1. Measuring QE with LEDs LED Carousel (See Slide 3) Focusing Tube Focuses LED light onto photocathode in vacuum chamber. Cap (See Slide 2) Thomas Nevins 07/09/2013

2. The Cap Large Outer ring covers vacuum window to block light. Two interchangeable inner rings. Focus tube is attached here. Seen above is the configuration for measuring the edges of the photocathode. Outer Cap Rotating Inner Disk Centered Inner Disk

3. The Carousel Top disk rotates to switch between 10 different LEDs. Lower disk is fixed and holds onto the focus tube. Lovely tape labeling to distinguish between different LEDs. For the moment we change which LED is on by manually connecting the wires. Top Disk Bottom Disk

4. Current Setup With no vacuum chamber to attach to yet, we are currently using the above, shorter tube. This only requires us to switch the photodiode at the left with a photocathode to get measurements which can give us the QE. Shorter focus tube Photodiode or Photocathode

5. Results Above is for a “standard” Hamamatsu photocathode. The blue line was measured from a precise monochrometer, and the red dots from LED data. Error is small and the few that are inaccurate are likely due to the wavelength spread and the QE drop-off. Next step is to figure out a wavelength dependent scale factor to account for this.