Here is an estimate of the BRDF for ETMx and ETMy from the recent scattering images we took.
| BRDF | Ps (W) | Pinc(W) | |
| ETMy | 2.3e-02 | 8.4e-07 | 21 |
| ETMx | 2.5e-03 | 8.0e-08 | 19 |
BRDF=Ps/solidAngle/Pinc/cos(theta_s), in W/steradian , as quoted by Hunter Rew in P1400197. Ps is power incident on photodiode, solidAngle is that subtended by the photodiode, Pinc=incident power (here the IR power inside the arm), and theta_s is the scattering angle, which is the angle of the photodiode from normal incidence. SolidAngle-A/L where A is the area of the photo diode and L is its distance from the test mass.
Ps=calibration_factor*Intensity/Exposure, where calibration factor is in (W*microseconds/counts) and is defined by David Feldbaum in LLO alog 12627, intensity is the total pixel counts on the CCD, and exposure is the camera exposure in microseconds.
Pinc is calculated using Dan's calibration of the TMS photodiodes outlined in alog 15431.
A is as given by the camera specs = 1.0209e-05 m^2.
Rick Savage and Joe Gleason helped me get the distance and scattering angle of the camera from the mirror. L is 6.0452m and theta_s is 4 deg for the ETMS. .
The other paramters used are:
| UTC Measurement time | Exposure (microsec) | L ((m) | theta_s (deg) | |
| ETMy | 14-12-11-03-00-00 | 10000 | 6.0452 | 1 |
| ETMx | 14-12-12-02-00-00 | 500000 | 6.0452 | 1 |
Because these images are taken at small angles, its not valid to assume the BRDF is constant, so its not meaningful to integrate the BRDF to get an estimate on total scatter in ppm. I wanted to compare numbers to the ITMs, however I can't because the analogue gain setting was different (1023 not 100) for the ITM pictures. Once ETMy is clean and we have the Y-arm back I will re-measure ETMy and also add numbers for the ITMs.