Autism Spectrum Disorder (ASD) includes a range of similar conditions that affect a person’s social interaction, communications, interests and behaviour. Recent research has found that the social deficit in ASD is secondary to neuronal overactivation and the painful perception of environmental stimuli.  ASD is in part caused by dysfunction of perceptual information processing . Visual stress that is caused by the hyperactivation of the visual cortex leads to pain and misperception of visual stimuli in the environment.
Individuals with ASD report and are observed to have symptoms of other conditions such as learning disabilities, attention disorders, tic disorders, epilepsy, anxiety, and sensory processing difficulties.[NHS.uk] It has been suggested that some individuals with autism suffer from a co-occurring disorder called visual stress. Individuals with ASD or these other conditions report visual processing abnormalities such as hypersensitivity to lights and colours and experiences of visual distortion. These distortions may, for example, alter the perceived dimensions of rooms or faces. Visual distortions can also result in difficulties writing on printed lines and maintaining appropriate spacing between letters and words.
Visual stress is a neurological condition characterized by hyperactivation of the visual cortex leading to symptoms of visual perceptual distortions and eyestrain when viewing patterned images in the environment, including text. The symptoms that are alleviated by individually prescribed coloured filters. 
Research has shown that precision filters may reduce these symptoms in individuals with ASD. Current findings are important in showing that perceptual abnormalities in a significant proportion of children with autism benefit from the use of an overlay, not just in respect of reading ability, but also as regards improvements in perception of facial expression. This provides further evidence that low-level perceptual abnormalities may be responsible for their difficulties attending and processing facial expressions for individuals with ASD. 
“Sensory processing problems can cause real pain; even non-verbal individuals with autism can have a problem with sensory overload. Some people are really helped by coloured lenses.” - Temple Grandin, Ph.D.
“I know of several children and adults (with Asperger's Syndrome) who have reported a considerable reduction in visual sensitivity and sensory overload when wearing Irlen (Precision Tinted) lenses." - Tony Attwood, Ph.D., Associate Professor at Griffith University in Queensland, Australia, The Complete Guide to Asperger's Syndrome.