Build a camera adapter for capturing analog oscilloscope screen-shots.
This blog serves a narrow but illustrious audience: the photographically inclined electronics hobbyist. Yes folks, I'm going for the masses here:) Kidding aside, if the line above describes you, or even if it doesn't but you find yourself having to illustrate an article here and there with an analog oscilloscope screenshot, then this article is for you. Of course, most digital scopes these days have USB ports to where you can easily export a screen shot in popular formats such as jpeg or bmp. But many electronics hobbyist (myself included) still have an old, trusty analog scope they use now and then. What do you do when you need a screenshot with your "old" analog scope? Photographing these screens without help can be tricky. The CRT's glass is reflective and often you capture not only the waveforms, but also an overhead lamp, or a self-portrait. While the latter might serve well the most narcissistic among you, it is not often the result most people look for. Don't know about you, but I don't think I look that good on a CRT screen. The solution is to cover the screen with a non-reflective "box" of sorts and adapt it to your camera of choice (in my case a Canon Rebel DSLR). I was considering building a "custom" box when it hit me that I could easily adapt the nice, plastic box for some of my favorite cookies:) And so the OscilloCam was born! (Note: i briefly entertained the even catchier OscilloScam name but later decided against it.
The cookies in questions are made by this slightly disturbingly named "Croley Foods" company (crawling food anyone?) as depicted in Figure 1. (They come in different flavours; the pineapple variety is very good). In California where I live, there are easily found in any Asian food market. I often re-use these boxes to store assorted components since the build quality is not too bad and has a nice lid that keeps the contents protected from dust and humidity. As it turned out, the box was about the perfect size for this project. All I had to do was paint it in black (more on that soon) and cut a circular hole in the bottom toattach the lens. The box even has a circular marking on the bottom that is pretty close to the diameter of my Canon 18-55mm EF-S lens so very little trimmiing was needed.
Figure 1 - Sunflower Crackers Box
I decided to paint this box in black to minimize reflections and clearly improve the aesthetics of the project. I happened to have a Rust-Oleum black high-heat paint (Amazon.com link) that worked fairly well. One advantage of this paint is that it is very flat and non-reflective which is obviously one of the objectives for the OscilloCam. On the other hand, it is a bit messy, leaving some residue even when fully dry. It is conceivable other paints work just as well or better but this is the one I had handy and worked fairly well.
Figure 2 - The Paint
Figure 3 shows the box already painted and with the hole.I used a dremel with a cutting bit to cut this hole and a file to trim it afterwords. make sure you make it just large enough to put the lens through it but so that it is not lose when the lens is inserted through it.
Figure 3 - Bottom View
Figure 4 shows the final arrangement. The lens fits snugly against the hole so there is no need for additional support. The plastic box is very light and doesn't exert much pressure on the lens mount. Got to say, it actually looks pretty cool on that camera:)
Figure 4 - The Final Setup.
As mentioned in the introduction, the problem we are trying to solve are the nasty reflections one can get when photographing a CRT without any aid. Figure 5 illustrates the problem. Note how my overhead fluorescent lamps ruin an otherwise beautiful sine-wave!
Figure 5 - The Problem
It's time to pull-out the OscilloCam. Figure 6, shows the contraption in action. Note the box is large enough to cover my Tektronix TAS 465 screen and I suspect most other scopes. I've also used it with the slightly larger TDS784D without any problems. I expect this size to be adequate for most older Oscilloscopes out there.
Figure 6 - Using the OscilloCam
Figures 7 and 8 show two images obtained with the OscilloCam setup. As you can se, no more reflections! Some notes on exposure settings and photo technique. Exposure when photographing a CRT can be tricky. I found it better to set the camera to "Manual Exposure" mode than one of the automatic exposure modes. An exposure in the range of 1/40s at f/4.5 worked well for most cases, though this is highly dependent on the screen's brightness for your particular scope. However, these values should be a good starting point. Figure 7 and 8 were shot with slightly different exposures and white balance thus the different renditions. With Figure 7, I covered the CRT completely with the box, whereas in Figure 8 I kept it a few inches away from the screen plane. I've found that you don't necessarily need to completely cover the CRT to block most of the reflections. In fact, leaving some space between the box and the screen allows you to use a longer focal length (say 35 mm instead of 18 mm) thus reducing the barrel distortion introduced by most lenses at their wide-angle setting. This distortion is particularly noticeable with oscilloscope shots because of the square grid. Though Photoshop can compensate for this in post-processing, it saves you time to get the photo with minimum distortion to start with, straight from the camera.
Figure 7 - Test Screenshot 1
Figure 8 - Test Screenshot 2
Though the examples above were shot using an oscilloscope, one could easily use this tool with any other CRT-based test equipment such as spectrum or network analyzer. I could also see uses when photographing other screens prone to reflections such as cell-phone screens or small tablets. And I'm sure readers will think of other uses. The OscilloCam became another commonly used tool in my lab, especially when gathering data for my blog posts. Go and buy yourself some crackers before "Croley Foods" raises their prices:)
The OscilloCam with Cover
Comments, questions, suggestions? You can reach me at: contact (at sign) paulorenato (dot) com