Eyewitnesses and Dark Vision

eyewitness-19061Question: An eyewitness has departed a brightly-lit café at mid-night and walked down a street dimly-lit by weak street lamps. 30 minutes after leaving the café, they witness a hit-and-run. The eyewitness has given a description of the incident and the vehicle involved to investigators. How would their account be expected to differ from a day-time incident? Would their report be more or less detailed if they had seen the hit-and-run only five minutes after leaving the café? Why?

Answer: Cone receptors in our eyes are adapted for colour vision, daytime and detailed vision, whereas rod receptors are adapted for vision in dim light. Dark adaptation is the gradual improvement in the ability to see in dim light, as the rod receptors (needed for the dark) regenerate their retinaldehydes (regenerating molecules) slower than cone receptors. This is how we can move into daylight (or a brightly-lit café) and our vision adjusts virtually straight away, as opposed to suddenly enter a dark area (dimly-lit street at mid-night). With this in mind, the eyewitness would be able to see clearer 30 minutes after leaving the café than if it were only 5 minutes after leaving as it takes between 5 – 30 minutes for full dark adaptation to occur.

Something to consider with eyewitness testimony in the dark is that colour perception after twilight is unreliable to non existent, and the Purkinje effect can change the relative brightness of colours (e.g. reds appear darker and blue-greens appear brighter) which can make testimonies more unreliable. Red light however does not interfere with scotopic (dark adapted) vision.


About Stu Dunn

I have three major professional areas in my life: Team Dunn Real Estate (Harcourts Real Estate with my beautiful wife Bridget), SDL (Behavioural Science Consulting) and Qian Li Dao Academy NZ (Wing Chun Kung Fu). Stu & Bridget Dunn Real Estate: Providing awesome service and results through our approachable, professional and effective real estate services. http://stubridget.harcourts.co.nz/ SDL: SDL helps our clients to understand non-verbal communication and to avoid deception. We do this by providing online courses, training and specialist advice in areas of behaviour, emotional analysis and detecting deception. Both my wife Bridget and I are also FACS certified. I have also written an International Best Seller, True Lies; A Guide To Reading Faces, Interpreting Body Language & Detecting Deception In The Real World. www.MicroExpressions.co.nz, www.facebook.com/sdlmicroexpressions & www.StuDunn.com Qian Li Dao Academy NZ (Wing Chun Kung Fu): The Wing Chun System, known for its incredible speed, powerful straight punches & Chi Sao (contact reflex training), relies on angling & body mechanics rather than brute force. www.QianLiDao.co.nz
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