1. Bare LEDs & existing lamp reflector
As discussed earlier, the beam angle of the existing CFL fixture and the LEDs are very similar. So, one available option is to use no secondary optics. This option provides the lowest cost and lowest optical loss for the system. Using fewer components and less labour makes the luminaire easier and cheaper to assemble.
The drawback is the multiple-source shadow effect, explained later. Also, if the light distribution of the LED is significantly different than the target luminaire’s distribution, then this option is not available.
2. LEDs with secondary optics & existing lamp reflector
Secondary optics are optical elements used in addition to the LED’s primary optic to shape the LED’s light output. The general types of secondary optics are reflecting (where light is reflected off a surface) or refracting (where light is bent through a refractive material, usually glass or plastic). Secondary optics are available either by buying a standard, off-the-shelf part or by designing a custom optic through ray-trace simulation with an optical source model.
By using a secondary optic per LED, the beam angle of each LED can be customised to provide the exact light output pattern necessary. For instance, the beam angle of each LED can be narrowed to make the luminaire optimized for spot lighting instead of general lighting.
There are several drawbacks to this approach. First, the luminaire will have higher cost because of additional components and more complicated assembly. Second, since the optics are attached to each LED, there may still be multiple-source shadowing. Finally, the secondary optics will reduce the optical system efficacy.
3. Bare LEDs, existing lamp reflector & diffuser
Instead of using one optic per LED, a diffuser can be used over the entire LED array to spread the light. The benefits of this approach are a wider beam angle than is possible with the bare LEDs and eliminating the multiple-source shadow effect.
As with Option 2, the drawbacks are higher cost and reduced optical system efficacy. This is also not an option if the light distribution must be narrower than the bare LED, since diffusers can only spread light, not collect it.
Illuminance distribution, the multiple-source shadow effect, and aesthetics will usually drive the decisions on the optical system. Option 2 is the only option if the light output must be narrower than the bare LED. If not, Option 1 is better in terms of cost, efficacy and brightness. However, both Options 1 and 2 will exhibit the multiple-source shadow effect.
Also, users looking up at Options 1 and 2 will notice each individual LED. Users of Option 3 will see only a diffuse, uniform light source.
Multiple-Source Shadow Effect
Multiple-source shadow effect is a phenomenon where an object placed between multiple light sources and a surface will create multiple shadows. Most people have seen multiple light bulbs mounted above a sink in a bathroom. If you have noticed multiple shadows of yourself on the wall behind you, then you have seen the multiple-source shadow effect.
LEDs placed close together create multiple shadows that are close together. The appearance of these close shadows may be undesirable in the target application. It is the designer’s job to determine how important the multiple shadow effect is for the target application and whether it is worth additional optical loss to add a diffuser to minimise this effect.
b = ( a x L2 ) / L1
a = LED spacing
b = Shadow spacing
L1 = Distance between LED & object
L2 = Distance between object & surface