Learning Optics with Austin Archives - Shanghai Optics

Lesson 20 – Fisheye, Telecentric, and SWIR Lenses

Shanghai Optics — Mon, 24 Oct 2022 18:23:25 +0000

In this final post on types of lenses, we will be investigating fisheye, telecentric, and short-wave infrared (SWIR) lenses. The field of view for lenses is normally less than 60 degrees. The field of view for fisheye lenses, however, can go up to 175 degrees, which is very close to 180 degrees. Fisheye lenses are modeled after the actual eyes of a fish, and hence also where they get their name. If one looks at the eyes of the fish, they tend to bulge out, which is related to why the outermost fisheye lens is huge. Such a wide field of view is useful in security cameras where seeing the entire environment is needed. Fish probably have eyes with a wide field of view, so they are not eaten!

Telecentric lenses, contrary to the name, are not the lenses used in telescopes (in the “Focal Length and Applications of Lenses” post, we talked about how telescope lenses are composed of multiple different lenses). They are also not used in taking pictures. Rather, their application is in areas where the image size remains the same regardless of distance. Thus, the length of the pencil viewed through a telecentric lens will remain the same as the length of pencil in reality.

SWIR lenses have a cool application in being used in heat-detection cameras. This application results from the special property of infrared glass. Since regular lens material have a low transmission for infrared light (see “How Does Light Travel? Part I” on transmission), infrared glass is used to increase the transmission. SWIR lenses are used for light ranging from 0.9 to 1.7 um. As heat is a form of infrared light, SWIR lenses are used in appliances detecting heat.

As this is the 20th and final post, I want to thank all my readers for reading my blog posts. I also want to thank Glenn Brunelli, our Marketing Director for working with me to get the blog ready for publication every two weeks – it’s been a great experience! While this is where my lessons in optics come to an end, these lessons barely scratch the surface of optics (ha – get it?). While I hope this blog serves as a good introduction to optics, the more you get into the field, the more you realize there is a lot to learn! The Deep Dive into Optics blog will continue on with more advanced topics. The field of optics has so many applications in our lives, yet we may never realize it until we look at the image displayed from a fuzzy projector. Such is the behind-the-scenes importance of optics. May your study in light continue!

About Austin

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Lesson 19 – Projection and Zoom Lenses

Shanghai Optics — Tue, 11 Oct 2022 14:53:42 +0000

Projection lenses are another common application of optics. They differ from microscope lenses in that microscope lenses magnify what the human eye cannot see. Rather than zooming into an image, projection lenses enlarge the sample and PROJECT it. Just as with microscope objectives, there are certain design factors that must be taken into consideration when creating a projection lens. One of these factors is the conjugate distance which is the length between the sample and the image (calculated by sample – image distance). The projector at an IMAX movie theater would have a very different conjugate distance than a classroom projector!

Another factor to consider in designing is magnification. The magnification for a projection lens is calculated by dividing the image size by the sample size. Thus, the larger the image is “blown up”, the greater the magnification. The strength of the light source is also positively correlated with magnification- the higher the magnification, the more light is needed. And just like microscope objectives, the working distance is also taken into consideration when designing projection lenses. Focal length, field of view, and relative aperture are also key characteristics of projection lens design.

Zoom lenses have a unique characteristic where the focal length of the lens is adjustable in a range. Changing the focal length changes the field of view of the system. This is a useful property in capturing scenes at different distances and different ranges.

Next: TBD

About Austin

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Lesson 18 – Microscope Objectives

Shanghai Optics — Wed, 28 Sep 2022 14:21:24 +0000

Microscope objective lenses are a classic example of optics in our lives. The function of the microscope is to enlarge objects our eyes cannot see. Unlike telescopes which enlarge far away objects, the sample observed by the microscope is close to the lens. Microscopes also correct aberration, which otherwise would lead to blurry images. Achromatic (doublet) lenses only correct for aberration of two wavelengths of light whereas apochromatic (triplet) lenses correct for 3 or more wavelengths.

One of the factors that go into designing an objective lens is the magnification. The colored bands on the outside of the microscope indicate the magnification of the lens. The standard magnification bands are as follows: red band = 5x, yellow = 10x, green = 20x, blue = 40-60x, white = 100x. Thus, if a lens has a green and yellow band, the magnification would be 30x.

Another factor to consider in designing an objective lens is the working distance (WD). This is defined to be the distance from the front of the objective to the sample when in sharp focus. Working distance is related to the numerical aperture (NA) which is calculated by the formula NA = n * sin(θ_a), where n is the index of refraction. When in air, n = 1. To obtain a greater refractive index and increase the numerical aperture, sometimes the objective is immersed in a liquid such as oil or water. While aberration, magnification, working distance, and numerical aperture are not the only variables to consider when designing a microscope, they are key characteristics that one should look for.

Next: TBD

About Austin

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Lesson 17 – Quantitative Descriptions of a Lens

Shanghai Optics — Mon, 29 Aug 2022 16:48:08 +0000

In theory, the entire lens would be up to the qualifications set by the customer. After all, if a customer ordered a 150mm lens, the whole 150mm lens should be up to specification… right? Wrong. This is because in the real world, nothing can be manufactured precisely as specified. While on paper the entire lens works, in reality, the edges of the lens are not to specifications. To account for this error, clear aperture is the area of the lens that is up to specification. Usually, it is written as a percentage of the lens (such as > 95%) or an amount such as 145mm.

Surface quality quantifies the irregularities of the lens surface. We do not want the lens surface to look like the surface of the moon with various spots and streaks and craters. To quantify the smoothness of the lens, scratch measures the size of streaks across a lens. The thicker the cut, the higher the scratch quantity. Similarly, the dig measures the size of spots or bubbles in the lens. The greater the crater or bubble, the higher the dig quantity. Normally, the scratch is greater than the dig.

Surface flatness measures how close the lens surface is to its specifications. The ideal measurement would be 0 which means the manufactured lens is exactly to specifications as the lens on paper. As with clear aperture however, nothing is perfect and thus the product will deviate from the plan. Surface flatness is measured at a certain wavelength in waves (λ) with values closer to 0 being the best and larger values having greater deviations. For example, λ/4 at 400nm would be better than λ/2 at 400nm.

Next: TBD

About Austin

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Lesson 16 – Optical Systems II: Spherical Lenses

Shanghai Optics — Fri, 12 Aug 2022 19:46:40 +0000

We briefly discussed spherical and aspherical lenses in “Intro to Optical Components”. Spherical lenses can further be broken down into different categories. Plano convex lenses are flat on one side while spherical on the other. Biconvex lenses are spherical on both sides. Similarly, for concave lenses, plano concave lenses are flat on one side and concave on the other. Biconcave lenses are… you guessed it- concave on both sides. Finally, there are meniscus lenses which do not have a flat surface. This is a more in-depth discussion of convex and concave lenses that began in “Intro to Optical Components.”

biconvex, plano convex and meniscus convex, respectively

Shape is but one way of describing spherical lenses. Quantitative descriptions of lenses include radius, diameter, thickness, focal length, surface flatness, clear aperture, and surface quality. While one might think that the lens diameter is merely double the lens radius, this is not the case for lenses. The radius is the distance from the surface of the lens to the center of curvature. The diameter on the other hand is the width of the lens from top to bottom. Thickness has two main measures: thickness of center (TC) and thickness of edge (TE). The radius, diameter, thickness, and focal length are given an error bound (+/-) of a certain amount to indicate the tolerance of the lens.

Radius vs. Diameter of Lens (R=radius, D=diameter)

For example, a customer wants a lens with a diameter of 150 mm. While the requirements are clear on paper, it is impossible to craft a lens to the exact specifications due to manufacturing limitations and accuracy. Thus, there might be a tolerance of +/- 0.05 mm so that the final product has a diameter in the range of 149.05 mm and 150.05 mm. Of course, a lens with a diameter tolerance of +/- 0.01 mm would be more expensive than a lens with a diameter tolerance of +/- 0.05 mm as more accuracy is needed and thus the difficulty of manufacturing the lens. In the next post, we will discuss the remaining 3 quantitative descriptions: surface flatness, clear aperture, and surface quality.

Next: TBD

About Austin

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Lesson 15 – Optical Systems I

Shanghai Optics — Mon, 01 Aug 2022 18:19:00 +0000

Lenses come in all different forms, not just through the process of manufacturing. In fact, our own eyes are an optical system. Four main “components” to our eye are: 1) muscle, 2) lens (crystalline), 3) Pupil, 4) Retina. Closing your eyes when there are bright lights is an act of the muscles while the pupil dilates (enlarges) when it is dark so more light can be captured. The lens in our eyes is a convex lens that focuses the light onto the retina.

Our eyes are not perfect systems. Cataracts form on the retina and clouds a person’s vision. Myopia, otherwise known as near-sightedness, is when light falls short of being focused on the retina and thus images far away appear blurred. To fix this, a concave lens is needed which is the type of lens for glasses worn by near-sighted people. Presbyopia is the opposite condition when light has not been focused before reaching the retina. This causes things nearby to appear blurred which becomes a problem for tasks such as reading. To remedy this, a convex lens is used to correct the image.

For all the problems associated with our eyes, cameras are modeled after them. The main parts of a camera are: lens, prism, and mirror. The lens of a camera are equivalent to the lens of our eyes while the mirror has no counterpart but shortens the length of the camera. Cameras mounted on systems such as an unmanned aerial vehicle (UAV) have additional considerations such as resolution, weight, and cost. While a lower cost is preferable, a better resolution and lighter weight usually correlates to a higher cost and thus must be taken into consideration when designing such systems.

Next: TBD

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Lesson 14 – How do Atoms Emit Light?

Shanghai Optics — Tue, 12 Jul 2022 20:44:59 +0000

Light sources such as the sun give off light, but why? The equation E = hf relates the wavelength of light to energy where E is energy, h is Planck’s constant, and f is the frequency. Since we know from “How to Describe Light” that f = c / λ, we can also rewrite the equation into E = hc / λ. Thus, as λ (aka the wavelength) increases, energy decreases. From this relationship, we observe that the wavelength of light is related to its energy levels.

Rewriting the Planck Relation

To understand how different energy levels relate to light sources, we must look at matter at the atomic level. In the planetary model of the atom, electrons orbit a nucleus in rings. Each ring can be thought of as an energy level where electrons closest to the nucleus are at the lowest energy level and electrons farthest away from the nucleus are at the highest energy level. Electrons further away from the nucleus are also more unstable.

Planetary Model of the Atom

Suppose that an atom has 2 rings and that there is a single electron on the outer ring. If the electron were to drop from the outer ring with an energy level of E₂ to the inner ring with an energy level of E₁, then the change of energy can be written as Δ E = E₂ – E₁. Since energy is neither created or destroyed (the law of conservation of energy), the loss of energy must go somewhere. In fact, the energy is released as a photon which is what gives off light.

Absorption and Emission

If the photon is represented by ΔE, then substituting it into E = hf and rearranging the terms results in f = (E₂ – E₁) / h. The significance of this equation is that it relates the color of the light being emitted to the energy of the photon! As ΔE gets larger (the change in energy levels), the frequency of light also increases which trends towards the UV end of the visible light spectrum. As ΔE gets smaller, the frequency of light decreases which trends towards the infrared end of the visible light spectrum.

Next: TBD

About Austin

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Lesson 13 – Diffraction Grating and Wave Plates

Shanghai Optics — Tue, 21 Jun 2022 19:33:51 +0000

With the knowledge of interference and polarization, it is time to talk about the practical applications of both! Diffraction gratings are used to change the path of light and there are two types: transmission grating and reflection grating. A transmission grating utilizes diffraction to break light apart into different colors. A reflection grating has a jagged edge that utilizes reflection in changing the path of light. While the methods differ, both types of gratings are used in spectrometers measuring light waves.

Transmission (left) and Reflection (right) Gratings

Birefringence is a phenomenon where a material distorts light in a way that a single line appears to become two lines when viewed through the material. Calcite exhibits this phenomenon by splitting the light that passes through into ordinary light and extraordinary light. Wave plates are used to reverse this distortion so that a single line continues to appear as a single line.

Birefringence

Wave plates come in three types: quarter-wave plate, half-wave plate, and full-wave plate. Quarter-wave plates make circular polarized light into linear polarized light. Half-wave plates change the direction of linear polarized light. Full-wave plates usually have no effect on the polarization of light unless the wavelength of both the light and wave plate are different, in which case, linear polarized light becomes elliptical polarized light.

Quarter-Wave Plate

Next: Lesson 14 – TBA

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Lesson 12 – Polarization Revisited

Shanghai Optics — Mon, 06 Jun 2022 15:40:43 +0000

Now that we have explored light waves, we can dive more in depth into polarization. In how to describe light, we mentioned that sunlight is a form of unpolarized light or natural light. But what exactly causes light to become polarized? As light is an electromagnetic wave, it has both an electric field and magnetic field. The polarization state is defined by the electric field which can be linear, elliptical or circular.

“Linear Polarization

Linear polarized light travels in a wave perpendicular to the direction of propagation. This wave looks like a line when viewed from the perspective of the light coming at you. The light oscillates along this line in a wave. For circular polarized light, light travels… you guessed it- in a circular fashion. Depending on whether the light travels clockwise or counter-clockwise, we label the polarization as right-hand circular polarization or left-hand circular polarization respectively. For elliptical polarization, the same naming convention goes except for light traveling in an elliptical, rather than circular fashion.

Circular Polarization

A simple example again goes back to the 3D glasses in movies. If you pop out one of the lenses and hold it to a computer screen, you will see that turning the lens 90 degrees will either make the lens opaque or transparent to the computer light. This is because unlike sunlight, the light from a computer is polarized. Hence, holding a polarized lens at different angles changes the polarized states that are able to pass through.

Elliptical Polarization

If natural light hits a surface and reflects, it becomes linearly polarized light. Natural light that is refracted becomes partially polarized light. Assuming that the intensity of the original unpolarized light is I₀, the linearly polarized output would have an intensity of 50% of the original, or I₁ = 0.5I₀. If the linearly polarized light passes through a polarizer again, the intensity of the output (I₂) is equivalent to I₁ cos²(α).

Next: Lesson 13 – Diffraction Grating and Wave Plates

About Austin

2 responses to “Lesson 12 – Polarization Revisited”

Salvatore J Macedonio says:
June 7, 2022 at 11:01 am
I enjoyed this very much Austin !
After 30 yrs in /with Microscopy, Polarized light was always a hard one.
Thanks for the detailed yet understandable content.
Austin Wang says:
June 16, 2022 at 7:59 pm
Thank you Salvatore! My post is just the tip of the iceberg- there is still so much I don’t understand or know when it comes to polarized light. The 3 states are just the basics.

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Lesson 11 – Fringes and Interferometers

Shanghai Optics — Fri, 13 May 2022 15:25:19 +0000

We left off in the interference post using water waves to represent the interference that occurs between light waves. A well known experiment having to do with interference of light is the double-slit experiment. In this experiment, light passes through two slits before reaching a screen. Rather than just one band of light, bands of light are separated by dark bands. These light and dark bands are called fringes.

Double Slit Experiment

Interference explains this phenomenon because light from the two slits results in both constructive and destructive interference. The bands of light indicate areas of constructive interference while the areas absent of light indicate destructive interference.

Constructive and Destructive Interference

A follow up question to fringes would be whether or not the width of fringes change due to wavelength. It turns out that the longer the wavelength (red), the wider the fringe and the shorter the wavelength (violet), the narrower the fringe. While the fringes from the double slit experiment are visible to the human eye, some fringes require special instruments to observe and measure. Interferometers use interference in measuring different types of scientific phenomenon. The LIGO interferometer is used to detect gravitational waves while the Michelson interferometer measures small movement.

Zygo Interferometer

The Zygo interferometer is used in precision optics to measure the surface of a lens. This practical application of interference tests the surface precision of the lens (discussed in lens requirement post). In reading a Zygo interferometer, PV = maximum – minimum which gives the range. The RMS in turn is the root mean square which indicates a smoother surface the smaller the value. Since differences on the optical scale cannot be spotted by the eye, interference allows precision optic companies to remain precise in their work.

Next: Lesson 12 – Polarization Revisited

About Austin

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