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 Sun May 26 21:47:07 2019, Kruthi, Update, Cameras, CCD Calibration Sat Jun 29 03:11:18 2019, Kruthi, Update, Cameras, CCD Calibration Sun Jul 14 00:24:29 2019, Kruthi, Update, Cameras, CCD Calibration
Message ID: 14708     Entry time: Sat Jun 29 03:11:18 2019     In reply to: 14639     Reply to this: 14757
 Author: Kruthi Type: Update Category: Cameras Subject: CCD Calibration

Finding the gain of the Photodiode: The three-position rotary switch of the photodiode being used (PDA520) wasn't working, so I determined its gain by making a comparative measurement between ophir power meter and the photodiode. The photodiode has a responsitivity of 0.34 A/W at 1064 nm (obtained from the responsitivity curve given in the spec sheet using a curve digitizing software). Using the following equation, I determined the gain setting, which turned out to be 20dB.

$\dpi{50} \large Transimpedance\ Gain (V/A) = \frac{Photodiode\ reading (V)}{Ophir\ reading (W) * Responsitivity (A/W)}$

Setup: Here a 1050nm (closest we have to 1064nm) LED is used as the light source instead of a laser to eliminate the effects caused by coherence of a laser source, which might affect our radiometric calibration. The LED is placed in a box with a hole of diameter 5mm (aperture angle = 40 degrees approx.). Suitable lenses are used to focus the light onto a white paper, which is fixed at an arbitrary angle and serves as a Lambertian scatterer. To make a comparative measurement between the photodiode (PDA520) and GigE, we need to account for their different sensor areas, 8.8mm (aperture diameter) and 3.7mm x 2.8 mm respectively . This can be done by either using an iris with a common aperture so that both the photodiode and GigE receive same amount of light , or by calculating the power incident on GigE using the ratio of sensor areas and power incident on the photodiode (here we are using the fact that power scattered by Lambertian scatterer per unit solid angle is constant).

Calibration of GigE 152 unit: I took around 50 images, starting with an exposure time of 2000 $\dpi{50} \LARGE \mu s$ in steps of 2000, using the exposure_variation.py code. But the code doesn't allow us to take images with an exposure time greater than 100 ms, so I took few more images at higher exposures manually. From each image I subtracted a dark image (not in the sense of usual CCD calibration, but just an image with same exposure time and no LED light). These dark images do the job of usual dark frame + bias frame and also account for stray lights. A plot of pixel sum vs exposure time is attached. From a linear fit for the unsaturated region, I obtained the slope and calculated the calibration factor.

Equations:      $\dpi{50} \LARGE Power (P)=\frac{Photodiode\ reading(V)}{Transimpedance\ gain (V/W) * Responsivity (A/W)}$                    $\dpi{50} \LARGE Calibration factor (CF) = \frac{P}{slope}$

Result: CF = 1.91x 10^-16 W-sec/counts  Update: I had used a wrong value for the area of photodiode. On using 61.36 mm^2 as the area, I got 2.04 x 10^-15 W-sec/counts.

I'll put the uncertainities soon. I'm also attaching the GigE spectral response curve for future reference.

 Attachment 1: calibration_setup.jpg  4.633 MB  Uploaded Sat Jun 29 04:13:33 2019
 Attachment 2: CCD_calibration_2.jpeg  4.858 MB  Uploaded Sat Jun 29 04:13:45 2019
 Attachment 3: GigE_spectral_response_curve.png  48 kB  Uploaded Thu Jul 4 04:47:18 2019
 Attachment 4: 152_calibration_plot.png  41 kB  Uploaded Sun Jul 7 15:37:45 2019
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