A lot of scientific and industrial equipment uses small video and digital cameras to look at things so a computer can analyze or measure something. These are called machine vision cameras, and they are sometimes nothing more than good quality cameras without viewfinders and other human conveniences. These cameras are commonly used in telecines as well, but they are expensive – the cheapest ones start at around $300 and they go up quickly into the $1000’s.
Webcams are the poor man’s machine vision cameras. They are small and simple, yet include a computer interface (usually USB) and have lots of software support. It’s fair to say that the major challenge of this project has been to squeeze the most out of a very cheap camera!
I started with a Logitech Quickcam Communicate Deluxe. It has a 1.3 MP CMOS imager that provides very nice images when used as a live video or snapshot camera (e.g. when used as a webcam). But the CMOS imager is a little short on dynamic range for film scanning (see Dynamic Range article below). The CMOS image sensor is noisy, and the Logitech processing includes a denoising algorithm that mangles the detail in the shadows.
So I did some more searching and found the Philips SPC900NC webcam. It’s a discontinued model, but it has a Sony CCD in it, and is very popular with amateur astronomers so there is a lot of info on the web about modifying it. This camera produced a noticeably better image than the Logitech. There is still noise in the shadows, but it’s very random and the image doesn’t display the artifacts of aggressive noise processing. The camera has a bit more dynamic range (probably actually 1 bit more), and provides a gamma adjustment that is very helpful for telecine. These cameras are still sometimes available on Amazon and Ebay – mine was only $30. The earlier Philips ToUCam Pro 740, 750 and 840 used the same CCD and processor chip and are a suitable substitute.
To get a significant improvement in image quality would require a higher res machine camera which would cost about $600, so the cost of the machine would quadruple, but it might be worth it. There is room in the lamphouse for a little bigger camera, so it’s a possibility.
Here are samples from the two webcams I tried. The top image is from the Logitech, captured at its full 1280×960 resolution. The bottom image is from the Philips, also captured at its full resolution of 640×480. Click on the images to see them full size. The higher pixel resolution of the Logitech does not make up for its faults.
Here I adjusted the levels so you can see the shadow detail. Again, the Logitech sample is on top. It’s easy to see here that the lower resolution Sony CCD actually preserves more detail than the higher res CMOS imager. The CCD noise is random and resembles the grain of the film, so it looks better than the noise cleaning artifacts of the CMOS imager. Of course, the best of both worlds would be a 1280×960 CCD!
Like most webcams, the Philips offers a range of frame rates, from 5fps to 60fps. But these rates are not all created equal. The camera has only a USB1.1 interface, not the much faster USB2, so the data rate is pretty limited for streaming video. Above 5fps, the frames are compressed with increasing harm as the rate is increased. At 10fps and 15fps, the image shows severe 4×4 blocking, apparently from subsamping the color channels. At 20fps and higher, it looks like the camera only transmits a 320×240 image and the Windows driver upsamples to deliver 640×480 resolution, but the image is naturally very soft. So I am limited to 5fps to get good telecine quality.
Fortunately, with frame by frame transfer, the speed doesn’t matter. The playback speed is determined by the frame rate set in the AVI file, not the rate at which the capture is done, so the only penalty for slow running is that it takes longer to do the transfer. At 5 fps, a 400 foot reel takes about an hour and 45 minutes. That’s not too long to sit an watch, and of course, the scan is only done once per movie. But this is another reason why a machine vision camera is superior. Using a Firewire interface camera, it’s possible to run the movie faster than the normal 18fps viewing speed and still get high quality frame by frame transfers.