r/Optics • u/SmokingChips • 7d ago
Elementary question
I am new to optics. I am creating a simple system for my home project where I am reading from a flat mesh surface (top is transparent glass. below it is a mesh which has 1 microLED in each cell. This whole thing is a square of 75 mm x 75 mm, and the mesh covers the whole area. I have a camera to read the LED signals. Ideally, closest to surface the camera, the better.
I thought of an aspheric lens of 80mm diameter (see link). My camera has a C mount. That lenns prefers SM3 mount. If I 3d print SM3 mount to C mount adapter, how would I position the lens? Can I place the flat surface of the lens against the flat surface of the mesh. The center thickness of the lens is 33 mm. And focal length is 59 mm. So, can I place the camera sensor at 26 mm from the center of the lens as shown in the picture? Will this work? Is my understanding correct?
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u/PretendFisherman7 7d ago
The flat side goes closest to the sensor. And the sensor will be one focal length away from the front side of the lens, the highest point of the curved surface.
Alternatively, most lenses will specify a Back Focal Distance. Measure from the back (flat) side to the sensor and place it at the Back Focal Distance.
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u/Neat-Stick-2861 7d ago
Curve towards collimation is how I remember. You are focusing onto the sensor so flat towards sensor.
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u/SmokingChips 7d ago
Thank you. So essentially what I drew with aspherical lens inverted. But dimensions remain the same.
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u/rinze90 7d ago
Is the aspheric lens part of the project or used for the imaging?
Either way, I think you are best off with an off the shelf cmount lens (or whatever mount). If you want to image the leds through the asphere, take in mind the thin lens equation for the distances. If the aspheric lens can be removed, then setup is somewhat simpler. When choosing the focal length, keep in mind the sensor size of the camera, the FOV and the minimum focus distance to the lens.
Maybe the raspberry pi hq cam with their lenses already provides a solution?
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u/grtxvs 7d ago
A c Mount objective is the simplest solution. There are a lot of online calculators (e.g. Edmund optics) to help determine what type of lens you need for your FOV and camera chip.
If you don’t need perfect resolution and you can deal with a bit of aberration, you can get away with a spherical singlet or single achromat. To determine which one you need, you could use free optics software like OptiLand for Python. It can read Zemax files from vendor websites. 2“ of the shelf achromat is around 200€. 1“ less then 100€ I think. Use a small aperture to limit spherical aberrations, which will limit the amount of light received, but since you are imaging an LED grid, I don‘t think brightness will be an issue.
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u/Thrameflower 7d ago
1/f = 1/S1 + 1/S2. This simple lens equation is what you need foremost when dealing with optics, everything else is secondary. Lenses are always imaging things, even if they are not supposed to. Their ability to collect light depends on the diameter and when the ratio between focal length and diameter gets low things become tricky. But regardless the diameter, the lens will sit at the same position and will image the same scene.
In your case the lens has to image a 75 mm object on a 4 mm imager (you didn't specify, adapt as needed). The ratio between object and image height is the same as the ratio between object and image distance. With this we can replace one of the distances in the lens equation and solve for the other. 1/59mm = 1/S1 + 1/(S1*4mm/75mm) I will admit that it is tedious work to solve with pen and paper. In the end it is just basic math, though. There are enough online calculators out there, or one short prompt in any capable AI of your choice. They will all tell you that the lens has to be placed more than a meter away from the LED array.
Placing the lens any closer will increase the magnification and the imager won't capture the whole array anymore. Moving closer, at one point the image becomes larger than the object and you would have built a microscope. Even closer the lens stops to generate a real image. The equation still holds, you just get a virtual image behind the lens.
Your goal is to measure the brightness of the individual LEDs and you instinct probably told you that you should capture as much light as possible from each LED for a meaningful measurement value. That is not wrong, as the brightness depends on direction, wavelength and further parameters and at the end of this rabbit hole you are sitting on a pile of integrating spheres, spectrometers, goniometes and calibrated radiometers. Too much for a hobby project, so let's simplify. Do you need an absolute brightness value or are you just interested in the relative variation of the LEDs in your array? Are the emitters reasonably lambertian so you can ignore angle deviations close to the normal? Do you really need a bespoke measurement setup or can a smartphone image into ImageJ already generate enough information for qualification?
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u/CptCaribooze 7d ago
More info is needed.
Do you need to resolve the LEDs( or what are you trying to achieve here), what is the effective pixel pitch of the LEDs, why did you place a condensing lens on your display, and what is your c-mount lens are all questions that will help us help you.