Summary for the lazy folks:
Blue-light blocking "block mobile phone/computer death rays" - nah.
Violet-light blocking - yes, as an everyday lens, since violet light is just next to ultra-violet (UV) on the electromagnetic spectrum, and therefore has higher energy.
You've probably heard about blue-light blocking lenses or anti blue-light lenses - are they really any good, or are they just a marketing myth meant to scare you into spending more? Have you heard about violet light, High Energy Visible Light (HEV420), or 420nm? Let us make it clear for you, because we don't just sell you glasses.
(And no, not that 420...)
Lenses that filter out Blue-Light
Several years ago, the trend of blocking blue light started. All sorts of claims were made about it - blue light from computer screen, mobile phones, and digital devices causes headache, harms your eyes, increases myopia, etc.
These first-generation lenses came with a blue coloured coating, which reflected blue light, hence reducing the transmission of blue light (letting less blue light through the lenses).
This resulted in a slight yellowish tint - some lenses even claim to block 80% of blue light. Yikes!
Eventually, science caught up, debunking many claims of such lenses.
Blue light from electronic devices is not going to increase the risk of macular degeneration or harm any other part of the eye. - Harvard Health (2019)
Many ophthalmic lens manufacturers are currently marketing blue-blocking filters, which they claim will reduce symptoms of Digital Eye Strain (DES). However, there is limited evidence to support the proposal that DES results from the blue light emitted by electronic screens. - Rosenfield, M., Li R.T., Kirsch, N.T. (2020)
The Academy does not recommend any special blue light-blocking eyewear for computer use. - American Academy of Ophthalmology (2021)
HEV420 Lenses - Blocking High Energy Visible Light
Violet Light Blocking Lenses, also known as HEV420 lenses or violet-blue blocking lenses, block the deep-violet portion of visible light - approximately 400nm to 420nm. These lenses are not made to fully block the entire spectrum of violet light - which is why it is often labelled as "UV420" or "HEV420" rather than "blocks xx% of violet light".
This is often demonstrated using a laser-pen. The laser-pen emits at a peak of 405nm, hence a violet-block HEV420 lens will block most of the light from the laser (but not all).
We know UltraViolet is harmful, and since UltraViolet-A (UVA) is from 315nm to 400nm, violet light - being close to UVA - may also be harmful at larger amounts or with prolonged exposure. This is supported by studies , which you can also check out our informational poster at our physical store.
These lenses make use of the material itself to block violet light, hence do not require a violet (or blue) coating, thus they can come with the usual green coating we are all familiar with. Observant users may notice a slight residual greenish tint, since there is less violet light passing through the lenses, but this is often much better accepted than the yellowish tints in the first-generation blue-light blocking lenses.
The Purpose and Use of Violet-Block HEV420 Lenses
We suggest that violet-block (HEV420) lenses can be used as an every-day lens, since the largest source of violet light you are most likely to be exposed to is the sun  at 1600000000 nits brightness, compared to your mobile phone at 500 nits.
The demonstration using a laser pen is indeed quite dramatic - however remember that the greatest exposure to violet light for most of us folks is probably the sun and not from digital devices shooting lasers at you.
As these lenses gain popularity, some stores may (perhaps inadvertently) advertise blue-light blocking lenses, but actually be using these violet-light blocking lenses instead. Indeed, when the violet laser is shown onto a piece of paper that has whitener, the reflected light looks blue instead of violet (but remember it's still a violet laser).
There is probably no harm in being kiasu and using these lenses as a "blue-light blocking" lens - might as well just protect lah - but the real 'goodness' comes from it giving extra protection against violet light - hence as an every-day lens.
How do I tell between the two types of lenses?
If you put the lenses against a white background, like on a piece of paper, blue-light blocking lenses tend to look yellowish - could be more or less depending on how much blue-light is being reflected, while the violet-light blocking lenses tend to look slightly greenish.
This topic goes way deeper than this short article, but we hope you've gotten an insight into these blue-light / violet-light / HEV420 blocking lenses. We believe that consumers should make an informed decision when making a purchase, and hence provide this easy to understand article.
If you found this useful, please share it with others who will also benefit from it.
P.S. we added more information about these blue/violet light blocking lenses, you can read it here.
Note: We may update this article in the future as more research is published and as the consensus and understanding towards this topic changes.
Still reading? Check out this video:
 Grimm, C., Wenzel, A., Williams, T., Rol, P., Hafezi, F., Remé, C. (2001) : Rhodopsin-mediated blue-light damage to the rat retina: effect of photoreversal of bleaching, Investigative Ophthalmology and Visual Science , retrieved from https://pubmed.ncbi.nlm.nih.gov/11157889/
 Youn H.Y., Chou B.R,. Cullen A.P., Sivak J.G. (2009) : Effects of 400 nm, 420 nm, and 435.8 nm radiations on cultured human retinal pigment epithelial cells, Journal of Photochemistry and Photobiology , retrieved from https://pubmed.ncbi.nlm.nih.gov/19201202/ and https://www.sciencedirect.com/science/article/abs/pii/S1011134409000025
 Sapkota, R., Pardhan, S. (2016) : Eye Complications of Exposure to Ultraviolet and Blue-Violet Light, International Review of Ophthalmic Optics , retrieved from https://www.researchgate.net/publication/308844699_Eye_Complications_of_Exposure_to_Ultraviolet_and_Blue-Violet_Light
 Herman, J. (2010) : Giz Explains: Brightness, Gizmodo , retrieved from https://gizmodo.com/giz-explains-brightness-5649389