A number of monitor manufacturers (ViewSonic, for example) have recently added a ‘blue light filter’ to the list of features for their latest models. Designed to minimise eye fatigue when monitors are viewed for long periods, this feature is most probably a response to media reports claiming that extended exposure to high levels of blue light is linked to irreversible eye damage.

A web search on ‘blue light + LED eye damage’ delivers many hits on articles repeating claims that looking at mobile phone and computer displays can cause eye damage. These all seem to be related to a research paper by Dr Celia Sánchez-Ramos of Madrid’s Complutense University and others, published in the journal Photochemistry and Photobiology in 2013.

The paper’s abstract states:

Human visual system is exposed to high levels of natural and artificial lights of different spectra and intensities along lifetime. Light-emitting diodes (LEDs) are the basic lighting components in screens of PCs, phones and TV sets; hence it is so important to know the implications of LED radiations on the human visual system. The aim of this study was to investigate the effect of LEDs radiations on human retinal pigment epithelial cells (HRPEpiC). They were exposed to three light-darkness (12 h/12 h) cycles, using blue-468 nm, green-525 nm, red-616 nm and white light.

And goes on to say…

Fluorescence intensity of apoptosis was 3.7% in non-irradiated cells and 88.8%, 86.1%, 83.9% and 65.5% in cells exposed to white, blue, green or red light, respectively.

The reason for the choice of wavelengths in the Sánchez-Ramos study is unclear as they match neither the peak sensitivities for the human eye, nor the typical frequencies for red, green or blue LEDs. The human blue peak is actually significantly higher in frequency than that in the Ramos study and higher than the typical blue LED. These wavelengths are compared in the following bar chart (the shorter the wavelength and the shorter the bar, the higher the frequency):

The spectral make up of the ‘white’ light used in the Sánchez-Ramos study is not specified in the abstract. Neither is intensity of the light. It should also be noted that any source of light, natural or artificial, that appears white contains some light in the blue wavelengths.

According to the abstract, the research would seem to indicate that exposure to all light greatly increased the rate of natural cell degeneration compared to cells kept in the dark. White light, blue light and green light (not necessarily from LEDs) all caused high rates of cell death at close to the same levels. Even red light had a marked effect compared to the dark control.

It should be noted at this point that Dr Sánchez-Ramos is the principal scientific advisor for Alta Eficacia Tecnología, a spin-off company from the Complutense University of Madrid that’s involved in the commercial manufacture of yellow filter lenses intended to protect against Age-related Macular Degeneration (AMD).

The Madrid research was picked up by the house journal of ‘middle England’, the Daily Mail, in a May 2013 article entitled ‘Do environmentally friendly LED lights cause BLINDNESS?‘. In the tabloid article, Dr Sánchez-Ramos is quoted as claiming that: “This problem is going to get worse, because humans are living longer and children are using electronic devices from a young age, particularly for schoolwork.”

Previous to the publication of the Madrid research, in 2010, ANSES, the French Agency for Food, Environmental and Occupational Health & Safety, published a report entitled Lighting systems using light-emitting diodes: health issues to be considered. As is clear from the title, and as the English abstract shows, the French report focused on ‘white’ LEDs (WLEDs) used for lighting:

The principal characteristic of diodes sold for lighting purposes is the high proportion of blue in the white light emitted and their high radiance (“brightness”). The issues of greatest concern identified by the Agency involve the eye, due to the toxic effect of blue light and risk of glare. The blue light necessary to obtain white LEDs causes toxic stress to the retina. Children are particularly sensitive to this risk, as their crystalline lens is still developing and is unable to filter the light efficiently

As the French report says, adults develop a natural yellowing of the eye’s lens in reaction to gradual long-term exposure to short wavelengths, and this provides some filtering of these wavelengths. As far as white LEDs are concerned, the report appears to be talking about the type of very high brightness, clear-lens LEDs sold by companies such as Kingbright and Nichia for use at the component level in the electronics industry. Arrays of these LEDs are also found in some lights for domestic use and in LED torches. The point source light from this type of LED can certainly be painfully intense. However many WLED lamps sold as replacements for domestic incandescent filament bulbs now have filters and diffusers and are of similar brightness and point intensity to the incandescent filament bulbs they’re intended to replace.

Ultraviolet or blue light?

The Daily Mail article associates blue light from LEDs with ultraviolet (UV). The effects of UV light are well known and proven from practical experience. It’s known that sunburn can result from exposure to UV in strong sunlight, for example, and that photokeratitis can result from looking at welding arcs without eye protection, or that snow blindness can occur from UV in strong sunlight reflected from snow. The UV wavelengths are artificially divided into three categories: UV-A ranges from 315 to 400nm, extending down into the upper limit of human vision, while UV-B ranges from 280 to 315nm. UV-B has the beneficial effect of inducing the production of vitamin D in the skin. High-intensity UV-C (100-280nm) lamps are used for sterilisation purposes because the UV kills bacteria and viruses.

Exposing your eyes to UV, particularly to UV-C, is obviously not desirable, but the lower-frequency blue light is required for normal colour vision. Any attempt to markedly reduce blue light at the source, or to filter it out before it reaches the eyes, results in illumination, or in colour vision, with a yellow tint.

As described by Planck’s law, the amount of energy required to generate light of a particular frequency increases greatly as the frequency increases, and it seems likely that the truth about light damaging eyesight is simply related to energy and therefore to wavelength. The short wavelengths (and therefore high frequencies) of UV are known to be dangerous. As wavelength lengthens (and frequency drops) through violet and then blue all the way down to red, energy falls quite rapidly and there is considerably less stress placed on the cells in the retina.

White LEDs and energy-saver LED lightbulbs

The concern over LED light isn’t really to do with red, green or blue LEDs, or even with white light as such. It is specifically related to ‘white’ LEDs (WLEDs). Early flat-screen LCDs used CCFL (Cold Cathode Fluorescent Light) — and fluorescent lights pose their own problems and possible dangers — for their backlights. Today most TVs, desktop monitors, laptops and some mobile phones use WLED backlights. Although it’s possible to use red, green and blue LEDs to create a white backlight, this method is only found in the more expensive displays designed for wide gamuts or in OLED displays (unlike LCD screens, OLED [Organic Light Emitting Diode] and AMOLED displays [Active Matrix OLED] have no backlight; instead, each pixel has a red, green and blue OLED). Energy-saver LED ‘lightbulbs’ also use WLEDs.

Concern over WLEDs stems from the fact that in most products the primary light is generated by a high-intensity blue Indium Gallium Nitride (InGaN) LED junction. A phosphor (Yttrium aluminium garnet, or YAG) is added, close to the blue junction, which converts some of the blue light into yellow. The combination of blue light from the LED junction, with down-shifted, secondary yellow light from the phosphor, results in light that appears as white. This is because yellow light is a combination of red and green light. Along with the blue light from the LED junction, all three primary colours are therefore represented in the WLED output and the human visual system recognises this as ‘white’ light.

As you’d expect, our spectrographic comparison of the output of blue and WLEDs (above) shows the blue responses for both components are almost identical; a large and fairly narrow 460nm blue peak. For the WLED this is followed by lower green and red peaks from the YAG phosphors.

The white light from such LEDs exhibits a comparatively high level of blue because the secondary yellow phosphor conversion isn’t as efficient as the primary direct radiation from the blue junction. This is quite evident from the spectral plot for a typical WLED. The blue peak is usually twice as high as the yellow. WLEDs available as individual electronic components normally use either a clear or frosted plastic encapsulation, which does little to filter the light. However, it’s possible to produce WLEDs with a tinted encapsulation to reduce the level of blue, and most of the WLED array lamps produced for domestic lighting do have yellow filters, which can clearly be seen when these lamps are off. Osram’s plot for its Parathom LED lamp and our plot for a Verbatim 9.5W LED lamp show the blue level is much reduced by yellow filtering:

WLEDs without yellow filters are rarely used to provide direct light. All display backlights have several layers of light guides, diffusers, polarising films, pixel filters, glass and anti-glare coatings between them and the outside world. These intervening layers serve to reduce the level of blue, and the actual balance of blue to green and red is controlled by the colour balance settings. In addition, the current production WLEDs used for display edge backlighting now have built-in yellow filters.

Similarly, energy-saver LED domestic lightbulbs have a diffuser to reduce point intensity, which is usually coupled with a yellow filter to produce a warm white light. Although the response for these lamps does show a clear blue spike at around 430nm, most of the output is in the orange/red band, which can be seen by comparing our graph for the Verbatim 9.5W domestic LED lamp with the previous graph for the unfiltered WLED.

The domestic lighting industry regards the older incandescent filament lamps with their ‘warmer’ light — a close approximation of black-body radiation at 2,700 K — as a standard for colour rendering, and the majority of new lamp production is geared to making ‘warm white’ lamps that mimic the old incandescents as closely as possible.

Fluorescent lamps

Fluorescent lamps as we know them today were invented in 1934, although the experiments that led to them go back all the way to 1856. In the original long tube form they were easy and cheap to manufacture and they produced an energy-efficient and bright, diffuse light, which was well suited to manufacturing environments. They became very widely used in factories during World War 2.

Miniature cold-cathode fluorescent lamps were used for the first flat-screen backlights, while the compact fluorescent ‘energy-saver’ lamp (CFL) , invented in 1976, has been much promoted in recent years for its energy efficiency and long life.

However, fluorescent lights do have their drawbacks: they are filled with low-pressure, environmentally undesirable, mercury vapour and there is much debate over just how dangerous the very small amount of mercury in each lamp actually is. The primary light is generated in the ultraviolet by a plasma discharge through the mercury vapour and down-shifted into the visible by a mixed phosphor coating on the inside of the lamp tube, resulting in rather spiky output spectra. Early phosphor coatings emitted a particularly ‘cold’ and spiky light. Modern phosphor coatings have improved. The glass of the tube adsorbs most of the stray UV.

The amount of mercury in a CFL is very small (around 3mg in a 27W bulb), and it’s often overlooked that mercury is also emitted by coal-fired electricity plants. According to NRDC, a 100W incandescent light bulb generates twice the overall mercury emissions compared to an equivalent CFL over its lifetime.

Energy saving

Beginning in 2009, an EU Directive placed a phased ban on retailers selling incandescent light bulbs, initially with CFL lamps as the intended replacement. Similar legislation is in place in the USA.

More recently, and before the ‘blue light’ scare, LEDs — and WLEDs in particular — were hailed as the preferred successor to ‘energy-saver’ CFLs, with a lower environmental impact and even higher energy savings. The case for WLED lights is actually quite quite compelling, with potentially massive energy savings available from LCD backlights to domestic lighting and even street lights. A domestic WLED ‘bulb’ with the equivalent light output of a 60W old-style incandescent filament bulb consumes only 8W (a CFL equivalent consumes around 15 to 20W — there is some controversy over the accuracy of advertised illumination equivalence). As reported by Wired magazine, Roland Haitz, former chief technology officer, semiconductor products group at HP (later Agilent) has said that “20 per cent of the world’s electricity that’s generated is used for lighting. Three quarters of that can be saved by using LEDs; 15 per cent of today’s electricity consumption can be saved.”

Although they require more complex manufacturing techniques than incandescent lamps, LEDs also last far longer: the typical incandescent tungsten filament light bulb burns for between 1,000 and 2,000 hours, while LEDs burn for up to 30,000 hours. Current CFLs are rated at around 15,000 hours. (An interesting historical footnote is that in 1924 the major light bulb manufacturers of the time formed the Phoebus cartel, which came to a secret agreement that its members would all engineer their bulbs to fail before 1,000 hours of operation. Originally set to expire in 1955, the cartel’s operation was disrupted by World War 2.)

Reducing your blue light exposure

Several options are available if you’re concerned about the possibility of blue light damaging your eyesight. The ‘blue light filter’ feature of recent monitors is nothing more than another colour preset on the colour menu, so even if your computer monitor lacks an explicit ‘blue light filter’ setting, simply changing the colour preset from the manufacturer’s default to a lower colour temperature will reduce the level of blue light from the screen. If you regularly work with colour-critical images, establishing a colour-managed workflow, using a colorimeter or spectrometer calibrator to colour-calibrate your monitor is likely to reduce blue levels, particularly if you choose to calibrate to the lower ‘warmer’ colour temperature standard of D55 (Daylight 5,500 K). Similarly, most televisions can be adjusted away from the manufacturer’s default setting to a ‘warmer’ hue.

It’s possible to obtain glasses, either from your optician or from companies such as Gunnar or BluTech, that are designed to reduce eye strain caused by working at a computer monitor for lengthy periods. Such glasses include filter coatings to block UV light and often include a visible yellow tint to reduce the amount of blue light reaching the eyes. Viewing a screen through a yellow filter isn’t really a problem if you deal mainly with black text and/or numbers on a white background, but it will alter colour perception, which is undesirable if you regularly work with colour images.

Blue light and sleep patterns

Another concern about blue light is that it serves as a biological prompt for sleeping patterns, or circadian rhythms. Blue light levels in sunlight are high during the day and gradually fall as night approaches. Research has shown that there is a specialised photoreceptor in the eyes of all mammals, in addition to the rod and cone cells used for normal vision, that has evolved simply to track this change in daylight. These photoreceptors have a peak sensitivity to light at 488nm — around the middle of the blue band. Artificial lighting from fluorescent lamps, TVs, computers and phones, which produce high blue light levels and are often viewed late into the night, may disrupt sleeping patterns.

Applications are available that alter a device’s colour balance according to the time of day, including F.lux for Windows, OS X, iOS and Linux, and EasyEyes and Night Filter for Android.

Other alternatives for domestic lighting

Given the current environmental, energy and cost-saving issues, there is great incentive for companies to develop new forms of lighting. One technology that may provide an alternative to CFL or LED domestic lighting is Electron Stimulated Luminescence (ESL), which has been developed by VU1 and should soon be generally available. ESL lamps generate light by using a high voltage to fire electrons at a combination of phosphors, in a somewhat similar fashion to the old CRT display technology. They even resemble the early round-screen CRTs in shape:

ESL lamps should be cheaper than WLED but cost more than CFL. They aren’t as efficient as WLED, however: an ESL equivalent to a 60W incandescent filament lamp consumes around 18W compared to 8W for a WLED. It is claimed that, although they use phosphors (just as mercury vapour fluorescent lamps do), the phosphors are chosen for a ‘warm white’ response that’s similar to incandescent filament lamps. Since a mercury vapour plasma is not being used, the light from an ESL contains only low levels of blue light.

Halogen lamps have been available for many years, originally for 12-volt low-voltage systems, but are now being sold in 240-volt versions. Although they are billed as ‘energy-saver’ lamps, halogens are ‘improved’ filament lamps and while they do provide the preferred ‘warm’ low-blue-content light typical of filament lamps, they offer only a small energy saving (a 105W lamp is equivalent to 140W, with a typical lifetime of 2,000 hours).

Our thanks are due to Public Lab, a US non-profit organisation dedicated to monitoring the environment using citizen science. The spectral plots shown in this article were captured using a spectrometer constructed by the author, based on open-source hardware designs from Public Lab. Spectral analysis was performed using the open-source Spectral Workbench application, also from Public Lab.