Science Buddies: "Ask an Expert"

I am doing a project on determining how "stealthy" a shape is. Instead of using a radar to detect if the object is there, I am using a much less expensive method using a lux meter. An object is set inside a cardboard box, covered in black paper on the inside. A flashlight is also put through the box so that it can illuminate the object. Right under the flashlight is the lux meter's sensor. The light is shone at the object, and depending on how much light is reflected back to the sensor, the lux meter shows a certain reading of the illuminance.

Why is it that the cylinder showed higher readings of lux than the rectangular prism (whose edge was facing the flashlight -not the face)? Why did the cylinder show a smaller reading than the W shape (a paper folded into fourths and placed with the longer side down and the edges facing the light)?

I also constructed 2 planes for the experiment made of white paper (like the other shapes). A B-2 spirit bomber and an F-117 Nighthawk. The B2 Spirit Bomber showed a lower reading of illuminance, meaning it was more stealthy. Why is this when the Nighthawk is supposed to be?

Although, when I coated the shapes and planes with black paint, the readings not only significantly reduced, but this time the F-117 Nighthawk proved to be more stealthy (lower lux reading than B2 Spirit Bomber). Why is that?

Hello Nmurali23,

I read the experiment and noticed that the experiment is highly sensitive to your placement of objects in the box. When you find a good location make sure you mark it and measure all objects from that position. See "Finding a Good Location for Testing Inside the Box". If you don't control this variable your data may be skewed.

Based on my review of the experiment I would expect the cylinder to be less stealthy (more luminance or lux) than the rectangular prism. So your data makes sense.

However regarding the results for the planes, yes, that seems off. But it is highly dependent on how you constructed these planes.

To get best results you should "test different model airplanes of the same scale size" like the experiment suggests. That is buy model planes from a hobby store not construct them yourself out off paper.

Good luck!

Construction of the box and the placement of the components in it for this experiment can cause some significant issues with the interpretation of the results.
If the reading for no object present is not less than 3 orders of magnitude less than with your least stealthy object: your light is too bright, your box is too small, your box is too reflective.

I'm assuming your light meter is an incident light meter and not a small angle spot meter (since you didn't mention having to look through it line it up and get a reading). If light reflects off your test object and then bounces off the sides of the box back to the light meter, the object may have a significantly lower radar signature than what your test box indicates. In the radar application, there aren't large reflective walls to redirect scattered reflections back to the wave origin. If there was something like a mountain, then the radar setup would have "ground clutter" and a plane wouldn't have to be very stealthy to hide in the clutter.