K2 Luxeon Star LED - Update
by Howard Webb (St. Louis, MO, USA)
In March I wrote an article about my experience trying a K2 Luxeon Star LED for illumination, following the design suggested by Mike Andre in his December article. While the LED provided great illumination, the PWM (Pulse Width Modulation) power controller was causing a strobe effect with my digital camera. Several people were kind enough to write me with various suggestions on how to fix this problem. This article is an update on my recent changes and experiments.
For visual observations. The PWM power controller works just fine. However, since brightness is controlled by rapidly pulsing the LED on and off, the sensitivity of my Canon digital camera was picking up the pulses, and trying to adjust accordingly. At low illumination levels, it might take a light reading from when the LED was on, but capture the picture between pulses (giving a blank, black image). My work-around was to turn the PWM to full brightness when taking pictures. This is a workable condition, but definitely not optimal.
The company from which I bought the Luxeon LED (Luxeon Star LEDs*) also sells a series of power drivers, and it was suggested that I try one of these. After contacting the company for advice about which one would work best for my needs, and verifying that the dimming would not change the light temperature, I purchased a "LuxDrive BuckPuck 1000mA DC LED Driver". This 1inch square block controls the power, and the wiring harness includes a potentiometer to control dimming. It can take an input voltage between 5 and 32 V DC, and will power multiple LEDs - almost overkill when I have one LED. The drive and wiring harness set me back about $25 (US), which is a bit more than the PWM, and twice the cost of the LED itself. However, for a total cost of under $40 (US), this is still a very reasonably priced microscope illuminator (and very little assembly required).
I enclosed the driver in a small electronics project box, adding an on/off switch as well as both an input and output jack. Since I already had a plug on the LED, the output jack made it easy to switch power controllers. The input jack gives me the option of supplying power via batteries, or plugging in an old transformer. The transformer allows me to save batteries when I am home (however batteries seem to last forever with the LED), or with the batteries I can take the microscope anywhere without worrying about finding an electrical outlet.
Initially the puck didn't seem to improve things, or at least created a different set of problems. I tried the puck at 6V DC, and then switched to 4.5V (3 AA batteries, with which the PWM worked best). At the lower illumination levels there seems to be a bit of flicker, instead of a consistent, steady brightness; and the camera display seemed to drift across the whole range of illumination. I also found out that too low a voltage (4.5V) can damage the puck over time.
After some emails with the company, and more experimenting, I
did something that seemed counterintuitive (for me), and upped
the puck's input voltage to 9V DC. This seemed to stablize the
circuit, eliminate the flicker and give a steady image.
The brightness and steady color temperature have made me a real convert to LED illumination; and I would not consider going back to the original incandescent bulb illuminator which came with my scope. With an LED, there are none of the heat problems of incancescent (or halogen) bulbs, nor the problem of color temperature shifts when dimming the illumination. Short of a new microscope, I am not sure anything else could have made as big of an improvement to my system as this change to illumination.
I am really enjoying the new capabilities which the K2 has enabled (I have already started two more articles, based on the results so far). If you have an older scope with an open architecture that allows you to easily hack the illuminator, this is definitely something to try.
If you have a newer scope with a more integrated illuminator, the K2 may be too big for your space. However, there may still be some LED options. There is a new Luxeon Rebel LED with similar illumination and power characteristics, but at a fraction of the size (3mm x 4.5mm) which would probably fit within the space of about any existing bulb. If you can fit in the wiring (and possibly a heat sink), this might be an alternative to consider.
Microscope: Bausch & Lomb monocular, 10x ocular,
4x, 10x and 40x objectives.
Illumination: Luxeon K2 LED
Camera: Canon A540 (6 megapixel)
Software: Photoshop Elements, VirtualDub.
*I have no association with this company, other than being a satisfied customer. The Luxeon LEDs are manufactured by a division of Philips.
Comments to the author Howard Webb are welcomed.
Published in the June 2007 edition of Micscape Magazine.
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