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Definitely anecdotal.

This friend is hypermetropic, and uses reading glasses. He doesn't have major issues facing car headlights, or using a torch (the one with a bulb - not a white LED). Yet place him in front of a TV/Computer screen, and he'll develop an ache in the eye sockets in no time.

I understand an electric torch, an incandescent bulb, a car lamp are all DC types, whereas a TV/Computer screen is fast changing AC.

Are human eyes equally sensitive to light from an AC, and a DC lamp?

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Incandescent bulb also can use ac. Tv screens are different case altogether. It is a flickering light. –  WYSIWYG Jul 14 '13 at 18:55
    
I'm not great with electronics, but don't all devices use DC? Computers and TVs require AC adaptors to convert the AC power supply to a usable DC signal. –  Brandon Invergo Jul 14 '13 at 22:23
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Depends on the device. Many lamps work on AC. Fans and motors work on AC. Incandescent lamps work on the principle of electrical heating which both types of current can do. Semiconductor devices generally use DC because direction of the current is important. TV screens are not continuous light; it is a kind of flickering light. In a previous post, this has been discussed with respect to cats and dogs. You may find it useful. –  WYSIWYG Jul 15 '13 at 4:41
    
@WYSIWYG: The link to the other post was helpful. ty (+: –  Everyone Jul 15 '13 at 17:01
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I doubt that it's what kind of electricity provides the power -- rather what wavelengths of the light are emitted. –  kmm Jul 15 '13 at 18:56

1 Answer 1

Let's not confuse the type of power of a light emitting device with the type of light it produces.
This question is not about AC or DC current. It is about flicker frequency, light wavelength and their biological effects.

Flicker frequency

The term "ache in the eye sockets" probably refers to asthenopia.

Human eye cannot detect flicker with a frequeny above 75 Hz. But many computer displays have a refresh rate of 60 Hz [1]. Out of focus and low refresh rate monitors are known to cause eye strain and asthenopia [2], favored by existing hypermetropia (and other vision issues).

Visual discomfort has been related to 1) the presence of flicker; the possibility to regulate, 2) brightness, 3) height; and 4) inclination of monitor. Asthenopia has resulted statistically correlated to the presence of flicker and to the impossibility of regulating height and inclination of monitor for both sexes. The possibility to regulate monitor brightness has not determined a reduction of visual discomfort either in men or in women [3].

Wavelength

Eye sensitivity is different (see spectral sensitivity):

Cones SMJ2 E.svg
"Cones SMJ2 E" by Vanessaezekowitz at en.wikipedia / Later version uploaded by BenRG. - Based on Dicklyon's PNG version, itself based on data from Stockman, MacLeod & Johnson (1993) Journal of the Optical Society of America A, 10, 2491-2521d http://psy.ucsd.edu/~dmacleod/publications/61StockmanMacLeodJohnson1993.pdf (log E human cone response, via http://www.cvrl.org/database/text/cones/smj2.htm) Transferred from en.wikipedia to Commons by User:Richard001 using CommonsHelper.. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Light color and color contrast matter:

The study results showed that both visual acuity and the subjective visual fatigue were significantly affected by the color of light [4].

Images with excessive energy at medium spatial frequencies (Fernandez and Wilkins, 2008 Perception 37 1098-1113), or that have high color contrast and little or no luminance contrast (Wilkins et al, 2008 Perception 37 Supplement, 144-145) appear uncomfortable or aversive and can induce headaches in hypersensitive observers. Such stimuli are uncharacteristic of natural images [...] [5].

Blue light can also reduce melatonin levels and affect sleep and mood (the studies don't have a strong conclusion though):

Melatonin concentrations after exposure to the blue-light goggle experimental condition were significantly reduced compared to the dark control and to the computer monitor only conditions. Although not statistically significant, the mean melatonin concentration after exposure to the computer monitor only was reduced slightly relative to the dark control condition [6].

All light is not equal: blue wavelengths are the most potent portion of the visible electromagnetic spectrum for circadian regulation. [...] evening use of amber lenses to block blue light might affect sleep quality. Mood is also affected by light and sleep; [...] At the end of the study, the amber lens group experienced significant (p < .001) improvement in sleep quality relative to the control group and positive affect (p = .005). Mood also improved significantly relative to controls [7].


References:

  1. Wikipedia contributors, "Flicker fusion threshold," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Flicker_fusion_threshold&oldid=615159751 (accessed July 29, 2014).
  2. Wikipedia contributors, "Asthenopia," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Asthenopia&oldid=617227613 (accessed July 29, 2014).
  3. Rechichi C, Scullica L. [Asthenopia and monitor characteristics]. J Fr Ophtalmol. 1991;13(8-9):456-60. PubMed PMID: 2081858.
  4. Lin CJ, Feng WY, Chao CJ, Tseng FY. Effects of VDT workstation lighting conditions on operator visual workload. Ind Health. 2008 Apr;46(2):105-11. PubMed PMID: 18413962.
  5. Juricevic I, Land L, Wilkins A, Webster MA. Visual discomfort and natural image statistics. Perception. 2010;39(7):884-99. PubMed PMID: 20842966.
  6. Figueiro MG, Wood B, Plitnick B, Rea MS. The impact of light from computer monitors on melatonin levels in college students. Neuro Endocrinol. Lett. 2011;32(2):158-63. PubMed PMID: 21552190.
  7. Burkhart K, Phelps JR. Amber lenses to block blue light and improve sleep: a randomized trial. Chronobiol. Int. 2009 Dec;26(8):1602-12. doi: 10.3109/07420520903523719. PubMed PMID: 20030543.
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