Blocking Light Reflection
As hair loss progresses hair styling can become difficult to hide the light reflection off the scalp. One reason why many shave their hair to reduce the contrast between hair and scalp. This is a basic fundamental of a hair transplant procedure, but in reverse. Hair length is required to give the illusion of fullness and block the reflection of light off the scalp.
Hereditary hair loss (MPB) causes the hair shaft to shrink, become finer as well as lose colour or pigmentation, this process is known as miniaturisation. The effect is not immediate but as the miniaturisation increases more scalp becomes visible and this accentuates the reflection of light off the scalp showing clear signs that hair loss is progressing.
The longer the hair more shadow is created and more coverage, so there must be a minimum length of hair. This can be demonstrated with a blade of grass, and the shadow created.
More hair length more shadow, more coverage
It comes down to the law of physics, thick long hair will cover the scalp better than thin short hair. With thick, long hair the light does not reflect back as easily and less scalp is visible. As the goal of a hair transplant is to block the appearance of scalp there needs to be a compromise between graft density placed and the hair characteristics to achieve this.