High intensity LED lighting at MacCHESS

MacCHESS utilizes high power LEDs for illuminating mounted crystals for centering.

UPDATE 2010-01-07: Most of the lamp assemblies were replaced in 2009. The new assemblies include: The new LEDs use less current and the star is electrically isolated, so that the lamp assembly can be put together with thermal epoxy (Arctic Silver thermal adhesive). This is much easier than using bolts with mica washers. Like the Luxeon product, the Cree LEDs are available in colors (red, gree, blue, royal blue) although we are using only the white.

We control the brightness of the LEDs by supplying a voltage to the Velleman K8004 from a Parker CompuMotor motor controller.

Original article:

Our lighting setup is built around the Lumileds Luxeon V Portable Star (part number LXHL-LW6C, color white, Lambertian pattern). As seen in the above photo, the Luxeon star consists of the LED on a small circuit board with solder pads, mounted on a wafer of aluminum for heat dissipation. The star is attached to a heatsink (Aavid 500400B00000) using a TO-220 mounting kit with mica washer (Jameco #34139) and topped with an optical assembly (Fraen #FHS-HEB1-LL01-H, elliptical pattern). Power for the LED assembly is controlled by small unit built around a PWM (Pulse Width Modulation) control circuit (Velleman K8004), which is fed by a 9V, 1.2 A wall transformer.

The Luxeon V portable star provides up to 120 lumens and is rated at 1000 mA maximum, 700 mA suggested to extend product life and color balance. Since these devices provide more light than we need, we are running them at 500 mA to provide a further margin. In typical use, they are run at about half the available brightness.

The LED assembly is mounted near the collimator tower and below the crystal. It illuminates a piece of low density foam attached to the kapton tape which holds the beamstop, so that the crystal is backlit from the viewpoint of the video camera. The elliptical pattern of the optic is oriented vertically to best illuminate the foam strip regardless of beamstop position.

A wall-mounted display shows the view from the Firewire camera; the video and crystal-centering controls are also available outside the experimental hutch.

The Luxeon LEDs come in assorted colors. Here's a white one next to a red one which is so bright that it has overwhelmed the color balance on my cheap digital camera. The red unit is available on request at MacCHESS. Other colors would require advance notice for parts acquisition and assembly.

In the future, we hope to replace the control circuit with one designed for constant current rather than constant voltage, and to add computer control of the PWM dimming to enable remote control.

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