Which leaves the "
2) To aid homogenize the light power-wise.
Make power distribution of incident light more even.
Via light diffusion ,of course . " part for last ....
It has been proven that most plants benefit from diffused evenly distributed light ,both
power wise and spectrum wise ...
http://www.ncbi.nlm.nih.gov/pubmed/18028265
http://aob.oxfordjournals.org/content/early/2014/04/29/aob.mcu071
http://www.actahort.org/books/957/957_4.htm
Regarding the power -wise part ...
An attribute of a COB reflector should be the diffusion of light ...
http://en.wikipedia.org/wiki/Diffuse_reflection
With as less "cost" regarding light absorption ,possible .
Two ways-options open up here ...
White Paint Reflective Surface.
A flat (matte) /satin finished white layer is almost 100% diffusive regarding quality of reflection ..
But ,it presents ,some losses ,reflection -quantity -wise , due to light absorption from paint material.
Those losses can be as high as 20% ,in some cases ,
though at average range somewhere between 5-10 % .
Notes :
-Myself have measured temperatures reaching up to 90° ,10 cm in front of a CXA3070 LES (@2100 mA ).
Any possible paint used on DIY or commercial reflectors ,better be the "high temperature grade " kind .
It should be rated " 150° temperature " ,at least.
-Possible VOC outgassing from paint .
Aluminium Reflective Surface.
Shape software simulates the reflections ,as if the reflector has a reflective surface ,
resembling the one flat mirror-polished aluminium has ...
While the light losses from absorption is usually at the 2-5% range ,the diffusion is rather poor ...
A flat mirror polished aluminium surface is almost 100% specular reflective ..
Still,this type of surface seems to be the most favourable option of LED dedicated reflector manufacturers .
But ...
They seldom ,if not at all ,utilise flat mirror-polished aluminium /metalised poilymer surfaces ..
Either it is satin finished (brushed & anodised -transparent protective layer )
or it has some sort of "patterns" ,in order to diffuse the light .
(Honey-comb ,diamond,hammertone,etc .
More kinda like a "digital" version of the "analog" white paint diffusion....
By adding "texture" to flat mirror -finish .Multiple tiny mirrors ..)
More sophisticated solutions ,of that type , include more complex reflective / refractive surface designs , based on Fresnel phenomena ,where multiple "micro-scales" at the reflecting surface ,create a very even light distribution ,as light rays are "guided" with high precision ,power-wise .
(Utilising a design like that a reflector or lens can "disperse " concentrated light rays -say at the middle point of a LES ,for a lens -or on the contrary "concentrate " diffused light -at the edges of the same lens )
.A wide range of light power distribution patterns and shapes ,can be achieved with this way ,
while the reflector remains compact enough .
As a DIY option ,I think that is beyond reach ,to back-engineer ,copy,design and finally build .