Photodiode System Response vs. Wavelength
 A graph of the system response vs. wavelength for J, H and K bands, together with a blackbody curve showing what was expected. |
Method for measurement:
In the dark, a blackbody source was put at a distance of 145 inches away from the photodiode system and set at a temperature of 130 degrees Centigrade. The bias on the photodiode system was set to -0.175 mV, which is a point on the flat part of the load curve. We used a folded piece of aluminum foil to 'chop' the blackbody signal and thereby obtain an alternating current across the photodiode. These peak-to-peak voltages were recorded, as shown below.The blackbody source was then moved to a distance of 46 inches from the detector. The system response in H band was re-measured for calibration, and the system response in the J band, which has very little sensitivity to a 130 degree C blackbody, was measured.
| Band | Voltage p-p | Distance ('') |
| J | 0.1 | 46 |
| H | 2.5 | 46 |
| H | 0.240 | 145 |
| K | 16.0 | 145 |
The two H-band points were used to calibrate the measurements at the two different distances to each other
Analysis:
A blackbody curve of 130 degrees C was plotted and the data was shifted to lie on the blackbody curve; no absolute calibration of the diode's responsivity to flux was done, so absolute calibration of the data was not possible. The J, H, and K data points were placed at wavelengths of 1.22, 1.63 and 2.19 microns respectively, the so-called effective wavelengths of the bands.Most noticeable to us was the almost exactly log (base 10) apparent response of the photodiode system with wavelength. And while the H and K points lie quite close to the slope of the blackbody, the J point does not lie on this slope.
One possible reason for J's uncooperative attitude is the presence of reflected light in the room. Though all lights were off at the time, the possibility of near-IR light from somewhere else in the room, say, a computer, caused the incoming flux to be ~20-30 times higher than the expected flux from the blackbody. We couldn't think why else this flux would be so high...
Back to Main Page