Title: Optical Bandpass Filter at 550nm

1
Title Optical Bandpass Filter at
550nm Authors Affiliation

• Specifications
• Table 1Specifications of our project.
• Table 2Total cost of the unit.

Max Reflectance () 92
Bandwidth (nm) Measured at 50 reflectance 82
Thickness (nm,LiF) max 123.1
Thickness (nm,LiF) min 246.22
Thickness (nm,ZnS) max 71.24
Thickness (nm,ZnS) min 142.48
Max Thickness (nm) 194.34
Min Thickness (nm) 388.7
Max Fringe Reflectance () 28
Overview At the beginning of this project, our
team defined specific goals for our design to
meet. We wanted to maximize optical reflectance
at a wavelength of 550nm while minimizing
reflectance elsewhere. Also, we determined that
our design needed to have a delta that was less
than or equal to 150nm. Finally, we set out to
design a final product with a cost per unit of
8,000 because we decided that we would be given
a cap of 800,000 for funding. Our optical filter
design creates a reflectance of over 90 at a
wavelength of 550nm. At wavelengths other than
550nm, the maximum reflectance is only 28. Our
team reduced delta to approximately 82nm, which
was 68nm better than our goal at the beginning of
this project. We limited material use to four
stacks of alternating Zinc Sulfide and Lithium
Fluoride, which gave us a final cost of 4,612.40
per unit which is 3,387.60 less that what we
proposed.

• Results
• We measured the function of reflectance versus
  wavelength with 4 stacks (Graph 1) and read the
  reflectance at 550nm. We used that value to
  represent our maximum reflectance. To determine
  how many stacks we thought was necessary, we
  simulated the filter with 1 stack (Graph 2), 4
  stacks, and 10 stacks (Graph 3). From what we
  set our goals to, we determined that 4 stacks
  were the optimal amount. To find the thicknesses
  for the layers, we used equations stated in Table
  1.
• The main weakness of our design is a result of it
  using four different stacks. The evaporation of
  each layer must be done very precisely to ensure
  correct construction of the final product. As
  the number of layers increase, the probability of
  mistakes increases.
• The design could be improved given more time to
  research different materials and how they work.
  Our reflectance can be improved to about 100 and
  the delta could be minimized to around 50nm.

Evaporator Fee For 100 filters 6250
Per-layer cost 8 layers 375 100 300,000
Technician Fee applying layer 120/hr (.5 hrs 100 8 layers 1.5 hr) 48,180
Substrates 100 67.50 120/hr .5 hrs 6,810
Depreciation Cost 250/hr .5 hrs 100 8 layers 100,000
Total Cost 461,240
Cost per Filter 467,000 / 100 4612.40

• Background
• This project will allow the military to use a
  higher quality optical filter. It will be a
  great benefit to the soldiers who depend on the
  filters. Our design solves the problem of
  filters having filters with low reflectance and
  large deltas1.
• Our team has worked together to design sunglasses
  used by Russian astronauts in order to block out
  harmful UV radiation from the sun and other
  celestial bodies. Many similar filters are used
  for satellites and aeronautical equipment to
  enable long range communication3.
• Our team began this project by setting specific
  goals for our design. Next, we used a spectral
  response program in Matlab to plot index of
  refraction against wavelength for both Lithium
  Fluoride and Zinc Sulfide. Using a formula that
  related material thickness to the index of
  refraction and reflected wavelength, we obtained
  a starting point for our experimentation2.
  Finally we used the Matlab program to plot
  reflectance versus wavelength, and altered the
  layers thickness until we achieved maximum
  reflectance with a minimum delta.
• Our team evaluated our final design by looking at
  reflectance, cost, bandwidth, and reflectance not
  centered at 550nm. We used Matlab to measure the
  reflectance at 550nm, and measured bandwidth
  (delta) by finding the difference in wavelengths
  at 50 reflectance. We evaluated reflectance not
  centered at 550nm by finding the next highest
  reflectance peak in the spectrum of visible
  light. Finally, the cost was measured using the
  values and equations in Table 2. We measured
  these specifications because they were directly
  tied to our goals for this project. Our
  measurements show that we met the goals that we
  set at the beginning of this project.

References 1 C. Bunting. Case Study 1 of the
Actipad System. OSUs REAL LIFE Project.
Online. Available http//ecen3613.okstate.edu/R
esources/case_studies/CSOne.htm. 2 F. T. Ulaby,
Fundamentals of Applied Electromagnetics, 2001
Media ed. Upper Saddle River, New Jersey
Prentice Hall, 2001. 3 Barr Associates, Inc.
Innovator in Optical Filter Technology. Barr
Associates, INC. Online. Available
www.barrassociates.com/applications.php?appdefens
e