Beam Pattern / Optics / Reflectors / Refractors

Having an efficient light source only solves part of the problem; to be of any use the light produced must be directed to where it is needed.  In a traditional cave light, the halogen bulb is mounted in the focal point of a parabolic reflector.  Light that is thrown forward of the reflector generates a flood of light that cones out to an angle of approximately +/-45 degrees.  This allows the user to see close objects all around him/her.  The light that hits the reflector is focused into a tight beam allowing the user to pick out distant objects.  This type of beam is a compromise, in tight caves the spot is not needed, where as in large chambers, the spot may not be able to light up the distant formations.  If the spot is too bright Tunnel Vision can result, were the caver only see things lit by the tight ‘tunnel of light’.

LEDs are designed so that all the light is thrown forward of the junction.  In 5mm LEDs the beam is defined by the refraction properties of the encapsulation.  They are available in a wide range of beam angles from +/-10 degrees to +/-70 degrees, where the beam angle is defined as the angle at which, the light intensity has dropped by 50%.  LEDs with different beam angles can be mixed to produce a composite beam as desired.

Power LEDs typically produce a cone of light with a beam angle of around +/-70 degrees.  A caving light does not need a beam this wide, after all a caver can turn his/her  head to see objects to his/her side.  Spreading the light over such a wide area means the intensity will be much lower than if the light was focused into a narrower beam.  (If the beam angle is halved the intensity will be increased by a factor of 4)

Most power LED manufacturers specify secondary optics for their products. These secondary optics are typically made from acrylic and use internal refraction to tighten the beam angle.  There is normally a range of these optics that can tighten the beam to between 10 and 45 degrees.

Typical acrylic collimator

Few manufacturers specify parabolic reflectors for their LEDs.  When used with an LED, a parabolic reflector will give a different result when compared with a halogen bulb.  The halogen bulb is omni-directional so much more light will hit the reflector when compared to a standard (see below) LED, thus, the ratio between the spot and flood parts of the beam is much less when using an LED.

Parabolic reflector of the P60 module

Lumileds produce LEDs with three different beam patterns, lambertian (Standard), batwing and side emitting.  When used in a parabolic reflector the bat-wing device will produce a beam that is more like that of the halogen bulb.  In the side emitter the light is emitted from the sides of the LED.  If this were used in a parabolic reflector a very bright spot beam would be produced.

Polar plot of LED beam patterns