Daylight Energy Fatigue or D.E.F. 🌤️

Daylight Energy Fatigue or D.E.F. 🌤️

  • superbooks
  • 8 minutes
  • March 7, 2019

I recently stumbled on the following claim on the interwebs:

…that backlit displays can cause harm to the human eye whereas an electronic paper type of screen on a screenreader does not.

The user here was trying to upsell the idea that a screenreader with an e-ink screen is better suited for long haul reading than a tablet with an iridescent IPS panel—because of reflection, glare, or some sort of ‘blue light’ radiation from a backlit screen like that. As is usual on discussion forums online, the thread led to plenty of heated arguments on both sides of the aisle with each advocating for their case with opinions like:

  1. Not looking at the source of light directly,
  2. Because blue light could hurt our eyes,
  3. Because e-ink is almost like reading on paper and therefore…

For a moment I thought that if the paper was so much better than a digital screen and if it had all these magical qualities for the eyes then why aren’t people moving back from their smartphones and tablets to the way things to be with the dead-tree?

You know, line up textbooks on a shelf made of teakwood. 🤯

The answer first

As someone working on new book format for the web I had to find out. What are these people thinking and why do they feel that physical paper is better for their eyes? I put my search-ninja hat on and ventured into the world of science to get some answers:

To cut the long story short, none of the arguments in favor or against the benefits of an e-ink screen over an iridescent one are accurate or even remotely close.

If you look at both the products through the lens of science each has advantages or disadvantages depending on when and where you take-up reading. If your lifestyle is to sit on a beach under the Sun then you are probably better off with an e-ink screen. If you are a night owl who reads at night inside a blanket, then a self-illuminating tablet is certainly better.

As long as the contrast and flux of light are enough for your eyes to discern the text, your eyes will not be affected by the underlying technology in any way. But if the ambient-light is low, then there is a chance for your eyes to be hurting more with a screenreader than a tablet.

So now you might be thinking…

Then why does using a smartphone or an iPad all day feel so tiring? Does it really not matter if the surface was backlit or reflecting light in the room?

Enter the human eye!

[ Image credits:]

To answer the question above let us look at the human eye first:

A human eye is a simple optical receptor. Light rays enter from one end, through a transparent cornea and are then focused on to the other end, on the back of the eye, on to the retinal photoreceptors using a crystalline convex lens in the middle. The iris and the pupil take care of the incoming flux of light, meaning, the intensity, by expanding or shrinking to let-in only a safe amount of light for our photoreceptors to process.

This process of control of ‘incoming light rays’ by the eye is called exposure, and it works exactly the way exposure works on a camera. This is how a normal eye works.

[ Image credits:]

Unless we are forced to stare straight into the Sun for a considerable amount of time, our iris and photoreceptors are perfectly capable of handling the intensity of light (the visible part of the spectrum) safely.

Down to the last or single photon of light. It is that accurate.

In other words, the process of collecting light rays through a lens and projecting those rays onto the retinal surface on the back of the eye is a simple physical process and it follows the general laws of classical physics.

This mechanism is called optical refraction.

What matters to our eye is only the intensity of the incoming light and nothing else. Too dim or too bright are both harmful for us, but the source of light or its distance has no bearing on the process of viewing through the eyes. The incoming light could be coming from a primary source like a bulb or a phone or a secondary source like light being reflected off a wall or the ceiling or simply off a page on a book.

The nature of light does not change no matter what (Sun, mirror, or paper) or where (ten feet or millions of miles) the source of light is from you. Only the intensity varies, and intensity can be managed better on a tablet than a reader.

Reflected or direct, an electromagnetic wave (and the underlying physics) remains unchanged till infinity. As long as the intensity (or flux) of light is safe for our pupillary filters to pass through, we can stare at that source object as long as we want.

This solves the first piece of our puzzle:

Puzzle One:

That looking at the source of light directly (or indirectly) does not matter to our eyes. Only the intensity of light does. And therefore an e-reader has absolutely no advantage over a tablet since looking or not looking at the source of light directly is of no consequence.

One way to test this is by looking at the reflection of an iPad on your bathroom mirror. You are no longer looking at the source of light now but does it change the way content on your iPad appears to you?

If it had mattered, there would have been reflectors available on the market that were reflecting content off of the iPad so that our eyes didn’t have to look at the shining screen directly.


What’s that with the ‘blue-light’ then?

Another line that gets thrown around on the web often is that a ‘blue-light could hurt our eyes….’ I realize that this statement is common misinformation online, but it is often said in a way to suggest that this supposedly ‘blue-light’ poses some kind of danger to the human eye. Naturally, this claim was so outrageous that very little research was required to set it aside.

The first question here is to ask what blue-light is? And then does the color blue hurt our eyes in any way?

Well, let’s take a look at the full spectrum of light for this. The electromagnetic spectrum i.e “light” can be broadly categorized into the following categories:

| Alphabet    | Type or light            |
| ----------- | ------------------------ |
| G.          | Gamma rays               |
| X.          | X-rays.                  |
| U.          | Ultraviolet zone         |
| V.          | Visible spectrum         |
| I.          | Infrared zone.           |
| M.          | Micro waves              |
| R.          | Radio waves              |
| P.          | Power frequencies        |

The gamma rays, G., lie on the extreme left end of the spectrum (see figure below) and those have a very high frequency and thus penetrative power. Whereas the power frequencies (P.) sit on the extreme right end of the spectrum and those have a very high wavelength.

Do you see how tiny a sliver of the entire specturm is visible to our eyes?! The visible part, V., is merely a narrow range of frequencies that our photoreceptors are able to perceive. This range of frequencies is also known as the VIBGYOR part of the em-spectrum:

Assuming good-faith we can take it that the ‘blue light’ mentioned by those users online is the one represented by letter ‘B’ of the VIBGYOR, i.e the light range between 380 to 500nm respectively.

Does this blue-light pose any hazard to our eyes?

Nope, not to my knowledge, it doesn’t. In fact it is the other way round—blue-light is the healthiest range of light for our eyes and it draws our attention.

There is blue-light everywhere and it is not known to have harmed any animal or human being throughout history anywhere. It is part of the visible spectrum and the claim that it hurts us is just plain wrong.

But it does catch our attention…

Our retina functions much like a film on a camera. It is responsible for capturing all of the light rays coming through the eyeball, processing them into little light impulses through millions of tiny nerve endings, and then sending those light impulses over to the brain through an optic nerve connected with over millions of nerve fibers. The brain then finally processes these light impulses into our acuity called the vision.

It is literally our brain that sees and not the eyes!

In the morning, when the day breaks, we wake-up rapidly as soon as we flood daylight through our eyes. Our brain starts receiving a humongous amount of optical data or impulses through our eyes along with sensory data from other receptors like the auditory nerve and touch sensors under the skin, which were all in the state of rest during sleep.

The brain begins to use a lot more energy to process the amount of information coming in and then continues to operate at a higher energy level all day. In other words we operate on a higher state of consciousness during the day when we are awake and this can be tiring by the time we are done.

Daylight raises our senses and pushes our brain to process more data by being more attentive to our surroundings. This goes on throughout the day until the Sun sets and the nightsky prompts us to go back to sleep again.

It’s just the way our body works, in a cyclical circadian rhythm.

A modern iridescent screen of mobile or tablet works in a similar way. Its spectrum is similar to the morning light spectrum of daytime. The range of wavelengths hitting our cornea from a screen is similar to the one that we experience during the break of dawn. This blue-shifted morning spectrum grabs our attention span just like sunrise does.

It raises the dopamine level in our brain, which in turn helps us to focus more and be attentive during the day. Process the information quickly.

But we continue to look into the same screen closely way into the night, and our brains are thus tricked into thinking that it is still daytime. We are hooked to the the morning light spectrum coming off of our mobile phones instead of the Sun. This confuses us into not feeling that the day is over and we continue to remain alert and operate at a higher energy level inspite of being tired.

By not being able to discern when the day has officially ended, our brain literally loses the circadian rhythym and this leads us to screen fatigue by the time we fall asleep. Let’s call this screen fatigue Daylight Energy Fatigue or DEF, due to the lack of referenceable literature or vocabulary on the subject. Please correct me if I have misidentified the issue, or it has already labeled differently elsewhere.

Avoiding the DEF

To avoid DEF there is now an option of turning red-shift on on your smartphone. Apple calls this feature the Night Shift, which hints at using the warm end of colors on the spectrum (red-shift) that is used to prompt our sleep pattern on time. With the red-shift on, we can easily stop using the phone unless it is absolutely neccessary and this can help us keep our circadian rhythm healthy.

People can easily confuse the DEF with eyestrain, which is a completely different subject. Eyestrain deals with conditions like pupillary muscle atrophy, milky lens or focal degradation or retinal burn by staring at high intensity light source and it has nothing to do with how mobile phones or tablet work. Those medical conditions may need a separate treatment from a doctor whereas DEF is merely a fatigue of the mind.


Chances are that if you are a night owl you might be better off with a tablet. But if you are a daytime reader who prefers to reading outside in the Sun, then a screenreader or a physical book might be a better option.

Other than that, there is no difference between the two screens. If you are tired after your dayjob in front of a computer screen, do not pop open a book to read because your eyes, mind and the body need rest together.

About the author

Marvin Danig

I write code with my bare hands. 💪🏻 Yammer about Bubblin all day.