The race for LED manufacturers to improve the industry headline metric of lumens/watt is hotting up in the drive to improve the energy efficiency of lighting systems. But as sales of LED lighting systems displace those of traditional incandescent light sources the lumens/watt figure does not tell the whole story.
Design LED argues that it is the system brightness/watt figure that product designers should be more concerned about.
To create a functional, high-quality product that end users will buy, the lighting system designer has a number of objectives.
Firstly, an acceptable level of brightness in the illuminated area has to be achieved while maintaining an acceptable level of lighting uniformity. Bright spots have to be avoided and the light generated by the LED has to be directed to where it is needed. In short, the lighting system needs to include energy efficient illumination in exactly the right place and with sufficient levels of brightness and uniformity.
These challenges alone are not insignificant and yet, at the same time the system designer faces further challenges to complement the inherent efficiency of LEDs with other aspects of lighting system design.
Inefficient design
To achieve the project objectives, too often the design choices sacrifice the LED efficiency gains through inefficient system design and compromises. For example, if a light source emits in an unwanted direction it is more often than not absorbed by the system rather than being redirected to where it is needed. In addition, diffusers are used to reduce hot-spots and create a uniform area of light.
Diffusers often sit over the direct point source of LED light and “damp down” the differences between the LED hot spots of light and darker areas. However, they can create efficiency losses in excess of 50% of the emitted light and when this is added to other absorption losses efficiency can be reduced by more the 70%. It does not matter how good the original LED lumens/watt figure was if the lighting system design and choice of other materials reduces the overall efficiency.
Taking a new look at the materials used in the system and how emitted light is managed can make significant improvements to the overall efficiency of the system.
Lightguide
A different approach to using LEDs as sources of light is to inject the LED light into a lightguide and allow the gradual and controlled escape of the light throughout the length of the lightguide. This avoids the hotspots and the need for thick, light absorbing diffusers.
Design LED couples light from side emitting LEDs into a thin lightguide structure. The light travels through the lightguide, which also contains a pattern of printed dots. It is the location and density of the printed dots that controls the location and uniformity of escaping light. The design of the dot pattern is key to the success of the lighting circuit and is arranged to compensate for the natural tendency of light to escape closer to the LEDs and ensures good illumination uniformity.
It is important to note the apparent sources of light are the multitude of small, closely spaced dots and not a comparatively small number of bright LEDs. This significantly reduces the need for thick, energy diffusers yet also gives uniformity levels of 70% plus – beyond this figure the human eye cannot detect any noticeable difference. In addition, use of rear reflectors prevent the system from absorbing any wayward light but collect and recycle it to further increase efficiency.
With careful lightguide design and choice of lightguide materials, the designer can as much as halve their system losses and reduce the power consumption of the lighting circuit considerably.
Printed lightguides offer further benefits in relation to shape, size and brightness depending on each application. They also benefit from being thin (typically 1mm) and can be formed into simple curves.
Lumens/watt is the figure of merit preferred by LED manufacturers and is extremely relevant when measuring the total optical power produced by the individual LEDs.
As humans, however, when viewing an illuminated product we respond to its brightness, measured in candelas (cd/m2) and lighting levels are scalable according to the number of LEDs used. Therefore, system designers need to take a system level view of their lighting efficiency to ensure that light generated is usable, practical and pleasing to the human eye regardless of the lumens/watt figures of the individual components.
Iain Kyle is business development manager at Design LED Products