Many lens manufacturers and prescription houses are currently supplying spectacle lenses, which are described as ‘blue blocking’ or ‘blue protect’ or ‘blue control’. These lenses have a slightly yellow/amber appearance (of transmitted light) with a noticeably blue reflection from each surface (figure 1). They have been recommended to optical professionals and patients to guard against retinal damage caused by exposure to blue light.
This article describes the measurement of the transmission of these lenses.
Background
While large amounts of blue light are indisputably damaging to primate retina,1 there is no evidence that normal exposure to artificial light sources can deliver enough energy to cause such damage. This applies even to the light from mobile electronic devices with LED-backlit screens, despite the fact that they do emit more short wavelength radiation than other types of lamp, and are often viewed for many hours.
The International Commission on Non-ionizing Radiation Protection provides guidelines to safe exposure limits, which are based on extended periods of viewing (>10,000 seconds or about three hours).2
O’Hagan et al found that a clear sky on a summer day delivers approximately 10% of the recommended limit, whereas the screens of a wide range of computers, phones and tablets reached less than 0.5% of the exposure limit.3
The Advertising Standards Agency ruled in October 20154 that these lenses should not be advertised as protective, but a BBC Watchdog programme in November 2016 recorded optical practice staff making apparently unsubstantiated claims for the properties of these lenses.
Subsequently, Boots Professional Services Limited was referred to the GOC for a Fitness to Practice hearing, and was fined £40,000.5 Despite these well-publicised cases, a website sponsored by several well-known lens manufacturers seems to still be making the same claims (www.bluelightexposed.com/#bluelightexposed).
In view of the universal use by patients of all types of mobile electronic devices, it is also tempting for lens manufacturers to try to claim that blue light contributes to the eyestrain which can be experienced using display screens. This discomfort, which has been termed ‘computer vision syndrome’,6,7 and more recently ‘digital eye strain’,8 is multifactorial.
The general lighting level, or the brightness of the screen, are not, however, usually considered to be the problem. Instead it is the differences in lighting (and hence adaptation level in corresponding areas of visual system) between the screen and surround which can cause discomfort glare, or specular reflections from the screen which can cause disability (veiling) glare, which are more likely to be an issue. Neither of these problems can be significantly helped by tinted lenses.
Finally, there have also been concerns that blue light from display screens could have an influence on circadian rhythms. Each individual has an internal sleep/wake cycle, the circadian rhythm, which is set by exposure to light in the environment. Melatonin is the hormone which mediates sleep, and its release is controlled by the supra-chiasmatic nucleus (SCN) in the hypothalamus.9
The SCN receives signals from intrinsically photosensitive retinal ganglion cells (ipRGC) which contain a pigment called melanopsin which absorbs light.10 It is known that the peak of the absorption function of melanopsin is at 479nm,11 so artificial sources of light containing short wavelengths do have the potential to disrupt sleep, especially if the exposure occurs late in the day.
Despite the lack of any convincing need for these lenses, the lack of evidence is obviously disputed by some, and the lenses are still being marketed and sold widely. This investigation therefore took the alternative approach of measuring the lenses themselves to determine their direct effect on the wavelengths transmitted: what is the ability of these lenses to act as a barrier to certain wavelengths entering the eyes?
Methods
An internet search of lens manufacturer and supplier catalogues was conducted to identify lenses/coatings which were advertised as protecting from a blue light hazard, or controlling blue light, or being intended for use when viewing digital devices. Those which were obviously intended for outdoor use (‘sunglasses’) were not considered. Suppliers of the remaining lenses were contacted and samples were requested. No specific attempt was made to control the type of resin lens supplied: some were CR39 (n=1.5), but others were mid to high-index materials.
A Pye Unicam SP8-100 UV/Visible Spectrophotometer was used to determine the spectral transmission factor for wavelengths from 200 to 800nm for each lens sample. The spectral transmission factor (STF) describes the % of each wavelength transmitted after passage of light through the lens, assuming 100% incident on the front surface.
The usual measure of transmission which is quoted for lenses is the luminous transmission factor (LTF). This takes into account the variable amount of light at each wavelength that is emitted by realistic light sources (and the standard illuminant for this purpose is currently D-65),12 and the spectral efficiency of the human eye: this is the Vλ function which represents the photopic sensitivity (normalised to a maximum value of 1.0 at 555nm).
Multiplying these two functions together and summing for all wavelengths represents the luminous efficiency of the source. LTF is then a ratio of that function with and without the addition of the tinted lens. This calculation can be expressed mathematically as:
This represents the LTF for the full visible spectrum; that is, all the wavelengths for which the eye has a visual response.
The particular claim for the blue control filters is that they are designed to be used with LCD display screens with LED illumination. Therefore for comparison the LTF was calculated based on the amount of energy emitted from an LED source,3 weighted by the spectral luminous efficiency of the human eye for photopic vision (Vλ).
The LTF represents the visual effect of the tint, but non-visual effects were also determined. The current standard BS EN ISO 12312 also gives values for the blue light hazard function (Bλ) normalised to a maximum of 1 at 435-440nm. This time the ‘Blue Hazard Transmission’ is only calculated between 380 and 500nm, since there is no blue-light hazard outside that range. This was calculated for an LED source as expressed mathematically below:
The lower the % value, the greater the ‘protective’ function of the lenses.
With respect to any potential effect on circadian rhythm, the response is mediated by the pigment melanopsin in the intrinsically photosensitive retinal ganglion cells (ipRGC). The relevant non-visual response function on this occasion is the so-called melanopic sensitivity function, Vmλ, which is normalised to a peak value of 1 at 488nm.
Note that the peak of the function is shifted slightly from the peak of the melanopsin absorption function, because of the additional absorption in the human eye by the crystalline lens. The values of the Vmλ function are downloadable from the website of Professor Rob Lucas at the University of Manchester (http://lucasgroup.lab.manchester.ac.uk/research/). The function this time (if still considering the effect of an LED light source) is:
Results
As noted above, the ‘blue blocking’ or ‘blue protect’ lenses all had a slightly yellow/amber appearance (of transmitted light) with a noticeably blue reflection (figure 1). Not surprisingly, this can be seen in a spectral transmission curve which peaks at long wavelengths around 600-650nm (and in some cases reaches 100% transmission in this spectral region), and is lowest in the blue region (below 450nm) (figure 2).
Figure 2: The spectral transmission factor (as %) for wavelengths from 280 to 780 nm, of 4 different plano lenses: a colourless crown glass, colourless CR39, CR39 with UV400 filter, and CR39 with a typical blue control filter.
Table 1 shows the transmission of the lenses determined in several different ways. For comparison, standard uncoated crown glass, CR39, and uncoated UV400 lenses are also shown. The UV400 is a dyed tint on a standard CR39 lens, and has been available for many years. It would not be expected to have any particular effect on blue light, but it does tend to appear as very slightly yellow in colour.
The non-selective transmission of the colourless crown and CR39 lenses is borne out by the similar values in columns a to d. The transmission of around 90% is accounted for by the non-wavelength-selective loss of light by reflection at each surface of an uncoated lens of index n=1.5.
Comparing columns a and b, all of the lenses show a (slight) increase in transmission for light from an LED source compared to that from a standard illuminant. None of the ‘blue control’ lenses transmit less than 73% of the ‘hazardous’ blue energy from an LED screen, and two transmit over 90% (column c). Column d shows that the lenses all transmit over 85% of the energy which can potentially suppress the melatonin production.
Discussion
The most likely non-visual effect of display screens, over periods of extended viewing, is the potential to disrupt the circadian system. If light of the correct wavelength and intensity was present and could deliver sufficient energy to the retina, this might result in increased alertness during the use of the LED-backlit screen, and/or disturbed sleep following this exposure.
Oh et al13 investigated the (blue) light output of various smartphone displays to determine their photopic brightness, and their non-visual characteristics. They measured the output in a brightly-lit, and in a darkened, room, and varied the distance of the display from the measurement system.
The brightness was calculated as an MSV (melatonin suppression value) which could vary from 0% (if the ‘circadian illuminance’ was low), to 100% (total suppression of melatonin) in very bright conditions. Viewing web-pages on the device from 20cm at maximum screen brightness in a well-lit room creates an MSV of over 30%; but moving to 30cm in a darkened room drops this to around 2%.
However, the tinted lenses measured here are unlikely to help. The drop in circadian illuminance needed to go from 30% to 2% MSV is a factor of two, so this would need a tint of at most 50% transmission: to go from 100% MSV to 0% MSV would need the brightness to be reduced by a factor of 10, or a 10% transmission tint.
In time, manufacturers may be persuaded to adjust the spectral power distribution of smartphone displays to reduce their potential effect on circadian physiology. However, in the meantime, the advice for using a device before bedtime is to dim the screen, view it from as far away as possible, and dim the room lights. Of course, while optimal in circadian terms, this may not be good visual advice: the discrepancy in screen and surround luminance, and the reduction in angular substence of the screen detail, is more likely to cause eyestrain.
Conclusion
A casual inspection of the appearance and the transmission curves of these blue control lenses suggests that they have very little effect on incident light/energy. This is borne out by more precise measurements and calculations.
Practitioners should be careful not to make claims about lenses, tints and coatings which cannot be substantiated. Patients with symptoms of ‘digital eye strain’ would be better served by practitioners undertaking a careful investigation of refractive error, binocular vision stability and tear film dynamics.
Chris Dickinson is Professor of Clinical Optometry at the Division of Pharmacy and Optometry, School of Health Sciences, Faculty of Biology, Medicine and Health, University of Manchester, Manchester Academic Health Science Centre, Manchester.
Acknowledgements
The author is grateful to Fiona Cook for sourcing the different lens types, and to the lens manufacturers and suppliers for providing samples of the lenses.
References
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12 British Standards Institution (2015) BS EN ISO 12312-1 2013 + A1: 2015 Eye and face protection – Sunglasses and related eyewear BSI Standards Limited
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