objects in different light conditions.

The Three Types of Vision

The Anatomy of the Eye

Dark Adaptation

Human eyes take several hours to fully adapt to darkness and reach their optimal sensitivity to low light conditions. This is called dark adaptation. This relates to the physical and chemical changes within the eye that take place when your vision is adapting to see in the dark.

The three parts of the eye that plays a part in dark adaptation are:

The Pupil

When in low light conditions the iris expands pulling the pupil as wide as possible to let in more light.

The pupil is the dark hole in the middle of the eye. Its function is to control the amount of light that enters the eye, like the aperture on the lens of a camera.

The pupils contribution to dark adaptation takes from a few seconds to a minute to be complete.

The Cone Cells

The cone cells are situated at the back of the eye along the retina and are responsible for colour vision. Cone cells contain a substance called Rhodopsin, which is a light sensitive chemical. It is the primary chemical used by the cones when seeing in low light conditions. Cone cells take about 10 minutes to fully adapt to the dark. Rhodopsin is deactivated by sunlight.

The Rod Cells

The rod cells in our eyes are responsible for night vision, they cannot recognise colour only black and white. The Rod cells also contain Rhodopsin.

Each rod cell is 100 – 1000 times more sensitive to light than a single cone cell, once fully dark adapted.

Rod cells respond slower to light than cone cells, collecting information over a longer period, meaning it takes from 30 minutes to several hours to become fully adapted to the dark. Experiments have found that when people are kept in a dark room until their eyes adapt to the darkness, a single photon of green light will trigger receptor cells in the rods of the retina. That is why the longer you sit in darkness the better you will be able to see. 

Transitioning Back from Dark to Light

Once eyes have adjusted to the dark environment, the rhodopsin chemical is active in both cone and rod cells. However, once exposed to light the Rhodopsin is deactivated and the dark adaptation process must start all over again. 

Viewing VizLite DT Phosphorescence

Viewing phosphorescent material is best done in an area made as dark as possible. Even ambient light from smartphone and computer screens can affect the experience. It is best to remain in the darkness for a few minutes to allow your eyes to adjust to the dark conditions. 

Do not try and  view phosphorescent material in a draw or cupboard or view from a lit area. This will not work as your eyes are not dark adapted.  

The best way to test the long-lasting afterglow is a simple test that we recommend takes place at home. Before you go to sleep, take the sample and expose to light either artificial or daylight for 5 -10 minutes. 

Place the sample in your bedroom in a prominent position. When you wake if your room is dark, your eyes will be fully dark adapted, and you can see that the sample is still glowing many hours later. 

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