"but i have to say that i am still confused; because if it is true that the normal density is more than 100 FU per sq/cm i still do not understand how you can have a good coverage with just 40/50 FU"
An average person will not have 100+ in there donor area. 115 FU/cm2 is a normal density in the CROWN. This area of the scalp contains a spiral pattern (the whorl or swirl) of hair growth. Hairs have multiple directions in this area, more so than the frontal area or back of the head. This is not advantageous for coverage! I believe that nature or God has made up for this deficit by placing more hair in this area. So, again, 100+ is not a representative number for the head. 80 probably is.
Concerning coverage and densityâ€¦.
Let's say you have 80 tall trees growing in a small area of land behind your house. Your tree density is 80 trees/area of your property.
This means if you step outside you may well bump into a tree trunk with every step! For a bird flying overhead, the ground is imperceptible. All the bird sees is branches and leaves and tree top coverage. If someone cuts down ALL the trees, the "bald" ground will be apparent to you and this bird. If someone chops down 40 trees and leave 40 trees growing, you will notice from the ground. What about the bird? Well, if the tree branches still criss-cross and the leaves still provide shade, the bird may find the ground imperceptible as if 80 trees were still standing. So losses and gains in density are not always apparent to the eyes. Such is the case with hair on the head.
FU's are like trees. Some have branchesâ€¦2,3, or 4 hairs per FU. If the hairs have a curl or wave, the braches can criss-cross and give nice coverage. Density is related to coverage but they are different concepts. Density is a relationship between a quantity of something (FUs) and area. Coverage is more of a quality.
If you pluck a hair out of your donor area, you will lose density. You will not lose coverage. If you pluck another hair within the same cm2 area, again you lose density but not coverage. If you puck ALL of the hairs, density AND coverage is lost. So, somewhere between 1% and 99% of the original density will provide coverage. Realistically
, it is about 50%, more or less.
So, if you have good donor hair with a donor density of 80-100 fu/cm2, a density 40-50 fu/cm2 might provide good coverage in the recipient area.