Optional Extra Credit Opportunity #3 - Camera Obscura

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Dec 6, 2023

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Optional Extra Credit Opportunity #3 – Camera Obscura and Security/Privacy, due July 29 at 6pm. Worth up to 6 points. Read the 3-page paper that appears below, watch the YouTube video that is mentioned in the paper, and answer these questions: 1. What is the main point of the paper? 2. In what sense is this paper novel? 3. What 2 things in the YouTube video impressed or surprised you?

Camera Obscura for Obscuring Cameras* Roger G. Johnston, Ph.D., CPP Right Brain Sekurity https://rbsekurity.com Introduction It's possible to form images without lenses. A full color image can be formed using only a pinhole. Pinhole imaging has been undertaken for hundreds of years by artists and solar eclipse observers using the camera obscura —literally, "dark room". This is a darkened room (or box) with a small hole in the wall. Objects outside the room in the bright light are imaged on the room's opposite interior wall. The images are upside down and reversed. One of the intriguing things about pinhole ( camera obscura ) images is that they have an infinite "depth of field". This means that all objects in the field of view—near or far—are in focus, unlike photographs taken using lenses with limited depth of field. When artists and hobbyists display their film pinhole images, people looking at the images are often startled or disoriented by what they see. They are used to imaging using the human eye, which has a relatively narrow depth of field. Pinhole photography is usually done using high-sensitive film because of the small amount of light that comes through the pinhole. The recent availability of off-the-shelf, low-cost, low-light video cameras, however, makes it easy to produce camera obscura color videos. I demonstrate this in a recent YouTube video .[1] The video shows color images made with a Spinel 2-megapixel, HD video camera with 0.001 lux sensitivity using only a pinhole, no lenses. The camera costs $48 retail and runs on 5 volts. What is shown is only the raw video with no image processing to improve brightness, contrast, graininess, or image quality. Presumably more expensive low-light cameras, and those to be developed in the future, will allow higher-quality images. [Note that vendors of video cameras often talk about their "pinhole" cameras but these are not the camera obscura . They are merely video cameras with a small diameter lens or lenses. The small diameter of the lens(es) does tend to allow a good depth of field, but not as good as the true camera obscura .] _____________ *This paper was not peer reviewed.

Security Implications There are 4 reasons why a true pinhole video camera might have security interest. Consider, for example, the surreptitious surveillance of a conference room or office using a covert video camera hidden in the wall. The first advantage of the true pinhole camera is that only a very small hole is needed in the wall for the camera to view the proceedings. My YouTube video demonstrates the use of a pinhole only 100 µm in diameter.[1] That small a hole in the wall would be hard to find. The second advantage of using a pinhole video camera for hidden surveillance is that everything in the field of view is automatically in focus, including objects very close to the camera. The third advantage of a pinhole video camera for covert surveillance is that one low-tech method often recommended [2-4] to find a hidden video camera in a room is to turn off the room lights. A flashlight is then used to probe the room, looking for retro-reflections off the camera lens or off the imaging sensor of the camera behind the lens. With a camera obscura , there is, however, no such lens reflection because there is no lens. Any reflection off the imaging sensor behind the pinhole is very difficult to discern because the pinhole prevents any significant amount of light reflecting off the interior imaging sensor from exiting the pinhole. The final advantage of a pinhole camera for covert surveillance is that systems to "blind" video cameras using lasers have been proposed as countermeasures.[4] The laser light is so bright that it swamps the desired image by saturating the photo sensor. This is a more problematic approach with a pinhole camera. While I was able to somewhat saturate the image of my pinhole video camera with a 5-mW green laser, this required almost perfect angular alignment with the axis of the camera, something that might be difficult to do reliably in practice, especially in a large room. It might also be possible for the bad guys to recover partial video images despite laser saturation by using image processing on their covert video images. When coherent laser light passes through a pinhole, it creates circular interference rings called "Airy's Disc".[5] The camera pixels at the diffraction ring minima are not saturated. Also, because laser light is monochromatic (one color), it is possible that a usable image could be obtained by keeping only the red, green, or blue pixels in the color video, depending on the laser's color. Conclusion Security professionals should be aware that covert video cameras do not require lenses, and can operate with a very small viewing aperture as small as 100 microns (or smaller). Moreover, it is important to recognize that conventional detection or countermeasure schemes that work for covert video cameras with lenses may not work for pinhole cameras ( cameras obscura ).

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References 1. RG Johnston, “Camera Obscura", YouTube video, https://youtu.be/u3mlQzaKAIo 2. E Harper, " The Secrets to Finding Hidden Cameras", April 10, 2019, https://www.techlicious.com/tip/the-secrets-to-finding-hidden- cameras/comments-/CP2/ 3. S Rosenblatt, "Suspect a hidden camera in your Airbnb or hotel?", May 21, 2019, https://the-parallax.com/2019/05/21/hidden-cameras-airbnb-spying/ 4. KN Truong, et al., "Preventing Camera Recording by Designing a Capture-Resistant Environment", https://www.cc.gatech.edu/fac/summetj/papers/truongk- ubicomp2005.pdf 5. Hyperphysics, “Circular Aperture Diffraction”, http://hyperphysics.phy- astr.gsu.edu/hbase/phyopt/cirapp2.html

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