BLOG: Blue light affects everyday vision

Many researchers claim that contrast sensitivity is a much better measure of one’s functional vision. While visual acuity is quantitative, contrast sensitivity is qualitative.

Visual acuity measures the smallest detail one can resolve, using a chart that has 100% contrast, black on white. However, our world is not black and white. We all have patients with 20/20 vision or better who still complain that things just are not sharp. These patients range from young to old, from seemingly healthy to those who are post-surgical, have early cataracts or have other mild corneal or retinal pathologies.

On the other hand, contrast sensitivity measures spatial vision, our ability to resolve an image regardless of its size. Another way of thinking of this is that contrast sensitivity is a measure of “faintness appreciation,” a term I picked up from a lecture given by Stephen Beatty, MD.

Contrast sensitivity utilizes targets with high and low spatial frequencies. High spatial frequencies represent abrupt changes such as edges and fine detail. Low spatial frequencies represent global information about an object such as shape, proportion and orientation. Quality of vision depends on our ability to recognize both high and low spatial frequencies.

Blue light is detrimental to our quality of vision for several reasons. First, blue light is myopically defocused in front of our retina. Think about refraction: when we perform a red-green duochrome test, when balanced, red light is focused about +0.25 D to +0.50 D posterior to the retina, with green light focused on the retina. Blue light, being of a shorter wavelength, will be focused up to -1.00 D (at 420 nm) in front of the retina, creating a myopic blur circle.

Second, only 2% of cone photoreceptors in the macula are blue, while 65% are red, and 33% are green. On top of that, the fovea contains no blue cone photoreceptors. So, not only is blue light out of focus, we are also lacking in our ability to process it. This definitely has an effect on contrast.

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My last blog, "The cocktail napkin talk," discussed that macular pigment is located pre-receptorally, in perfect position to filter blue light before it is processed visually. In a paper that looked at the impact of macular pigment augmentation on visual performance using different carotenoid formulations (Loughman, et al., 2012), subjects that took a supplement containing all three macular carotenoids (lutein, zeaxanthin and mesozeaxanthin) had a significant increase in contrast sensitivity, along all spatial frequencies tested under both mesopic and photopic glare conditions. Interestingly, those who did not take a supplement containing mesozeaxanthin did not show statistically significant improvement.

Now consider our daily vision. Think about our ability to see a baseball or golf ball against a blue sky. How about our ability to see an animal in the road while driving at dawn? What about a child next to a parked car ready to chase a ball at dusk? A police officer’s ability to see a weapon in the shadows? A soldier’s ability to see a camouflaged enemy sniper? All of these examples represent targets that are large enough to see from an acuity perspective. However, depending on conditions and our ability to discriminate contrast, the end visual result may range from the loss of a game to, at worst case, the loss of life.

If you have not already done so, perhaps it is time to consider routinely measuring macular pigment optical density and/or contrast sensitivity. The results of these tests can tell us a lot about the vision of patients who have good visual acuity but still complain about their vision. I can attest that putting patients who test low on supplements to build their macular pigment can significantly improve their quality of vision.

As well, blue filtering spectacle lenses improve contrast sensitivity. Last year our clinic beta-tested a new, soon-to-be-released device for measuring contrast sensitivity. We saw significant improvement in scores for patients taking macular carotenoid supplements. We also saw scores improve by simply putting blue filtering lenses over a patient’s habitual correction and re-testing.

Blue light affects quality of vision. Setting yourself and your practice apart by addressing visual performance and taking steps to improve on it will assuredly result in happier patients. And it certainly won’t hurt your bottom line!

References:

Loughman J, et al. J Optom. 2010;3(2):74-90.

Loughman J, et al. IOVS, 2012;53(12):7871-7880. doi: 10.1167/iovs.12-10690.