“Let there be light” takes on new meaning in the era of LEDs. Light from solid state light sources can be focussed and shaped by optics and lenses, and it is significantly easier to obtain specific colours or adjust the colour of the light. The innovations brought about by LED lighting technology are described in our Lenses and Optics guide.
The term ‘optics’ refers to components that are used to collect, shape and direct light in order to produce a usable light source with effective light output. Asymmetrical lenses are being used more and more in applications such as street lighting to direct the light beam effectively without generating stray light or glare. There are essentially three types of LED optics:
- Primary: attached directly to the LED. Primary optics are usually part of the LED package, with the lenses integrated into the LED package, especially with SMT and high-power devices. This may cause the luminous flux emitted by the LED to have a particular radiation pattern.
- Secondary: fitted directly over or around the LED. Secondary optics consist of lenses and reflectors that are fitted on top of the LED to provide the desired form of light output.
- Tertiary: external devices. Tertiary optics include a wide variety of optical devices used to diffuse or direct light. A typical example is a light guide.
Lenses and reflectors
Lenses are usually classified as secondary optics and are fitted directly on the LED. They usually produce a light beam with a large optical viewing angle and a circular cross-section. Reflectors are additionally fitted on lenses to improve the bundling and steering of the light beam. This results in highly focussed light, such as a spotlight. High-quality reflectors are characterised by low stray light. In applications such as pocket torches, moveable reflectors can be used to adjust the focus of the light beam.
Light guides are external LED lighting technology devices classified as tertiary optics. Their characteristics help optimise the use of LEDs. Light guides are used to diffuse, direct and transport light. LEDs can be switched and modulated very quickly by controlling their operating current. The high modulation rate of LEDs is important for use in optoelectronics and in data transmission using fibre-optic light guides. Fibre optics are usually made from glass, quartz or plastic.
Optical viewing angle and radiant power
The optical viewing angle is one of the key characteristics of LEDs. Incandescent lamps radiate light in all directions, but LEDs emit light in a cone-shaped beam with a specific optical viewing angle. LEDs always illuminate only part of the area around them. Multiple LEDs are necessary for all-round lighting.
Typically, when we think of LED optics, what comes to mind in a plastic lens of some sort used over the LED itself to focus or disperse the light. We need to take a step back and realize the LED itself has a protective casing called the primary optic. The primary optic serves two purposes: to protect and to shape the light output of the diode.
So why aren’t optics used everywhere then? Well there are disadvantages of secondary optics with some of them being pretty significant:
- They can be expensive. They typically add between 25%-50% to the cost of each LED.
- With few exceptions, they create a harsh transition from bright to dark. In other words, the “edges” of the cone-shaped light pattern can be very distinct. This creates a “flashlight effect”, where everything within the light beam is very bright, and everything just outside the beam is very dark.
- They hinder color blending. Since most of us like the actinic effect of all blue lighting, and since most of us prefer a cooler color than the coolest white LED available, we must use a combination of royal blue and white LEDs in our systems. When secondary optics are added, you may see obvious white and blue spots in your tank. This effect can be reduced by reducing the center to center spacing of each LED, so that the cones effectively overlap, but it may still be very apparent as our corals grow towards the top of the tank. Also, our rock formations and corals cast shadows, which may be distractingly blue or white under secondary optics.
We expect the market for LED’s to continue to grow and other advanced lighting and we will continue to bring you more quality information on the advances in this promising technology.