These days almost all spectacle lenses are anti-reflective. They are designed to prevent as much light reflecting off the lens as possible, resulting in a better cosmetic result and prevention of artificial light from entering the eye of the user.
This in turn makes things like night driving less of a nuisance to spectacle wearers, and also makes the lenses more transparent since otherwise a proportion of light is reflected per surface.
That proportion depends on the material the lens is made out of.
How do anti-reflective lenses work?
To give a full treatment, quantum electrodynamics is required, but classical physics will do for a transparent explanation…
Please forgive the pun!
Let’s start by looking at the early days of anti-reflective lenses.
These were designed with a single film to eliminate the reflections from a specific frequency (i.e. colour) of light. Their performance was still good for light frequencies in the neighbourhood of the target frequency, but the performance dropped off as the light frequencies deviated from the target.
Some might remember the blue/purple bloom from these lenses, caused by the selective reflections from the surface.
To perform optimally, the film needs to satisfy two conditions.
1. The path condition
The first is called the path condition and stipulates that the optical thickness of the film needs to be a quarter of the wavelength of the light to be eliminated. The reason is that the incoming light will travel quarter of a wavelength before some of it is reflected. The reflected light then travels a further quarter of a wavelength making it out of phase with the incoming light.
This means the reflected light now destructively interferes with the incoming light and is eliminated. Because energy cannot be destroyed, the light is thus fully transmitted through the lens (a small amount is absorbed by the lens, but we need not worry about that).
2. The amplitude condition
The second condition is called the amplitude condition and stipulates that the refractive index of the film needs to be the square root of the refractive index of the main lens. This ensures that full brightness of the reflected light is eliminated rather than just dimming the reflections.
So that’s how a single film anti-reflective treatment works, but these days more sophisticated treatments are employed.
Broadband anti-reflective lens
It is now commonplace to have what is referred to as a broadband anti-reflective lens.
That is a system designed to eliminate reflections over most of our visual range. They work by having a stack of different metal layers that alternate between high and low refractive index and amidst this magic, almost all reflections are eliminated. What remains is a dim green reflex when light hits the lens.
Multipurpose lens treatments
Next came the so-called multipurpose lens treatments.
These lenses are made with a hard layer, an anti-reflective stack and a thin silicon layer. This means they have smudge resistance and water repellent properties. They are also made in a charge-free environment so that they don’t attract dust.
Now we have night-driving oriented, anti-reflective lenses which are designed to eliminate some of the incoming light from car headlights, by selectively cancelling out some of the light frequencies that we are more sensitive to at night time.
Computer ray-tracing simulations
Previously all anti-reflection designs were oriented around preventing reflections from direct light. With more sophisticated computer ray-tracing simulations, we are now starting to see more sophisticated lens treatments designed to eliminate reflections from all around us.
Hopefully we will see other industries progress with anti-reflective designs.
It would be nice to have commonplace anti-reflective car windscreens or smart phones, but of course these industries have different challenges such as overcoming surface degradation due to wear.