Questions and Answers  

    Questions and Answers on Light Pollution webinar

    SPD is preferred over Kelvin to describe blue content. That being said, the typical 3000K should not be considered "Warm" as the typical 3000K LED has substantial blue?

    SWL: We understand that Correlated Color Temperature (CCT, Kelvins) does not adequately describe the ‘non-visual’ response to light which is better described by melanopic lux (Lucas et al., 2014 or the melanopic / photopic lux ratio (see  for examples). We did not have time in the talk to review this issue, unfortunately.

    SPD is of course better, but Kelvin is often used. 3000 Kelvin is generally indicated as upper limit for colour temperature; but of course lower is better. It’s all about avoidance of blue light for species active at night.

    When you mention "dark" does this include moon effect (if any)? What about the effect of full moon; have you done any studies for the change of behaviour?
    With ‘dark’ I meant every situation without artificial light. Of course, there’s a significant variation in natural light at night, and this certainly has an effect on the activity of species. For example, it’s reported that activity of several species of bat is reduced by moonlight.
    Would it be possible that the blue component of the LED sources might have a stronger influence on night activity, just as for human being?
    Yes – any blue light. However, it’s of course possible to produce LED sources with less or no blue light.
    Do you have any findings for Amber light with Animals? I am finding more requests for amber lights in the national parks but curious on what that impact is having versus white, green & red spectrums­?
    We did not test amber light, however, it resembles Low Pressure Sodium (LPS) light (although Amber light is not monochromatic like LPS) which is for example known to attract less Pipistrelle bats (species from the agile, non-light shy group). This is probably because it attracts less insects. Not much is known on the effects of Amber on light shy, slow flying bats, but its effects can be expected to be lower than broad-spectrum white light. Compared to the light we tested, the Amber peak is however much closer to the peak sensitivity of the M-cones of nocturnal species.
    I am from India and we have good sunlight all-round the year. However with rapid urbanization, unfortunately most urban spaces are dense and don’t receive adequate sunlight. We've millions of people with acute D Vitamin deficiency leading to such diseases as Diabetes. While one could rely on vitamin supplements, are there any low cost light sources that could replace sunlight­?
    The non-visual effects I was describing are all through detection of visible light by the eyes. Vitamin D synthesis is stimulated through a particular ultraviolet light exposure to the skin and is a different system. I am not aware of light sources that would help and you should not expose yourself to UV light without supervision.
    In the Melatonin "activation" graphs with the 3 scenes: light all evening, dimmed light in the evening, and dimmed light all through the night. Could you please expand on the third scenario? Is complete darkness not a requirement for the melatonin cycle?
    No. The rhythm of melatonin is internally generated by the circadian clock regardless of any light or dark input. (I have studied many totally blind people, including people without eyes, who have a perfectly normal melatonin rhythm; see Lockley et al., 2007 What light does is two-fold; i) light resets the circadian clock every day which in turn resets or synchronizes the the melatonin rhythm and all the other rhythms the clock controls; and ii) light directly suppresses melatonin if the eyes are exposed to light at night. So it is correct that darkness at night is needed to see the full expression of the melatonin rhythm and amount but the rhythm is generated internally, not by the light-dark cycle.
    What is about the blue light in the new LED-TV's. Do they have the same effect as a "normal" light?
    All light, from any source, can have an effect on human physiology. The eye detects photons and it does not matter what the source is. While there are other factors, the biological impact of the light will depend primarily on the amount of light reaching the eye and the spectrum. The closer a light source it to the eye, the greater the intensity and greater the effect (see The more 480nm short wavelength blue light in the spectral power distribution (SPD) the greater the effect. The blue light emitted from LED TVs, or from any source, is likely to have an impact.
    How sensitive is a person when sleeping with his eyes closed? How much light is needed to suppress melatonin with eyes closed?­
    About 95% of the light is cut out when the eyelids are closed but, if the light is bright enough, it is possible to get suppression of melatonin through the eyelids. The minimum about of light needed is hard to detect as we are limited by the methods we use to measure melatonin which does not allow very small changes to be seen. Levels of light in the tens of lux is sufficient to suppression melatonin and possibly lower but most normal indoor light levels, unless very bright, would not have a major effect on melatonin suppression through the eyelids. To avoid it, however, turn off the light, use black out shades if outside, or use an eye mask.
    At what wavelength or color temp can you actually do physical damage to the eyes?
    Any light can be damaging if the amount is very high or its concentrated in a small area (e.g., a laser). There are safety standard defining how much light is safe, based in the wavelength and energy. Most commercial light sources used in the home are very much below these safety standards although we do check the safety of non-typical experimental sources when we use them. We work with Dr David Sliney, an expert in light safety, and one of his talk is here: and a recent video is here:
    Are the night time modes in electronic devices that remove bluer light effective at mitigating the impact on sleep?
    There is a lot of variability in the night time modes, with some being better than others, so it’s not possible to give a general answer – we would need to study each one. In general, however, anything that reduce the amount of light and the short-wavelength content of light around 480nm emitted by electronic devices is likely to be helpful in the long-term, but the effect will not be reduced completely. You can look at how different light sources and filters effect the biological ‘strength’ of the light here: and by using the ‘Filter’ tab at the top of the page.