Anatomy of the Eye:

Cornea - transparent outermost layer that does a lot of the refractive work (~60%), getting light to bend onto the vision cells at the back of the eye

Lens - This bit does the rest of the refractive work and lets you change your focus depending on the distance between your eye and the object

Retina - This is where the vision cells are that turn light into signal for your brain to interpret, it's the innermost layer and attached on the inside

Globe - Not labelled but refers to the whole shape of the eye so a global rupture would mean damage that compromises the eye leading to leakage/draining of the fluids inside

Finding a useful paper:

Limitations:



Normalised Energy for an Elite Dart:

Putting it on the graph:

What do all those injury names mean?

Hyphema is bleeding inside the front of the eye, resulting in a pooling of blood in front of the lens. For the strong stomached among you, it looks like this. Needless to say, you'd want to get that checked out by a doctor pretty quickly

Lens & Retinal damage cover a wide range of conditions from rossette cataracts to dislocated lens to detached retinas and are very certainly something you want to avoid. Remember, your eye isn't hollow, there's a lot of squishy fluid in there so any damage done to the front can create a shockwave that will ripple back and damage your retina, too

Global rupture, I pretty much defined that above but it's any sort of tear or damage to the sclera (the white bit) that might compromise the shape of the eye - this one certainly needs surgery because chances are you're going to be getting a glass eye.

I forgot my eye pro, what's the worst I can expect at a UK game?

What about the other end of the scale?

Many moons ago, I did a brief literary survey on the science of eye protection and the risks associated with not wearing any. The initial survey focused on one paper and had a few short comings. First, it covered only metal fragments and second it was a very narrow study. In doing some research for my upcoming series on lesser thought about aspects of Nerf, I thought I'd have another look at the literature and see if I could gather some more in-depth and relevant data.First up, a little bit on the anatomy of the eye. Our friends over at Moorfields Eye Hospital (the UK's leading eye hospital for eye injuries), provided this handy diagram on their website:The main areas we are concerned with are the Cornea, the Lens, the Retina and the Globe.Now, darts to the eye can affect any part of your eye but the four listed above are the most pertinent. The two main issues you're going to get are blunt force trauma from darts and scratches to the cornea as a result of foreign bodies present on the tip of a dart. I couldn't find any in-depth data on corneal injuries so I've elected to leave that for a separate article so I've focused primarily on blunt force trauma.So, what did I find? Well, I found this very handy literature review from the Centre for Injury Biomechanics at Virginia Tech in the USA . It looks at a range of studies and in reviewing them they came up with this very handy graph:So what does all that mean? Well, they took a bunch of studies and realised that one of the big problems is comparing different type of projectiles. It's hard to compare a baseball with a shard of metal. In order to fix that, they converted the kinetic energy and velocity into 'Normalised Energy' which is basically kinetic energy per unit surface area. Their research also told them that the total energy of impact is far more important than the speed in determining the damage that gets done. Helpfully, we as a community already work with Joules so we can break out the old calculations as follows:Kinetic Energy (in J) = 1/2 x mass (in kg) x velocity ^2 (in m/s)Normalised Energy (J/m²) = Kinetic Energy (J) / Impact surface area (m²)What's even more helpful about this review that I hadn't quite twigged is that the European Union Toy Safety Directive states that toys shall have a Kinetic Energy Density of 0.05J/mm² which is the same as Normalised Energy but just needs a slight metric unit conversion. That's a useful tool to determine if something can still have a CE mark stuck on it and be marked as toy safe.Before I continue, there are a few key limitations here that need to be acknowledged. First, the four conditions listed on this graph need to be diagnosed by a qualified clinician. In the real world, we see more corneal abrasions, swelling and redness from eye injuries more than anything else and that's probably linked to the third limitation below. The second, and probably most critical limitation is the fact- that would waste energy as heat and other types of energy and not transfer it to the eye. Effectively, each and every one of these calculations is for an FVJ dart that will transfer nearly all of its energy into your eye. If I find a way of determining the energy lost then I will re-run all of the calculations and update this post. Third, most of the studies done involve firing projectiles directly at a pig or similar eye using an air gun. That fails to account for weird trajectories and things like your eye lid getting in the way. The data provided below, therefore, is for that one dart in a thousand that comes directly at your eye and you don't get time to blink or shift away.So, I want to work out the Normalised Energy for my Stryfe. I know that the dart impact head is 10.8mm across and that the area of a circle is Pi x Radius ^ 2If I calculate the area of the impact circle in m², Google tells me it's: 9.16×10m² which is pretty small but a square meter is pretty darn big so it's not surprising.Taking that number forward, I stick it into the Normalised Energy Equation:Normalised Energy (J/m²) = Kinetic Energy (J) / Impact surface area (m²)NE = 1 / 9.16×10m²NE = 10917.031 J/m²Now, just over 10 Kilojoules might sound like a lot but remember that you're looking at that energy being distributed over a whole square meter. I'm pretty sure even Hasbro aren't going to make darts that large in an effort to get them to hit a target!So we know have our Normalised Energy for our waffle tip dart, we can put it on the graph.Based on the maroon lines, you're looking at a 40% chance of Hyphema and ~7-8% chance of lens or retinal damage at 1J for a direct impact to the eye.The various injuries are summarised below:As you can see, at the top end of most UK games (1J) getting caught with your goggles down isn't going to blind you. Hyphema is pretty self limiting, albeit painful and will resolve itself in around a week under your doctor's supervision. Lens and retinal damage are much rarer but still enough to warrant enforced eye protection at all times.Right, assuming in some magic universe you get onto a field at a UK game and the foam starts flying, how bad can it get? Well, Foam Flinging Skirmish is current the top rated UK game at 300FPS. Assuming you're looking at a 1.3g dart then your kinetic energy is on the order of 5.4J. Crunching the numbers through the equations above produces a NE of 59326J/m².As you can see, FFS comes off the end of the graph's scale. This isn't to scare anyone but to point out that mandatory eye protection exists for a very good reason at higher end games. This little detour offers an insight into 'what could be' and backs up the notion that eye protection stays on at all times and the notion around no magazines being inserted in blasters in safe spaces. Unlike lower energy levels, the risk from a single round here is catastrophic which makes it a very sensible policy.Now, there are other ends of the scale where there are younger players who are perhaps not as reliable with their eye protection and energy levels are concomitantly lower. What's the worst you can expect there? Well, again assuming a 100FPS limit on a 1.3J dart then you're looking at ~0.6J of kinetic energy. Crunching that through the equations gives me a NE of 6591J/m².Now, the Hyphema line stands at around 15% but the lens and retinal damage curves have barely gotten moving in this case. Game organisers tend to be pretty good at reminding kids to keep their goggles on and stopping them from shooting each other in designated safe zones.I'll go have a poke at bone stock NEs and the like when I've had a chance to locate the specific section of the Toy Safety Directive.So there you go, that's what I found when I was digging through the literature for a better idea of the risks of not wearing eye protection at games. All BritNerf organised games mandate eye protection but I always like to have a little science underpinning common and best practice. I'm happy to discuss the implications and outcomes of this below. I put this together with a view to giving game organisers a handy 'here's why we have eye pro' article but also to satisfy my own curiosity. I've only focused on a single literary review here so if people are interested, I can go into more detail going forward.Comments, thoughts, discussion are welcome.