We all know it takes time for the eyes to adapt to the dark, but have you ever wondered why? It is all down to the chemical process that occur within certain receptor cells in our eyes (known as rods) when we ‘see’ low levels of light. The following article gives a simple explanation of dark adaptation and provides some simple ideas for keeping your eyes dark adapted.
Rods and Cones
Any standard school biology textbook will describe the structure of the human eye. Of particular relevance to dark adaption is the structure of the retina which contains two types of cells, rods and cones. Rods work at very low light levels but cones require much more light to function. Both rods and cones contain proteins that undergo a chemical change called “bleaching” when exposed to light. Sensitivity of the eye to light is dependent on the amount of protein that is present. The human eye is said to operate in two modes:
- Photopic in bright conditions, and
- Scotopic in low light conditions.
Keeping your rods fully charged
What this means for the astronomer’s ‘dark adapted’ eyes is that in low light conditions their vision relies upon the more sensitive rod cells, and is dependent upon these cells being “fully charged” with a protein called rhodopsin. Rhodopsin is what gives the retina its purple colour. Light causes a small portion of the protein molecule to be split off and at the same time a signal is sent to the brain along the nerve cells. Exposure to a bright light destroys the rhodopsin (the rhodopsin immediately photo-bleaches). Thus exposure to bright light causes a temporary loss of dark adaptation until the rhodopsin has been regenerated.
Light Rage?
Why do astronomers get so annoyed when someone switches on a bright light or carelessly shines a white light in their eyes? When eyes become ‘light-adapted’ (photopic) the dyes in the rod cells rods are fully bleached and are unable detect faint light. The change from a dark-adapted eyes to light-adapted eyes happens very quickly indeed but it then takes the eye a considerable amount of time (30 to 60 minutes) for the rhodopsin to regenerate and the eye to become once more dark-adapted (scotopic).
Why do you astronomers use bright red torches and lights at night?
Rhodopsin in the rods absorbs green-blue light most strongly. The upper wavelength cutoff for Rhodopsin is around 600nm and photo-bleaching only takes place at wavelengths shorter than this. Therefore, in red light (wavelength around 700nm) the chemical structure of the rhodopsin in the rods remains unaffected and dark adaptation is maintained.
My eyes aren’t what they used to be!
If you are not the spring chicken that you once were then, like me.you will be noticing some of the effects aging has on your eyesight. There are many changes that can affect our vision as we get older – the maximum pupil dilation shrinks, yellowing of the eye’s optics changes colour perception. With age also comes a noticeable decrease in the rate of dark adaptation.
Sorry, there’s not much good news here (beyond possible eligibility for a free bus pass from your local council to get you to your astronomical society meetings).
Etiquette around astronomers:
Generally none of this will occur to the non-astronomer. So rather than ‘light rage’ it needs patience and a bit of education:
- Torches should always shine a red light (red LED or a white light torch with red film over the glass).
- If you have a head-mounted light DON’T let it shine in peoples faces when you talk to them or into the objective lens of their telescope – even if its is a red light!
- Be careful with your mobile phone – it is surprising how bright these can be when they light up.
- If you are arriving at an observing site by after dark, consider parking your car well away from the observing area so that your headlights don’t cause problems.
- If your car is parked on the observing site be aware that lights may come on automatically such as interior courtesy lights when a door or boot is opened and indicators when locking/unlocking your car, etc.
- Smokers – if you must light up please shield the lighter flame.
What can I do to stay dark-adapted:
It’s simple – avoid bright lights! Here are some ideas:
- Common sources of bright light at an observing sites include common sources of light pollution such as nearby PIR household floodlighting and street lighting. If you cannot chose better site then be careful not to stare directly at the source of the light or try put up some sort of shield. I have grown plants with thick foliage to shield bright lights on neighbouring properties.
- Install an app on your laptop or portable device that can be used to both and redden the display – it is quite effective. Some packages provide an ‘observing’ or ‘night vision’ mode which reddens the display but you can still be caught out by pop-ups and windows from other applications running in the background.
- If you need to go indoors to get a hot cup of tea then consider using a pair of red goggles when you go indoors. Better still, get your husband / wife / partner to bring a cup of tea out for you.
- Each eye reacts separately to light, so it is possible to keep one eye dark adapted by using an eye patch – though your friends could be forgiven for thinking that you are in fact rehearsing for a part in the local pantomime. So what! They already think you are quite mad for staying out in the freezing cold all night.
Finally, to produce Rhodopsin your body needs Vitamin A from dietary beta-carotene – so get munching on those carrots. If anyone asks “What’s up doc” you can at least reply sensibly with something like “M42, the Pleiades, M31 and Jupiter is just rising ”.