JunMultiLED
Introduction
Multispectral imaging has enabled identification and analysis of targets across sectors, from agriculture to defense. This project focuses on the practical engineering-related aspects of creating an illumination system for future use in a portable multispectral imager. Specifically, this work describes efforts to characterize and evaluate the non-uniformity of high-power LEDs for this illumination system, as well as efforts to mitigate the associated challenges from running such a system.
Background
Photographers use LED-based ring lights to control illumination of a target. These ring lights take the form of an annulus with an inner radius sufficiently large enough to accommodate a camera lens. One example of such a product is shown below:
The illumination system for the multispectral imager will have a similar form factor to the above figure in order to enhance the uniformity of the output light.
Experimental Setup
Power LED: iPixel LED’s 3W “warm white” (3000-3300nm) power LEDs packaged on an aluminum-backed PCB base were chosen for their availability and ease of use. According to the provided datasheet, each LED had an expected forward current of 750 mA with an absolute maximum of peak pulsed forward current of 3A. In addition, the worst-case forward voltage drop across the LED is 3.8V while the minimum voltage drop is 3.2V.
With a 5V power supply, the circuit’s ballast resistor needed to accommodate a 1.2V drop with 750mA of current. Using standard values and +/- 5% tolerances, a 1ohm power resistor was selected. The circuit schematic follows below.
Figure
Under these test conditions, resultant current from the test circuit was 1.1-1.3A with worst-case fluctuation of 20 mA. Furthermore, significant heat was emitted to the point that some adhesive used to secure the LED had begun to warp. Although these findings were expected, both the current fluctuation and emitted heat had to be controlled before further work could be done.
Mounting Frame: Duron hardboard was originally used as the mounting material for its ease to machine and versatility. Being composed of treated wood fibers however, Duron offered little to no dissipation of heat. To accommodate the mechanical requirement of an annulus-shaped frame while also improving its thermal dissipation, the frame’s material was changed to plate metal. To accommodate the range of radial distances needed for LED mounting, the frame was fashioned by securing two Lazy Susans together. For stability, four metal angle braces were added to ensure the frame would remain standing. A picture of the mechanical frame and the associated camera (described later) are shown below:
Figure
LED Driver: A DC-DC step-down converter, alternatively known as a buck converter, was used to limit and control the current supplied to the LED. Although many options were available, the AL8805 was ultimately selected. Its 6V to 36V input was within the 12V range of the lab’s power supply, eliminating the need to purchase additional test equipment. The control voltages for the IC were 2.6V to 5.5V, meaning most micro controllers could directly interface with the IC. Most importantly though, the AL8805 could tolerate up to 1A of continued switch current but could be set to a lower output current based on the feedback resistors interfaced to it. The IC was set to output approximately 350mA of current, lowering the total dissipated heat. The IC and supplementary passives were available as a PCB from Sparkfun; the board’s schematic and picture follow below:
Figure
Figure
Camera: The Point Grey Flea3 USB camera was used to capture images. Two separate configurations were used:
Configuration 1: 0.102ms shutter exposure interval 0.000dB gain 60.0000 fps
Configuration 2:
0.051ms shutter exposure interval
0.000dB gain
60.0000 fps
A second configuration was necessary due to four of the experimental conditions causing saturation of the image. To correct for this, the shutter exposure interval was reduced to 0.051ms. Although this change means there is no uniform set of camera settings, we are not aware of any work demonstrating that shutter exposure interval causes changes in uniformity of output light.