I have a circular fluorescent screen with 25 mm diameter. When the electron beam pass, it produces visible light.I want give beam image with CCD camera (1.2")(1032*779 pixel, size of pixel 4.65 micrometer)(6.4*4.8 mm2 chip dimension). how can i choose best lens? how can i determine working distance,focal length and field of view?


1 Answer 1


I will attempt to tackle this problem however, be aware that I often output gobbledygook. Nevertheless, this is my best shot. Your subject is 25mm in diameter. You’re imaging chip measures 4.8mm height by 6.4mm length. I would shoot for an image with a diameter of about 80% or the height of the imaging sensor = 4.8 X 0.8 = 3.8mm (rounded value. In other words the goal is an image of 25mm object that measures 3.8mm on the chip. This will be a magnification of 25 ÷ 3.8 = 6.58 (written as -6.58X). The value is negative because the resulting image is smaller than the original object.

Suppose we place the lens 200mm downstream from the object. We can construct a triangle drawing imaginary lines from the ends of the diameter to the center of the lens. Inside the camera, an image is projected on the imaging chip. Additionally we can trace out an imaginary image triangle. The base of this triangle is equal to our goal of 3.8mm. The height of this triangle can be calculated because the object triangle and the image triangle have identical angles and the height to base ratio is also identical. Thus 3.8 X 8 = 30. This math tells me that a 30mm focal length lens will accomplish this task.

Let me add, this ratio assumes a simple thin lens. The lens you choose will not be simple because it will be constructed using several lens elements. Nevertheless, this math will work; you will merely be forced to make small adjustments to the image distance.

OK – you mount a 30mm lens on your camera – the angle of view will be 9.1° vertical (math not shown).

You will need to check my work, I have been known to output gobbledygook. That being said, I think a 30mm lens will do this trick provided the camera’s focusing mechanism allows you place the lens so that its rear nodal is about 30mm from the sensor. Best of luck! Alan Marcus

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    – Eric S
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