Despite concerns that staring at devices putting out high amounts of the blue light wavelength could damage human retinas, a recent study finds that most devices put out less of that light than the blue sky on a clear day.

"Even under extreme long-term viewing conditions, none of the low energy light bulbs, computers, tablets and mobile phones we assessed suggested cause for concern for public health," said lead author John O'Hagan, head of the Laser and Optical Radiation Dosimetry Group of Public Health England in Chilton, U.K.

As people are using computers and phones more often and low-energy lighting like fluorescent and LED bulbs is becoming more common, the types of light human eyes are encountering is changing, the researchers point out in the journal Eye.

Compared to traditional incandescent bulbs, electronic screens and low-energy light bulbs tend to emit more blue light, which has long been known to be toxic to the retina, they write.

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Based on that toxicity research, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) has proposed a safe exposure limit, below which blue light is unlikely to harm a viewer's eye.

O'Hagan and his team measured the blue light emitted by several sources, including mobile phones, tablet computers, laptops and lamps, over time periods similar to the way people use the devices. Then they compared the emissions to the ICNIRP's exposure limits.

"The aim of our study was to measure blue light from a variety of sources, see what levels of blue light people were likely to be exposed to and determine if it was appropriate to issue advice on the public health aspects," O'Hagan told Reuters Health by email.

After comparing multiple colors on device screens, the researchers found that a white screen had the highest blue light emissions, so they used a white screen set at maximum brightness for their measurements.

They also compared the blue light emissions from various devices to the levels people would encounter when looking at a clear blue sky in summertime in Chilton, in southern England, and also to an overcast winter sky in the same location.

The blue light exposure on a clear day in June was around 10 percent of the ICNIRP safe limit. A cloudy day in December produced around 3 percent of the limit.

Comparing these natural exposures with light from lamps, computer screens and mobile devices like smartphones, the study team found that the artificial light produced even lower exposures than people normally encounter outdoors. That is, provided they're staring just at the sky, not directly at the sun, which is well known to damage eyes.

Even considering that people may stare at computer screens for hours in the course of work or play, the study team concludes that it is not likely to damage their retinas.

They caution, however, that the amount of light that gets transmitted from the surface of the eye to the retina is age-related, so children may be more sensitive to blue light. Light sources that are comfortable for adults could be distressing for children, the authors warn.

Meenu Singh, a light spectrum researcher at the National Tsing Hua University in Hsinchu, Taiwan, told Reuters Health he still has concerns about the safety of blue light.

"In this time the whole world is going towards digitalization and people are likely to be spending more time on display devices," Singh said by email.

The U.K. study did not look at other questions regarding the effect of blue light on human health, such as its potential to disrupt circadian rhythms and sleep.

Singh noted that this is an important consideration. "Displays like laptops, tablets, and phones should not be used for a long time at night because its bright emission suppresses the melatonin," he said.

To reduce eye strain, Singh said, people should be careful about how close their eyes are to screens, how long they're using the device, and in particular, the brightness of their screens.

"Everyone is different and so individuals are best placed to know what to do to minimize their own eye strain, including consulting an optometrist, if necessary," O'Hagan said.