I measured the scatter from the eLIGO beam dumps as best I could. The experiment setup is shown in the attached diagram. 

After familiarizing myself with the equipment in the morning I noticed three issues with the setup

1 - around the minimum scatter the back scatter from the beam dump is very susceptible to the incident angle (makes sense since the Si plate inside the beam dump at Brewster's angle when there is minimum scatter).

2 - The mirrored plug (Part 20 in D0900095) which is suppose to be used for alignment is not very effective. It moves around too much in its hole in the front face of the beam dump. Just by touching it I could make the reflected beam jump around by about 0.1 radians.

- I think to align these properly we'll have to partly assemble the dumps. If we leave off the front plate of the horn then we can measure the reflection off the Si. If we measure this with a power meter then alignment becomes a simple matter of rotating until this reflection is minimized.

3. - For this measurement the incident beam was a small (~ 1mm diameter) central beam with a small amount of spray of laser light beyond that central region. This spray was hitting the aluminium front face of the beam dump and was scattering back to the photodiode. This was clearly the limiting factor in the measurement. Most of this light was spread horizontally so I placed a couple of pieces of black glass on either side of the aperture, just blocking the edges a little. This reduce the background reading at the minimum scatter from 17.0uV to around 4.5uV with still a little bit of light hitting the top and bottom of beam dump face.

The incident power on the beam dump fluctuated a little but was in the range 20.5 to 22mW. The response of the PD is approximately 0.2 A/W and the transimpedance is 7.5E4 V/A.

The SR830 Sensitivity was set to 1x1 mV.

It was difficult to measure the actual angle of incidence. The dump pivoted about a point directly under the input aperture at the front. By measuring the displacement of a point on the back of the dump as I rotated it and knowing the distance between this point and the pivot point I was able to make a reasonably accurate measurement of a range of angles about the minimum.

The measured scatter (in V measured directly by the PD and as a fraction of the incident power) is shown in the attached plots.

I think I can do a better job cleaning up the incident beam - so these numbers only represent an upper limit on the scatter.

attachment 1: beam dump assembly

attachment 2: experimental layout

attachment 3: scatter measurement

attachment 4: BRDF - (scatter divided by the solid angle = 1.1 m steradians)

attachment 5: (slightly blurred )photo of dump - overhead view