During astronomical observations, our eyes need to be dark adapted in order to perceive faint distant celestial objects whose photons have travelled millions and in some cases billions of light years before reaching Earth. In this blog article, I aim to demystify the mechanism of dark adaptation, the importance of dark adaptation, how long does dark adaptation take, how to preserve dark adaptation and also recommend red LED flashlights to use during astronomical observing sessions.
Let’s dive right in.
In order to understand the mechanism of dark adaptation, we need to first understand the structure of the human eye. As seen in the diagram below, the retina is a tissue located at the rear of the human eye that collects light falling on the eye. The retina contains photoreceptor cells called rods and cones that convert the light into signals that are transmitted to the brain by the optic nerve.
There are roughly 120 million rods and 6 million cones in a human retina. Rods perceive black and white and are responsible for nighttime vision whereas cones perceive colour and are responsible for day time vision. Rod and cone cells get their names from the shape of their outer segments. Outer segments of rods are rod shaped and those of cones are cone shaped.
Rods contain a photopigment called rhodopsin also known as visual purple because it is purple in colour. Rhodopsin most readily absorbs wavelengths in the region of 507 nm. This doesn’t mean rhodopsin and hence rod cells will not absorb other wavelengths. It only means that the readiness of rods to absorb wavelengths other than 507 nm will be lesser. Rods are sensitive to dim light and as a consequence are extremely useful in low light conditions i.e. at night. Activity in rod cells increases in proportion to the intensity of incident light. But in day light, rods are completely saturated and hence don’t make a significant contribution to visual perception.
Cones cells are divided into three types based on the photopigments they carry. L-cones are more sensitive to long wavelengths (red light) and carry the photopigment erythrolabe. L-cones show maximum readiness to absorption of wavelengths in the region of 565 nm. M-cones are more sensitive to middle wavelengths (green light) and carry the photopigment chlorolabe. M-cones show maximum readiness to absorption of wavelengths in the region of 535 nm. S-cones are more sensitive to short wavelengths (blue light) and carry the photopigment cyanolabe. S-cones show maximum readiness to absorption of wavelengths in the region of 430 nm.
Human eyes are able to detect only a small segment of the electromagnetic spectrum. The range of visible wavelengths goes from 380 nm up to 700 nm. In this visible portion of the electromagnetic spectrum, the wavelength of light is associated with the perception of colour.
As shown in the diagram below, if a beam of white light is incident on a prism, the refracting mechanism of the prism deviates this light into it’s red, orange, yellow, green, blue and violet components. Each component has a different wavelength and as consequence is deviated at an angle corresponding to it’s respective wavelength.
Human vision can be classified into 3 categories namely photopic vision, scotopic vision and mesopic vision.
Cones may provide better detail than rods, but rods are more sensitive. Formulated differently, cones have better visual resolution whereas rods have a higher visual sensitivity. This is mainly due to the manner in which rods and cones are connected to post receptoral elements of the retina. Rods are connected in such a way that they sum up information over space. This results in better sensitivity but mediocre resolution. Cones on the other hand are connected in such a way that they maximise visual resolution at the cost of sensitivity.
The following illustration given by Steven H. Schwartz in his book the Visual Perception : A Clinical Orientation is an elegant demonstration of the difference in sensitivity and resolution of scotopic vision i.e. rod cells and photopic vision i.e. cone cells.
As illustrated by the diagram shown above, an important distinction between scotopic and photopic vision is the number of rod or cone cells that communicate with a single ganglion cell. For sake of simplicity, let us assume 10 quanta of light are necessary for a single ganglion cell to detect light, that light is delivered on the rod and cone cells via sources of light containing 5 quanta each separated by a distance of x and that all quanta of light are absorbed by these photoreceptors.
In scotopic vision, the two sources of light produce 10 quanta absorptions and this information is summed up by a single ganglion cell to produce a signal and transmitted to the brain. The ganglion cell signals the presence of a single source of light. This information is lost due to spatial summation of scotopic vision. Thus the scotopic system demonstrates a high level of sensitivity because the stimulus is seen but demonstrates poor resolution because only one source is seen.
In photopic vision with the same stimuli, neither of the two stimuli is seen. Only 5 quanta are absorbed and communicated to each ganglion cell. But the threshold being 10 quanta for a single ganglion cell, none of the ganglion cells are triggered. Suppose we double the number of quanta in each source of light i.e. 10 quanta each. The photopic vision detects two signals because the threshold for two ganglion cells is reached. The scotopic vision collects 20 quanta of light but cannot distinguish between the two sources of light because all rods cells in the example communicate with a single ganglion cell whose threshold is crossed and hence the signal is detected.
In summary, photopic vision has poor sensitivity but a powerful resolution whereas scotopic vision has excellent sensitivity i.e. spatial summation but poor resolution. This is the reason why we can see faint stars, galaxies and nebulas at night thanks to our scotopic vision.
Dark adaptation is defined as the gradual improvement in vision after exposure to a bright-adapting light (Visual Perception : A Clinical Orientation, 5th Edition Steven Schwartz McGraw Hill Education).
Dark Adaptation happens mainly due to several mechanisms in the human eye namely :
Post receptoral factors are out of the scope of this article and hence will not be covered in this article. Let us study the first two factors in detail.
During the process of dark adaptation, the diameter of our pupil increases in proportion to the decrease in ambient light of the surroundings. This increase in pupil diameter is one of the mechanisms deployed by our body to help us see in the dark and thus adapt to dim light conditions at night. But this is not the only biological mechanism that our body uses for dark adaptation.
It is understood that the human visual perception mechanism operates over a range of 10 log units. Steven Schwartz in his book Visual Perception : A Clinical Orientation gives a very eloquent quantification of the role of pupil size in the dark adaptation mechanism. Let’s consider the change in pupil diameter of 9 mm in dark conditions at night to a diameter of 3mm in bright day light conditions. According to Steven Schwartz, using the formula for the area of a circle we can calculate the reduction in light collected by the retina.
Area of a circle = πr2
For a pupil of diameter 3 mm, we have an area of light beam entering the human eye of Ab = 2.25 π
For a pupil of diameter 9 mm, we have an area of light beam entering the human eye of Ad = 20.25 π
Ad / Ab = 9 or 1 log unit
This means that out of the 10 log units of human visual perception the pupil size is responsible for 1 log unit and the rest i.e. 9 log units are due to the rod and cone photoreceptor cells as well as other post receptoral factors.
The graph above is useful in understanding the role of rods and cones and their photopigments in the process of dark adaptation. When a person enters a dark environment for example a movie hall from a brightly lit environment, for roughly 12 minutes, her photopic vision is working i.e. cone cells are working to constitute her vision. Cone photopigments are playing their role in this process. As discussed above due to low sensitivity of photopic vision mediated by cone cells, photopic vision contributes to visual perception only for a duration of roughly 12 minutes in the above case. After 12 minutes in a dark environment, we reach a point called the photopic threshold also called the rod-cone break. Beyond the photopic threshold, her photopic vision makes little to no contribution. This is reflected by the horizontal straight line just below 4 log units.
During this time, the rod cells are making little to no contribution to her vision essentially because the rod photopigment called rhodopsin has been bleached. When light is incident on rhodopsin, it absorbs some of the light. This causes the closure of sodium channels located at the end of rod outer segments which in turn causes rod hyper polarisation. The number of sodium channels on rod outer segments being limited, the maximum possible magnitude of rod hyper polarisation is also limited. When roughly 10 % of the rhodopsin is bleached, a critical number of sodium channels are closed. Further bleaching of rhodopsin doesn’t lead to additional hyper polarisation which means rods are saturated.
After roughly 12 minutes in a dark environment, rhodopsin starts to regenerate and rod cells take over the task of visual perception from cone cells. As we saw in the earlier chapter, scotopic vision mediated by rod cells is characterised by excellent sensitivity but poor resolution. The slow regeneration of rhodopsin gives a slow decrease in scotopic threshold or inversely slow increase in scotopic sensitivity. This is represented by a second plateau on the graph and is referred to as the rod plateau. After a period of approximately 35 minutes in the dark, the detection threshold of the human eye is reduced by 5 log units and most of it’s sensitivity is recovered.
After the rod-cone break, rods become more sensitive than cones. Before the rod-cone break, cones detect the light source and after the rod-cone break, rods detect the light source.
It is believed that basic dynamics of quantal absorption by cone pigments are similar to those by rhodopsin but the regeneration of bleached cone pigments takes place much faster than rod pigment.
One needs to remember that the time for dark adaptation is not identical for all wavelengths. The curve in earlier figure represented time for dark adaptation for a light source of wavelength 420 nm.The curve in the figure above represents the time for dark adaptation with alight source of 650 nm. Since the sensitivity of rods and cones for a light source of 650 nm is the same, there is no rod-cone break. When stimulated by a light source of wavelength 650 nm, rod and cone photopigments are regenerated and the human eyes recover their maximum sensitivity after spending less than 10 minutes in the dark.
It is worth noting that the process of dark adaptation doesn’t depend solely on time. It also depends on the size of the stimulus i.e.source of light as well as the location in the eye where the incident light is collected.
From what we learned in the chapter on types of human vision, rod cells are associated with scotopic vision i.e. night time vision and cone cells are associated with photopic vision i.e. day time vision. As you can see from the diagram below, rods are more sensitive than cones at all wavelengths except in the long wavelength red region of the spectrum. From this analysis, it is safe to infer that red light will excite our rod and cone cells the least and preserve our dark adaptation and help observe distant and faint celestial objects like planets, galaxies, nebulas, stars etc.
In their book titled Basic Vision : An Introduction to Visual Perception (Oxford University Press, 2012), the authors Robert Snowden, Peter Thompson and Tom Troscianko cite a very interesting example of how red light is used to maintain dark adaptation. During the Second World War, pilots flying on night missions were briefed in rooms illuminated by red light. This ensured that the photopigments of rod and cone cells was not bleached, their eyes were operating in scotopic vision and as soon as they left the briefing room for the dark of the airstrip and their waiting aeroplanes, their eyes required little to no effort for dark adaptation.
The US military standard MIL-STD-1472D (Human Engineering Design Criteria for Military Systems, Equipment and Facilities) advocates the use of red illumination or red filters to maintain dark adaptation. Here are the two most relevant extracts from this specification.
188.8.131.52 Display illumination and light distribution
184.108.40.206.1 Display illumination
220.127.116.11.1.1 Normal. When maximum dark adaptation is not required, low brightness white light (preferably integral and adjustable as appropriate) shall be used; however, when complete dark adaptation is required, low luminance [0.07 – 0.35 cd/m2 (0.02 – 0.10 fL)] red light (greater than 620 nm) shall be provided.
5.11.3 Optical instruments and related equipment
18.104.22.168 Lighting. Means shall be provided for illumination of internal and external scales, level vials, etc., under low light conditions. Continuously variable control of illumination shall be provided as required by weapon system characteristics. Illumination under low light level conditions shall be designed to minimally affect the dark adaptation of the observer. Red illumination or red filters should be used to maintain dark adaptation.
This is corroborated by the curves of threshold plotted against the time in dark. It can be observed from the difference between the curves obtained for a light sources of 420 nm and 650nm wavelength. For the light source of 420 nm i.e. violet colour (short wavelength), we observe that it takes an average human eye approximately 35 minutes for dark adaptation. On the other hand, for a light source of 650 nm i.e. red colour (long wavelength), it takes an average human eye less than 10 minutes for dark adaptation.
If we analyse the graph shown below that represents scotopic sensitivity as a function of wavelength, it reinforces our understanding of why red light is recommended for astronomical observation. The wavelength of most red LED lights on the market is between 630 and 660 nm. Red light in the visible electromagnetic spectrum lies between 625 and 740 nm. From this we can conclude that red light will prevent our rod and cone photopigments from being bleached, preserve our scotopic vision and maintain dark adaptation required for astronomical observations.
It is important to note that although red light is our best solution to preserve dark adaptation at night, we must maintain the brightness of red light at the dimmest level possible. An unnecessary increase in brightness will compromise the dark adaptation achieved earlier.
This is a popular choice amongst amateur astronomers. It has a 9V alkaline battery which can last 320+ hours at the dimmest setting and 12+ hours at the brightest setting. This is an additional incentive to keep the brightness of light at the lowest possible level besides preserving dark adaptation.
This flashlight carries two red LEDs that have long lifetimes, are robust and provide smooth illumination. According to Rigel Systems, this flashlight is equipped with an exclusive brightness regulation circuitry that keeps the LEDs shining brightly even when the batteries age unlike other LED flashlights that start bright but fade with time.
This flashlight is fairly water resistant, weighs 0.2 pounds or 91 grams and is 5.5 inches or 13.97 cm in length.
Best feature : Thumbwheel brightness control let’s you adjust the amount of light to the desired level of intensity. Dimming the intensity to it’s lowest level will extend battery life and preserve dark adaptation.
Skylite possesses all the features of the Starlite albeit a few notable differences :
Best feature : The possibility to switch between 2 red and 2white LEDs as the situation requires.
The Mini Starlites have the same features as the Starlites but two differences :
Best feature: Compact and cost effective.
The Mini Skylites have thesame features as the Skylites albeit a few notable differences :
Best feature: Compact and cost effective.
This is a 2-in-1 device possesses a handheld red LED flashlight as well as a power bank. This flashlight has three brightness levels that allow you to adjust the intensity depending on how dark your observing site is.
The PowerTank glow 5000 features silicone mount straps to attach the flashlight to your telescope tripod leg or any other convenient location so that you don’t accidentally trip over your equipment while moving around in the dark. A convenient wrist strap also comes along with this flashlight for handheld use.
After several hours of continuous use, planeterium apps can significantly consume your smartphone’s battery. Furthermore, your smartphone may not be fully charged when you begin night sky observation. These concerns are a thing of the past with the PowerTank Glow 5000. This compact power bank delivers a full charge to mobile devices including iPhones, Android phones and tablets with it’s 5000 mAh 5V DC lithium-ion battery. This gives you the freedom to scan the skies with your smartphone and share the experience with your friends and family on social media by capturing images through the eyepiece or live streaming your stargazing session.
The power bank has an USB Type A output port that can charge 5V DC electronic devices like smartphones, tablets and more. It’s input port carries the function of charging the internal lithium ion battery. This battery has a minimum lifetime of more than 500 charge cycles, which effectively means you can use it for observation for years to come. The battery’s input and output ports have covers to protect them when they are not in use. The PowerTank Glow 5000 also features overcharge protection. This means that it will not suffer any damage if inadvertently left recharging for an extended period.
With a single button you can control all the functions of the PowerTank Glow 5000. Short presses of the button turn the red LED flashlight on, cycle through three brightness levels starting with the dimmest and then turn the flashlight off. Pressing and holding the button switches device charging on and off.
The flashlight can be used even when the output port is charging an external device. Battery charge status is indicated by four red indicator LEDs and a green LED indicates when the power bank is providing power to a device.
Best feature : No need to change batteries when your flashlight needs power, just recharge the PowerTank Glow 5000.
This flashlight uses two red LEDs for even illumination and is powered by a 9V battery. It features a thumbwheel to vary the brightness as per your needs. It’s square shape prevents the Celestron NightVision flashlight from rolling off of your observing table to the ground. The attached lanyard makes it convenient for handheld use.
Best feature : Simple and cost effective.
This headlamp features a white LED spotlight and 2 auxiliary red LEDs. In addition to astronomy, this versatile headlamp can also be used for overnight hiking, camping, trail running and for general purpose outdoor use.
According to Meade, this headlamp has a rugged construction and is lightweight. It has an IPX4 rating for water resistance and also works in cold temperature making it suitable for year round usage.
This headlamp is powered by 3 x AAA batteries. On and off buttons give quick and easy control over auxiliary red and white spotlight LEDs. An elastic headband make this headlamp adjustable for all sizes.
Best feature : A hands-free headlamp makes it easy for you to set up your telescope in the dark. Nevertheless, you have the additional responsibility to not accidentally shine your headlamp directly in the eyes of a companion observer. This may compromise his or her dark adapted night vision.
The RedBeam II LED flashlight features a knurled finger wheel that allows you to adjust it’s brightness as per your needs. It has twin red LEDs which are powered by one replaceable 9V alkaline battery. According to Orion Telescopes and Binoculars, this battery can provide up to 600 hours of use.
The RedBeam II LED flashlight is 4-1/4" in length and can easily fit in your pocket. You can also hang it around your neck with the attached 20" looped cord.
Best feature : Knurled finger wheel for brightness control.
This is a multipurpose flashlight that allows you to choose between red light for astronomical applications and white light for increased brightness and visibility i.e. for non-astronomical applications. The selection between red light or white light can be done with the flip of a switch.
The Brightness wheel let’s you adjust the intensity of either red or white light from dim to bright. The DualBeam LED astronomy flashlight runs on one replaceable 9V alkaline battery and is only 4-1/2"in length. An attached 20" lanyard can be used for hanging the flashlight around your neck during observing sessions.
Best feature : Offers the possibility to select red light or white light as the situation requires.
This headlamp allows you to adjust the brightness of it’s red LED light from 5%, 10%, 50% to 100% illumination and also features a completely hands free mode that let’s switch the light on and off with the wave of your hand. A simple wave of your hand approximately 4 inches in front of the LED, motion sensor detects this gesture and turns the light on and off. You don’t have to worry about moving you head around and objects in front of you triggering the motion sensor. The motion sensor being focussed to 4 inches away, no object farther away will trigger the light. The motion sensor has the convenience of avoiding the need to search for the power button in the dark when it’s cold and one has thick gloves on.
The RedBeam motion sensing headlamp is powered by three AAA batteries which are NOT included. One can adjust this headlamp to aim straight out or angle it downwards towards the ground when worn.
An elastic adjustable head strap makes it suitable for all sizes and comfortably holds the flashlight in place.
Best feature : This headlamp frees both your hands to set up your astronomy gear or find lost parts if the need arises. You have to take care of not shining your flash light directly in the eyes of a companion observer and compromise his or her dark adapted night vision.
In addition to a red LED light to preserve your dark adapted vision this versatile LED lantern also has a brighter white light that you can use in the garage, around the house or on camping trips.
According to Orion Telescopes and Binoculars, this lantern is waterproof and dust proof. By clicking on the power button you can cycle through the different intensity settings depending on your needs. Red light can be adjusted to 5%, 10%, 50% and 100% brightness. White light can be adjusted to 10% and 100% brightness. When you no longer need light, you can switch off the astro lantern by pressing and holding the power button for approximately 3seconds. This can be done in any colour and brightness setting. A blue LED indicator light informs you about how much power is remaining and so that you can recharge when needed.
This lantern features an internal, rechargeable lithium battery and comes along with a micro-USB to USB cable, two magnetic clips, carabiner clip and wrist lanyard. The Astro Lantern also features a 2600 mAH (milliamp-hour) power bank function that can be used to recharge mobile phones in case of an emergency.
Best feature : The Astro Lantern includes two magnetic clips for easy attachment to telescope tubes, mounts, car hoods and other metallic surfaces.
It is also interesting to note that if a light source is coated with red tape, this tape will act as a narrow band filter and let only red coloured light to pass through it and serve it’s purpose for maintaining dark adaptation of eyes.
With this information in mind you can convert an ordinary flashlight into an astronomy friendly flashlight by covering it with thick layers of red cellophane paper or red tail-light tape. Rose City Astronomers suggest red construction paper or red fabric as other feasible options to cover standard flashlights for star parties.
Best feature : Cost effective.
We have covered a lot in this article and I hope this article has helped you to understand how dark adaptation works and what are the techniques to enhance and maintain dark adaptation for night vision. You don't need to implement all the techniques mentioned in this article to maintain dark adaptation. Even if you use dim red light during your astronomical observing sessions, it will go a long way in protecting your night vision. As you gain experience in observing the night sky, you can select the technique you are most comfortable with.