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Methods of Quantifying Experimental Results for Experimenters


-Some Methods of Quantifying Experimental Results and
Estimating Cosmetic Implications for Home Experimenters-

Hi Guys,

Here are some notes I would like to share with other
DIY Follica experimenters.

Thought I should write this stuff up; maybe you can
use some of the methods I’ve been utilizing, and
perhaps there is use for a thread on this and we can
pool some ideas there.

Included below are methods for determining hair
diameter and density (under certain conditions) using
readily available items, and some derived and collected
methods of estimating cosmetic implications of known
densities and diameters.

Here goes…

Since I’ve been seeing at least some results from a
combination of things I’ve been trying, it occurred
to me to ask the questions:

What type of regrowth would be required to produce
cosmetically significant effects?

How can I measure/quantify the results I am seeing?

If I think I am seeing results from my experimental
treatments, can I estimate/predict the cosmetic
implications of my results?

Assuming results are repeatable, how many treatments
would be required to achieve the cosmetic results I
desire with the particular method I’ve been using?

After reading a lot about this (it appears these are
really important questions when performing HT’s, so
they’ve been explored extensively for that purpose),
some of the most important factors appear to be:

-Density of hair (most commonly measured in number
of hairs per square centimeter) Individual hair
densities vary depending on a number of factors
but typically are around 200 hairs per square
-Average diameter of hairs (this plays a big role
in how much hair is required to produce cosmetically
significant results, thinner hair shafts requiring
more hair, thicker hair shafts requiring less)
This generally varies on an individual basis also,
in the range from around 20 to 200 microns.
-contrast between hair color and scalp color
(lower contrast here means less density is required
for good cosmetic results, higher contrast means
more density is required)

The last factor is inherently qualitative; however,
surprisingly, recently available technologies appear
to allow us to get good estimates for the other two
factors (see below for a discussion of this).

I was first trying to figure out how to
mathematically estimate the cosmetic implications of
particular combinations of the first two factors.

From the literature, it’s generally stated that
hairloss is first noticed once hair density
decreases to about 50% of it’s original value.
Since this value is roughly around 200 hairs per
square centimeter, one might conclude that a
pretty good restoration target for the average
individual would be around 100 hairs per square

I explored some other methods for predicting
"good" density targets:

As an initial first model, I tried to figure
out how many hairs of a particular diameter
and length would be required to completely cover
a field of view, assuming they were all lying
flat and aligned side by side (this is an
optimistic model, but it seems like it should
allow a good estimation of a lower limit for the
requirements). The exact equation I came up with
for this simplistic model is:

required density in hairs per square centimeter =

10000 / (hair length in cm * hair diameter in microns)

This is a lower limit, because it assumes the
hairs are lying side by side, and they are flat
in the field. In reality, the hairs obviously
rise off the scalp, and overlap each other in a
three dimensional region. On the other hand,
absolutely complete coverage is not necessarily
needed for the illusion of density, but maybe this
simple model is a good starting point.

I’ve measured my hair diameter to be on average
about 50 microns (I outline later a for method
for measuring this yourself). So for example,
in this simple model if hair were 3 cm long,

10000 / ( 3 * 50 ) = 66 hairs per square centimeter

seems a bit low, but certainly a lower bound…

I then did some searching, and found this
fascinating way to estimate the cosmetic effect
producible given particular diameters and densities:

For those interested in this topic, Dr. James
Harris presents a fascinating system for
making estimates at this link:


Here, he describes a way of calculating a number,
which he calls the HVI (Hair Volume Index) which
he correlates with the subjective visual hair
density given an individual’s particular density
and average hair diameter.

Essentially, his equation is:


hair diameter in microns * hair density in hairs per square centimeter / 100

which he relates to the following subjective ranges

thin appearance: HVI < 20
moderate appearance: HVI 30 to 40
dense appearance: HVI > 60

For instance, given my current measurements of
density (around 50 hairs per square centimeter)
and diameter (around 50 microns), I would

HVI = 50 * 50 / 100 = 25

To boost this to the “middle moderate” range
by increasing density, I would need to increase
density to:

hair density in hairs per square centimeter =
35 * 100 / 50 = 70 hairs per square centimeter

For “dense appearance”, I would calculate:

hair density in hairs per square centimeter =
60 * 100 / 50 = 120 hairs per square centimeter

(this might also be useful for those
considering HT’s and trying to estimate how
many hairs or grafts would be required to produce
a particular result)

So, now equipped with some methods of
predicting what the cosmetic impact of a given
density and diameter would be, how can these
quantities be measured? Surprisingly, there
appear to be a few methods which can yield
considerable accuracy available to us.

For density measurements, I have been most
successful using a digital camera with macro
focus and little strip of plastic cut to 1 cm
in length.

I’ve had the hair cropped short so it’s easy
to identify individual hairs once a photo is
taken of a region of the scalp. The plastic
strip is placed on the scalp within the field
of view of the photograph as a scaling
fiducial. When the photograph is examined,
hairs can be counted within any square region
having the same width as the 1 cm wide plastic
strip. In other words, I can zoom way in
until the plastic strip fills the width of
the field of view, and then pan around in the
image to estimate haircounts at any location
in the image.

That’s the best I’ve been able to come up with
for home methods of estimating density.

For measuring hair diameter, you can actually
do this quite accurately at home using a
laser pointer! I have done this and it does
work. Be careful not to point the laser
directly into your eyes, of course.

I tape a sampled hair across the beam where
the laser emerges from the pointer, then
point the beam at a wall a few meters away.
The laser light diffracts around the sides of
the hair forming a diffraction pattern which
basically looks something like this:

— — — —*--- — — —

The pattern is the brightest in the center of
the beam. I could write a lot about the
science behind this, but if you are just
interested in using the method to measure
hair diameter, what you want to do is measure
the distance between the pointer and the wall.
Then take a piece of paper and place it on the
wall on one side of the pattern and mark
the location of the minimum intensity points
in the lines which extend out to the side (the
places where it’s the dimmest).

Then take the paper and measure and figure
out the average distance between those
minimum points.

Then, use this equation to determine the
hair diameter:

hair shaft diameter =

laser wavelength * distance between hair and image
/ distance between minimum intensities

If you are using a red laser pointer, your
laser wavelength is 650 nanometers or
650 x 10^-9 meters. For example, say you
measured 1.5 meters between your laser and
your wall, and you determined the average
distance between your minimum diffraction
pattern intensities to be 1.4 cm
(or 0.014 meters).

Then the diameter of the hair which produced
the pattern would be:

diameter of hair =

(650 x 10^-9) * (1.5) / (0.014)

= 0.0000694 meters

or approximately 70 microns.

Because of variation in diameter really
necessary to do this for a number of
hairs and average the results if you’d
like to get a good idea of the average

See the following links for in-depth
discussion of this method and the theory
behind it, some useful photos, etc. :




I am impressed. The laser technique for measuring the diameter of a hair is really beautiful. I have just tried it, and I have seen the pattern on the wall. Not measured it, but I can see that it works. I will save your formulas.


Excellent post! Thank you!