An underwater, multispectral light source: Difference between revisions

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== Background ==
== Background ==
'''Circuit Design'''
Our system needs to drive 7 high brightness LEDs. We selected 6 Philips LUMILEDS series LEDs to provide 6 of the 7 wavelengths we need. The wavelengths of these LEDs have been chosen such that they are evenly spread out in the optical spectrum. We also added a UV (395nm) LED that would be used primarily for measuring fluorescence in planktons/detecting fluorescent behavior in underwater organisms. The high current requirement for these LEDs(~700mA) means that we need to use a driver circuit to drive these LEDs as micro controllers cannot sink/source more than 25mA of current. Since the brightness of these LEDs should be adjustable, we need to have some form of LED dimming capability incorporated in our system.
**** Digital section goes here
Our design uses an NMOS transistor to sink about 0.7A though a high brightness LED. By controlling the gate voltage of an NMOS device, we can control the current flowing through the LED and hence the brightness of the LED. We chose the ZVN4306A FET from Diodes Incorporated as our high current FETs. From hspice simulations, we found that, varying the gate voltage from ~1.7V to 3.3V led to a current sweep of 0.15A t 0.70A through the LEDs. This gave us ballpark estimates of the gate voltages we should be using. Since the Atmega168 cannot generate analog signals, we fed the output of the PWM pins to the gate via a resistor. This leads to low-passing the PWM signals; effectively generating an analog voltage at the gate of the FET. This is shown in the following diagrams:
[[File:lowpass.png]][[File:PWMtoanalog.png]]
Once we were satisfied with the design and had tested the system on a breadboard, we designed a 2 layer PCB for the controller in PCBExpress. Snapshots of the PCB are shown below:
[[File:pcb_gopro.png]][[File:pcb_fab1.jpg]][[File:pcb_fab2.jpg]]


'''Optical Design'''
'''Optical Design'''

Revision as of 08:02, 18 March 2015

Group members: Bhrugurajsinh Pradyumansinh Chudasama, Candice Murray, Anirban Chatterjee


Introduction

Background

Optical Design


Underwater light sources


Absorption of light in water

Water exhibits much higher absorption of photons than air at some wavelengths of light. This can be seen in the graph below. Increasing the depth of the water increases the absorption, which can be modeled by the absorption equation I(x)=I(xo)*exp(−α(x−xo)) where α is the absorption coefficient for water at the wavelength of interest and I(x) and I(xo) are the intensities at the final and starting locations, respectively.


Scattering in water'

Methods

LED Design

LED specifications from the manufacturer are shown in the table below. For full LED specifications, see [1].

LED Color Red-Orange PC Amber Lime Green Cyan Blue
Lumens @ 350mA 72 lm 78 lm 167 lm 102 lm 76 lm 41 lm
Lumens @ 700mA 134 lm 140 lm 313 lm 161 lm 122 lm 70 lm
Efficacy @ 350mA 98 Lm/W 73 Lm/W 174 lm/W 100 Lm/W 75 Lm/W 38 Lm/W
Efficacy @ 700mA 83 Lm/W 63 Lm/W 160 lm/W 68 Lm/W 51 Lm/W 29 Lm/W
Typical Wavelength 617 nm 591 nm 567 nm 530 nm 505 nm 470 nm
Wavelength Range 610 to 620 nm 588 to 592 nm 566 to 569 nm 520 to 540 nm 490 to 515 nm 460 to 485 nm
Beam Angle 125° 120° 125° 125° 125° 125°
Recommended Operating Current 700 mA 350 mA 700 mA 700 mA 700 mA 700 mA
Maximum Rated Drive Current 700 mA 700 mA 1000 mA 1000 mA 1000 mA 1000 mA
Typical Forward Voltage 2.1 Vf 3.05 Vf 2.75 Vf 2.9 Vf 2.9 Vf 2.95 Vf
Maximum Forward Voltage 2.8 Vf 3.51 Vf 3 Vf 3.51 Vf 3.51 Vf 3.51 Vf
Thermal Resistance 8 C°/W 10.4 C°/W 6.4 C°/W 10.4 C°/W 10.4 C°/W 10.4 C°/W
Max Recommended Junction Temp 135 °C 130 °C 150 °C 150 °C 150 °C 150 °C
Operating Temperature Range -40 to 120 °C -40 to 110 °C -40 to 135 °C -40 to 135 °C -40 to 135 °C -40 to 135 °C
Dimensions L x W x H 4.5 x 3 x 2 mm 10 x 10 x 3.7 mm 10 x 10 x 3.7 mm 10 x 10 x 3.7 mm 10 x 10 x 3.7 mm 10 x 10 x 3.7 mm

Results

Conclusions

References

1. H. Buiteveld and J. M. H. Hakvoort and M. Donze, "The optical properties of pure water," in SPIE Proceedings on Ocean Optics XII, edited by J. S. Jaffe, 2258, 174--183, (1994). [2]

2. K. S. Shifrin, Physical Optics of Ocean Water, American Institute of Physics, New York, (1988). [3]

3. "Optical Absorption of Water Compendium", [4]

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