FLIR - IR Window Transmittance - ir window
I made all of the distance measurements relative to the lens axis and to a spot on the lens that I guessed to be near the vertex of the red arrows shown in the diagram. You don’t really need to know this location, since these techniques will actually calculate the location.
Ghost imageryfree
The nearer wall distance setup is called ‘x’. The farther wall distance setup is called ‘X’. Similarly, the distance along the wall from the frame middle to the frame edge in the nearer wall setup is called ‘y’, while the farther distance setup is called ‘Y’.
Ghost images arise from reflections off of two surfaces in the primary optical path and can include a reflection from the detector itself. Light is then re-imaged back onto the image plane. If that light is near focus, then the resulting ghost image can be substantial. Ghost images can typically be analyzed by sequential ray tracing software since the surfaces involved are from the main imaging path.
Ghost imageryapp
You only need to take 6 different measurements to calculate everything! The trick is in setting up the targets to make those measurements accurate.
Quantumghostimaging
As shown above, I put some tape targets on the wall that point at the edges of the field of view and the exact center of the frame. I made sure that the distances on either side of the middle of the frame exactly matched. I also made sure that the lens was at exactly the same height as the middle wall target. Using high magnification via Live View or the viewfinder helps getting the targets more precise.
The mathematics used in the calculations involves both algebra and trigonometry. Any “scientific” calculator has the necessary features to do this math. You can always download a scientific calculator app onto your smartphone. Fear not the math.
Ghostpic girl
The diagrams above show the setup to align and measure your lens to get the information for calculating the focal length and field of view (FOV).
I happen to have a fun little laser that can make extremely accurate measurements over long distances. I used this device to get my targets placed within a millimeter, and to measure the camera-to-subject distance.
"Flare" is stray light that can arise from reflection or scattering off of non-optical path surfaces or simply by passing through surfaces which are not part of the designed optical path.
You can download the simulation files on SolvNetPlus (account required, please log in first). If you have any comments or questions, contact us at photonics_support@synopsys.com
Ghost imageryliterature
One reason you might like to measure this stuff, versus just reading the lens focal length stamped on the lens, is that manufacturers often lie about the true lens focal length. This is particularly true of long zooms, which might not be as long as claimed. It’s often the case that a lens focal length reduces drastically when focusing close (called focus breathing), and these techniques could attach real numbers to that.
Ghost imagerysoftware
Using these techniques, you could actually find out what an un-marked lens’ focal length is. You could also verify (or debunk) manufacturer claims about the real focal length of a lens.
As I often say, “garbage in, garbage out”. The more accurate measurements you make, the more accurate your results will be.
What isghostimage in Computer
The analysis and control of stray light, composed of ghost images and flare, is an important but complex task for the design of imaging systems. Ghost images arise from multiple reflections off of surfaces in the primary optical path. Ghost images that impinge on the image plane at or near a focus is of specific concern—flare can arise from light reflecting off of lens mounts, non-optical surfaces of the lenses (such as flats and edges), and as a reflection off of the detector itself re-imaged back onto the detector. Modeling light reflected off of the detector can be complicated by diffraction from the microstructure of the detector. In this application note, we will discuss various computational approaches to simulating stray light in an imaging lens. All computation was done with Synopsys software, specifically CODE V, RSoft Photonic Device Tools, and LightTools.
To get all of the necessary information, you’ll need to set up some targets on a wall (I used painter’s tape) and then take distance measurements at two different camera-to-wall distances.
My setup was on a tile floor, and I used the tile grout lines to make sure the lens axis was exactly perpendicular to the wall. You could also temporarily tape a little mirror against the wall and line up the camera until you see your reflection in the center of the camera’s field of view.
For both setups, the angular measurement from the lens axis to the frame edge is called ‘Angle’, measured in degrees. The FOV of the lens is twice the value of ‘Angle’.
Download "Ghost Image Reduction in Lens Systems with Nano-Structured AR-Coatings Using CODE V, RSoft and LightTools" PDF
The detector itself will reflect a portion of the incident light back into the lens. This reflected light can, in turn, be scattered or reflected back onto the detector, causing ghost images or flare. For solid-state detectors such as CMOS chips, the microstructure of the detector can lead to significant diffraction patterns in the reflected light which may have an effect on the returning stray light pattern.