"The light comes off the clouds straight into my eyes and makes it really hard to stay focused because I feel a kind of headache in my eyes before it goes into my brain," he says.
Another challenge is facing artificial light sources.
"It can almost feel like my eyes are going to melt after a good hour or so of watching TV or using a computer screen."
Irlen Syndrome is a hereditary condition, it affects both males and females equally and can skip generations.
It is not an optical problem, but is related to the way the brain processes visual information.
After learning of Coker's situation, Learning For You tutor Michaela Newman told learning support adviser Chris Robson, who put an individual support plan in place for him.
One strategy to overcome the Syndrome is the Irlen method of intervention, this identifies which colour or colour combination will filter the offending light waves for the individual.
Newman began providing Coker with different coloured paper options and he was able to identify mauve as the one that provided the most clarity.
Meanwhile, senior Irlen screener Prue Deighton contacted Robson to ask about students who could be helped by charitable trusts that are available to assist Irlen Syndrome sufferers.
Coker travelled to a Palmerston North optometrist twice – once for a full vision test and again for the Irlen diagnostic appointment, which determined his individual filter colour.
After a three week process, Coker took ownership of a bespoke pair of Irlen glasses, specially made with purple-coloured filters.
"The main thing I was stressing out about was how to get to Palmerston North, but Chris definitely made it easy," Coker says.
"I do love the colour purple a lot, I'm guessing because it is more calming to my brain. I was able to try a whole range of colours and keep testing until I found the combination that is perfect for me."
The effect of the Irlens glasses was immediate, Coker has found himself reading faster than he ever has before.
"It has been easier for my brain to process what I am seeing and Chris mentioned that my writing is looking better as well," he says.
"It definitely does affect my mood, my mum has noticed that I'm happier now. You don't really notice how good or bad your eyesight is or how things feel until you have the improvement."