Re: Quetions about optics of periscope

The question: "If the colour of the internal part of the periscope tube is black in colour, how does it affect the image? Why? If it is coated with a silvery surface, will we be able to see the image? How about a white surface on the internal part of the tube? How does the length of the tube of a periscope affect the image? Why?"

First, I will be discussing two broad kinds of periscope. The first will be a submarine periscope complete with prisms and lenses, while the second periscope will be a simple mirror-only one that can be made by anybody. There are other kinds of periscope, such as on a battle tank, but the two I will discuss cover all the bases, I think.

A really good discussion and some drawings of a submarine periscope can be seen at maritime.org. In that discussion, which you should read carefully, you will read that the length of a periscope tube is limited by some of the design parameters such as width of field and minimum exit aperture. There are some equations in that page which allow some calculations of the maximum length possible for given parameters.

The above discussion at maritime.org does not mention light scattered off the inner surface of the periscope tube. But modern optical instruments such as this submarine periscope incorporate what is known as baffles to limit unwanted scattered light. Baffles and flat black paint are commonly used in optical telescopes, such as in the discussion here and here, for example. At this page at meade.com the baffles are also called "field stops", inside the "primary baffle tube". This concept can also be applied to telescopic systems such as periscopes to limit stray light.

But what about a simple periscope such as the design at opticsforkids.com? Baffles can actually help in such a simple design! For instance, if a horizontal plate were fitted into the rectangular tube of this periscope, with a rectangular hole in it, a huge amount of stray light could be eliminated. Of course the field of view would be reduced, but I think a reasonable baffle could be introduced without serious reduction of the field. You can try it!

Flat black is the color of choice for the internal non-optical surfaces of optical systems. In well-designed systems with adequate baffling the color usually doesn't matter too much, and yet I would not design an optical system with white or silver surfaces! Such surfaces could be a serious problem for imaging systems that are used in high-contrast work, such as pointing to a scene close to a bright light source. You will have seem pictures of "flares" in pictures, caused by the sun's being just out of the field of view. Some of the flares are due to reflections from optical surfaces, even with antireflection coatings, but reflections from nonoptical surfaces can also contribute to reduced contrast. There is a good discussion of lens flare at cambridgeincolour and a discussion of common telescope problems at concreteairship.com.

Basically, scattered light that is not trapped by baffles can arrive at the exit pupil (or the image plane) in random places, adding light to places that shouldn't have any light, so dark areas can be lighter than they ought to be due to scattered light. This is the cause of the reduction of contrast.

John Link, MadSci Physicist