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Monday, August 18, 2014

Why Young and Old Don't See Things the Same

Human vision changes over time. When making visual assessments, we must understand these changes in order to better compare our assessments with others.

The chart above shows that transmission of light through the lens of the eye decreases with age. Even a newborn only has 92% transmission through the lens. This gradually decreases to around 30% transmission at 80 years of age.  Often, if the lens filters enough light out from reaching the retina where the rods and cones are located, people will have surgery to replace the foggy, old lens. This will increase the transmission of the lens as if it were 'younger'.

The chart above also illustrates that as people age the transmission of light through the lens of their eye changes. At 1 year of age, the spectral curve is relatively flat. This means that there is nearly the same amount of light transmission through the lens from 500-800 nm. There is some slightly lower transmission from 300-500 nm.  At 8 years of age, again, the spectral curve is relatively flat. However, the transmission is less at all wavelengths than age 1 year old. At 25 and 58 years of age, the spectral curve shows significantly lower transmission at all wavelengths. Also, the transmission drops off dramatically in the UV (300-400 nm) and blue (400-500 nm) part of the transmission spectrum. This all means that the lens is yellowing with age

All of this points out that people of different ages may see things differently due to the aging of the lens in their eye.  All people will age differently, too. So there is no way to mitigate these effects. It is important, however, to be aware that they exist.