Wavelength cancellation theory

Lenses transmit less than the maximum light because of surface reflections. There are a number of ways of reducing these reflections.

For ophthalmic use the only way is to use wave interference techniques that make use of the fact that light is transmitted in waves. Interference relies on the principle of superposition, which basically states that two waves of light (or even two waves of water) will add together if they are coincident in the same space. If the peaks and troughs in the two waves coincide exactly, then a wave of double the size is the result.

However the peaks of one wave are aligned with the troughs of another then they cancel each other out.

By putting a thin film on the surface of the lens, light waves from the reflections from the front and back of the film can be made to interfere so that they cancel out. In other words if the reflected waves from the front and back of the coated lens are exactly out of phase with each other there will be no visible reflections.

This is achieved if the difference between the back and front is exactly half a wavelength and to achieve this the coating must be one quarter of a wavelength thick.

So how do coatings affect the reflections?

A fraction of the incoming light is reflected from the top surface of the coating. Some more is reflected from the rear surface of the coating.

Light which has been reflected from the rear surface at point B travels in the V shaped path A - B - C. At point C it meets light reflected from the front surface. If the thickness and refractive index of the coating film are appropriate then at point C the peaks of light reflected from the front surface are coincident with the troughs of the light which has travelled A - B - C and the waves cancel out. This means that the reflections are suppressed and the surface is reflection free.

However, it is impossible to completely get rid of reflections across the visible spectrum because of the range of wavelengths, but with clever multi-layer designs very good results can be achieved.

Click here for details of the equipment used to create an AR coating