Technical Explanation

In whatever circumstances they are used lenses reflect a proportion of the light away from the surface of the lens. For many years camera lenses have had anti-reflection coatings and you may have noticed the residual colour or bloom on the lens. Indeed the camera lens would not work properly without an anti-reflection coating. Because each element of a camera lens assembly reflects at least 8% of the transmitted light the total transmission can be less than 50%.

Nowadays AR coatings are even put on the fibre optics cables used in telecommunications to improve the quality of the transmitted signal.

The reflection of the light away from the surface is caused by the change in the speed of light in the two media through which the light has to pass - the air and the lens itself. As the lens has a higher refractive index than air, any time that the light passes through the interface between the lens and air part of that light is lost at the interface.

The amount of light reflected is dependent on the refractive index of the lens and has nothing to do with its thickness. Bare surface reflections are called Fresnel losses from the equation that is used to calculate them.

These calculations are for a standard index CR39 plastic (organic) lens and you will see that the loss is 3.87% for one surface and 7.74% for two surfaces. Consequently the light transmitted through an uncoated CR39 lens is only 92% - and that's the most highly transmitting material used in ophthalmic lenses!

Other materials used in spectacle lenses transmit even less light in their untreated state -

Depending upon the material you've chosen for your lenses the transmission drops from 92% to just under 81%. The higher the refractive index of the lens material the greater the reflection.

An additional problem is that there are internal reflections within the lens as the light is reflected around inside the lens between the two surfaces.

The result is that the spectacle wearer (or the camera film) suffers from reflected ghost images, reduced light transmission and a blurring or lack of clarity in the perceived image.

However, all is not lost as it is possible to dramatically improve the transmission of light through the lens by adding an anti-reflection coating.

Click here for a description of how we can create a Reflection Free lens in your spectacles.