When the concept for HiLO Lens (a right angle lens) first came up, the obvious solution was to use a mirror. Simple solutions are often best. A mirror angled at 45 degrees redirects the camera through a right angle.
But we quickly ran into problems.
The iPhone has a field of view of about 60 degrees (which is quite wide for a camera). The camera is about 9mm from the edge of the iPhone, to avoid the back of the iPhone appearing in the final image, the mirror needs to be held a certain distance from the camera.
As the mirror extends away from the back of the iphone the camera’s field of view is getting wider and wider, thus the mirror needs to extend even further. The end result is that the mirror needs to be relatively large to capture the iPhone camera’s field of view. From an aesthetic point of view this looks terrible. From a practical point of view the accessory becomes too large.
Our first technical challenge was to decrease the size of the mirror. This requires some specialized optics. Once Alexander (our optical engineer) started looking at the problem the question of using a mirror or a prism was raised.
Prisms and mirrors both have their advantages. A prism can be made from various types of glass and this gives the optical design more flexibility. Mirrors have an exposed reflective surface, so they need to bel protected from dust. Prisms can have the hypotenuse surface coated and this reflective surface is against the glass so protected from dust. There is also the risk of a mirror getting damaged when it is cleaned. So we favored the prism.
When exploring the optical design an interesting optical property came up for discussion. This is called total internal reflection. To better understand this, lets consider a normal reflective surface, this could be a mirror or the hypotenuse of a prism. Any reflective coating will absorb some light instead of reflecting it. For example a simple aluminium coating could reflect 88% of the visible light, silver might reflect 98% and a dielectric coating could reflect over 99%. But total internal reflection achieves an unbeatable 100% reflection and happens when the angle of incidence of the light means that no light can pass through the glass/air barrier. For further details wikipedia is your friend !
These details make a huge difference in image quality. You’ll be glad to know that we are using total internal reflection in the HiLO Lens optical design.