Infrared with a solar panel

[Stefano]

Andy, No, sorry, I have not done any tests with RG1000 2mm + S8612 1.75mm, which would be below OD4 at 1200nm.

I guess Ninjin tested that also, but I have not.

 

Anyway, trying to use a OD4+ filter in a region where the sensitivity of the sensor ends doesn't make a lot of sense to me. I guess that the filter from Thorlabs rated at OD 5 is a better alternative.

[Stefano]

I did a test with a small prism I have.

 

Zoomable LED torch zoomed in, with 2 books to produce a narrow beam.

 

 

Spectrum at ~1.5 m.

 

My prism. The sheet below is to tilt it slightly.

 

Unless I use something like lasers or discharge lamps, I have no way to tell the exact wavelengths.

[Andy Perrin]

You have a laser. Use the laser to calculate the refractive index (you can measure the prism angle and use Snell’s law). Then you should be able to work out everything else, assuming the prism doesn’t absorb IR or have large IR dispersion.

 

Honestly it might be easier to just get a different grating.

[Stefano]

More than a year later, and I have a much better camera. Now I don't need to stack multiple exposures, I can control the lens and the sensor is much more sensitive. There's no comparison. So I tried again, with the exact same solar panel, and this time the images look much better. I didn't try the "dark water" experiment, I will do that with LEDs, but I just imaged the solar panel glowing in IR. The solar panel was turned on with my bench power supply, limiting the current at 0.75 A, and the voltage drop at that current at the equilibrium temperature was about 7.6 V, so that's 5.7 W of power almost completely converted to heat. It is very important to limit the current, as in semiconductors the voltage drop/forward voltage tends to lower as the temperature increases. So what can happen is that the solar panel gets warmer, the resistance becomes lower, the current increases and this heats it up even more. This can end up burning it if you are not careful.

 

Camera: Full-spectrum Canon EOS M

Lens: Soligor 35 mm f/3.5

 

I was in total darkness, but I used a Hoya R72 filter for all shots just in case. Also, I didn't go past f/8 as diffraction at 1000+ nm is greater than usual. Since I discovered the veiling flare phenomenon, I avoid using the lens at f/3.5, but I set it at f/4 instead. Also, I noticed my images were a bit dark, so I raised all of them by 50% using Windows Photo editor. Since IR is monochromatic there, I just set the camera to Monochrome, so the images have no color in them.

 

f/8, ISO 100, 30 s exposure

 

f/8, ISO 100, 30 s exposure

 

f/4, ISO 100, 8 s exposure

 

The bubbles you see are there because I once heated it up too much and the plastic on top slightly melted.

 

This solar panel can be used as a very weak light source, and can illuminate objects. As a proof of concept, I tried with my hand:

 

f/8, ISO 3200, 300 s exposure

 

I had to stay still in that position for 5 minutes, and it can be a little tiring. There is a ton of noise, but I did get an image.

[colinbm]

Good on you Stefano.

I seems funny taking a photo with the light (IR) of a solar panel...

[Stefano]

I had to lay on the floor and watch the time on the camera in BULB mode... things you do when you are young I guess...

[colinbm]

I had to lay on the floor and watch the time on the camera in BULB mode... things you do when you are young I guess...

 

Don't worry, you do sillier things when you get older too.....

[Andy Perrin]

Yup, as we showed it definitely glows. Worst LED ever.

[dabateman]

Yup, as we showed it definitely glows. Worst LED ever.

 

That almost sounds like a challenge.

The 370nm flood lamp Led I bought with polycarbonate focus lenses was close to the worst I have ever bought. You could cook eggs on it until I removed the lens group.