Cell Sensitivity, Bandpass & Grade
These are major factors that determine image brightness,
contrast, bandwidth and clarity of light transmitted out of the cell.
The first factor to consider is glass thickness. The applied magnetic field
has less effect on the active layer (Ferrofluid) as the distance from it increases.
You can see by the green line on the graph (above) how quickly the strength falls off a short distance away
from the magnet (inverse cube law). Thicker glass requires a more powerful magnetic field than thinner glass does to resolve an image.
A typical Neodymium magnet will exert much more field strength than can be obtained from a small coil around a core.
That's why an MRI machine is so big. To reach 2 or 3 Tesla, MRI coils have to be huge and they consume massive
amounts of current. Whereas a 2" x 2" Neo can have a surface strength of 2 Tesla simply and economically.
The only drawback is you can't turn a magnet off.
Thin cells are well suited for experiments using electromagnetic induction due to the reduced field strength required to activate one.
As the glass gets thicker, the amount of light passing through gets attenuated
by a greater amount. Various types of glass each have their own transmittance values.
There is a limit on how thin the glass can be for a Ferrocell. It's determined by its surface area.
A 100mm disk will flex if the thickness is 1mm or less. This will lead to light and dark patches
of fluid inside the cell after a few times of use. Glass that's too thin is also more likely to leak.
The thin edges make it difficult to evacuate and seal the cell properly.
A second factor is the type and quality of glass. Ordinary window glass works well,
however, its difficult to find this type of glass with an optical quality surface area.
An un-even surface results in dark and light patches of fluid inside the cell and
reduces the ability to mate both glass disks close together. As particle density increases
(thicker), light passing through the cell gets attenuated at an exponential rate.
For example, if the thickness changes from 10 microns to 20 microns, we will see 100 times less light
out of the cell. Along with this severe attenuation, the cell's bandpass response changes dramatically.
Violet, blue and some green will be lost if the thickness is excessive.
An extremely flat surface results in a cell with a broad bandwidth (red to UV) and high resolution.
The drawback is cost. A grade 1 glass disk with 1/10th wave surface accuracy will cost at least 10 times
more than a 1 wave for the same diameter. Grade 1 quality optical windows are normally made with a
surface accuracy within +/- 1 wavelength. Experimenter grade glass has a wider +/- 3 wavelength and falls into the
grade 2 class. This specification refers to the flatness of the glass face within 550nm
across the major portion of its area. We want a cell with maximum resolve and a highly detailed image
that falls within our eye's spectrum range. This is one reason I prefer a green LED or laser for experiments.
Green is within our peak sensitivity.
Cell grading is based on fluid thickness and transparency. A light grade cell is usually made from the higher
quality glasses. To create a cell with maximum transparency the disks must be as close as possible. The output of
a grade one 1/20th wave Ferrocell will look 99% transparent to the incoming light, except where the field potential shows.
This is an example of how clear a 1/20th light grade cell can display magnetic potential. Notice how transparent the cell looks. A 1.2T cylinder magnet is setting on the top face of Ferrocell. White LED 60mm below cell. White changes to blue (in a similar manner to our Earths sky) after passing through the cell and field. Its a property known as Rayleigh Scattering.
(note: blue LED's weren't available when this picture was taken (2006).
Grade 1 glass is available with many surface tolerances. 1 wavelength, 1/4, 1/10th, 1/20th are all within grade 1.
Most 1/4 wave Ferrocells are medium-light grade, but are classified as medium.
Grade 2 glass is usually used to create medium grade Ferrocells. In some instances, a light grade cell can be
made with grade 2 glass, as the spec's require the glass to fall between +3 & -3 wavelengths of flatness
across its surface area. Sometimes the occasional 'perfect' set of glass disks will mate together
to form a smooth, well distributed and thin layer. However, a normal medium grade cell appears translucent
orange-ish brown, filtering some of the green but most of the blue and violet out. This process was first noted by
Grade 3 glass is what I consider 'construction grade'. Windows in your home, garage or shed will fall into this category.
They are mostly flat across their surface, but they can also be a bit 'wavy'.