Alignment eye wear: This type of eye wear is used for low power visible laser beams. Alignment eye wear should not be worn during the operation of high power or invisible laser beams. Instead, safety eye wear that provides adequate protection should be worn.

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The potential for skin damage depends on the type of laser, power of the laser beam, and the duration of exposure. The type of damage may range from localized reddening to charring and deep incision.

The ability of eye wear to filter the laser beam is expressed in terms of optical density. Optical density, type of laser, and visibility required are all important factors in the selection of protective eye wear. Protective eye wear may not provide the same degree of protection for infrared as for visible light and ultraviolet laser beams. Goggles with side shields are preferred because they provide protection against back reflection and side entrance of stray laser beams.

In most robotic contexts, the primary use for imaging and visual-odometry is done in relatively short ranges with cameras that have short focal lengths, and the primary medium for light to travel through is air. Since there doesn't tend to be big atmospheric variances between objects that are close, and since light is all traveling through the same medium, there isn't much of an asymmetric refractive effect to characterize or measure. As a result, this kind of radial distortion isn't common when calibrating cameras for these kinds of applications. If we can't measure it, we shouldn't try to model it!

Consult appropriate standards such as CSA Standard Z94.3.1-16: Guideline for selection, use, and care of eye and face protectors or American National Standards Institute / International Safety Equipment Association (ANSI/ISEA) Standard Z87.1-2020 for guidance on selecting protective eye wear for your specific application.

The term "laser" is an acronym that stands for "Light Amplification by Stimulated Emission of Radiation". Laser light is a form of non-ionizing radiation. Laser equipment produces and amplifies light that has unique properties that cannot be produced any other way. The light that it produces is monochromatic - it is composed of one single colour at a specific wavelength. Laser radiation can be generated in different parts of the spectrum - ultraviolet (UV), visible light, and infrared (IR).

In the above figure, one can see how the lens being either angled with respect to the orthogonal axis of the image plane, or shifted, would project an image into a different spot on the plane. In most cameras used for robotics or automated vehicle applications, tangential distortion will usually be significant enough to model, but is often an order of magnitude smaller than e.g. symmetric radial distortion.

Every piece of laser equipment has built-in engineering controls such as protective housing, fail-safe interlocks, master switches, beam stops and attenuators (e.g., light absorbers) to prevent accidental exposure. However, eye protection is needed while using Class 3B or Class 4 type lasers to prevent harmful exposure from reflected and scattered laser beams.

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Neither of the above two scenarios are typical; as such, asymmetric radial distortion is an important aspect of modeling the calibration in applications when these scenarios are encountered.

There are two types of laser hazards: the laser beam hazards and the non-beam hazards. Laser beam hazards include eye and skin burns which are due to laser beam shining on a person's body. Non-beam hazards are associated with the laser equipment or the hazardous substances released from the laser equipment, and fumes emitted from materials exposed to laser beams, including laser-plumes produced during surgical procedures.

This type of distortion is typically tricky to visualize, as well as to quantify, because it is dependent on the environment. In most robotic and automated vehicle contexts, asymmetric radial distortion is not a great concern! Why? Well, the difference in distortions depends on the difference in distances between objects. This is usually because of some kind of refractive difference between two objects being imaged, or because the objects are out of focus of the camera (i.e. focal length is too large relative to the object distance).

The location of the damage depends on the optical nature of the laser beam. Lasers in the visible light and near infrared range focus on retina. Therefore the injuries produced are retinal burns. The infrared radiation is absorbed in the cornea and may cause corneal damage and loss of vision.

Tangential distortion is sometimes also called de-centering distortion, because the primary cause is due to the lens assembly not being centered over and parallel to the image plane. The geometric effect from tangential distortion is not purely along the radial axis. Instead, as can be seen in the figure above, it can perform a rotation and skew of the image plane that depends on the radius from the image centre!

In workplaces where a class 3B or Class 4 laser is used, a laser safety officer (LSO) must be on staff. The laser safety officer must do the following to ensure safe use of lasers.

The eye is the most vulnerable to injury from a laser beam. The potential for injury depends on the power and wavelength of the laser beam (light). Intense bright visible light makes us blink as a reflex reaction. This closing of the eye provides some degree of protection. However, visible laser light can be so intense that it can do damage faster than a blink of an eye. The invisible infrared laser beam, such as carbon dioxide (CO2) laser beam, does not produce a bright light that would cause the blinking reflex or the pupil to constrict and, therefore, chances of injury are greater compared to visible light beam of equal intensity.

At this point, we've covered the different kinds of lens distortion types one can experience when working with camera-based systems. We'll pause here, and return with Part II, where we'll introduce common distortion models that can be used to correct these distortions, for more accurate calibration models and overall system performance. Read Part II now.

While the two distortions might seem as if they are fundamentally different, they are in fact quite alike! The amount of distortion in the teal lines is greater in magnitude at the edges of our image plane, while being smaller in the middle. This is why the black and teal lines overlap near the centre, but soon diverge as distance increases.

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The last kind of distortions to characterize in a calibration are tangential effects, often as a result of de-centered lens assemblies. First, see the following figure for an (exaggerated) example of what these might look like:

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A wide variety of lasers are used in health care facilities. The type of laser depends on the purpose of use. Lasers can be used as knives or probes and for imaging techniques. For example, laser knives can make cuts that do not bleed. They can be used to smooth skin wrinkles or remove skin moles, cysts, tattoos, spider veins, and so forth. Some commonly used lasers are given in the following table.

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Before we get into the actual models themselves, it is good to first understand what lens distortion looks like. There are several distinct types of distortions that can occur due to the lens or imaging setup, each with unique profiles. In total, these are typically broken into the following categories:

Protective clothing (gown, cap, mask), gloves, and safety eye wear may be required for working near a laser. Consult manufacturer's operating procedures and check with the laser safety officer to determine the specific needs for personal protective equipment and clothing.

In the above figure, we represent the image plane as a grid. As we discuss the different kinds of distortions, we'll demonstrate how this grid is warped and transformed. Likewise, for any point \((x, y)\) that we refer to throughout this text, we're referring to the coordinate frame of the image plane, centered (with origin) at the principal point. This makes the math somewhat easier to read, and doesn't require us to worry about column and row offsets \(c_x\) and \(c_y\) when trying to understand the math.

Each of these is explored independently below. For each of these distortions, consider the base-case where there is no distortion:

Many lasers use high voltage and high current electrical power. The danger of electrical shock or electrocution arises when an untrained or unauthorized person tries to perform maintenance work without following the proper safety procedures. ANSI Standard Z136.3-2018 outlines electrical safety procedures applicable to laser equipment. Electrical safety requirements include the following:

This distortion is characterized as symmetric because it only models distortion as a function of distance from the centre of the image plane. The geometric effect of radial distortion is only in the radial direction, as characterized by \(\delta r\) in the above figure.

The CSA Standard Z305.13-13 (R2020) "Plume scavenging in surgical, diagnostic, therapeutic, and aesthetic settings" also requires that:

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Today, we want to explore another part of the camera modeling process: modeling lens distortions. One assumption behind our pinhole-projection model is that light travels in straight rays, and does not bend. Of course, our camera's lens is not a perfect pinhole and light rays will bend and distort due to the lens' shape, or due to refraction. If you've already completed this post, head on over to Part II.

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For example, the CSA Standard Z386-20 "Safe Use of Lasers in Health Care" specifies that facilities using lasers shall have a laser safety officer (LSO) and a laser safety committee (LSC) to perform risk assessments, and to ensure that laser safety policies and procedures are developed, implemented and maintained.

A fire can be started when laser beam or reflection of the beam strikes a combustible material such as rubber, plastic, human tissues, paper products, skin treated with acetone and alcohol-based preparations, human hair, and intestinal gases. Fire hazards are of particular concern in oxygen-rich atmospheres when oxygen or when nitrous oxide is being used.

Symmetric radial distortion is an after-effect of our lens not being a perfect pinhole. As light enters the lens outside of the perspective centre it bends towards the image plane. It might be easiest to think of symmetric radial distortion as if we were mapping the image plane to the convexity or concavity of the lens itself. In a perfect pinhole camera, there wouldn't be any distortion, because all light would pass through a single point!

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The ANSI Standard Z136.1 recommends a laser safety program for workplaces using class 3B or class 4 lasers. Following are the essential components of a laser safety program:

Another property of lasers is they are coherent light sources. This feature means that lasers produce monochromatic light (i.e., with a single or selected wavelength) in which the light “particles” or photons all travel in the same direction. This directionality allows laser beams to be very focused (collimated) so they do not fan out like the light beam of a flashlight. Since the light beam can be contained in a very narrow beam, it has a high radiant power per unit area. These properties enable laser devices to produce powerful laser beams that can cut metal. In health care, lasers are used for cutting, sealing and surgical procedures.

Plastic versus glass lenses: Protective eye glasses typically are available with plastic lenses. Plastic lenses are light weight and can be molded into comfortable shapes. However, care is needed because they can be affected by heat, and/or UV radiation which can darken the lens or decrease its ability to absorb laser energy.

Previously, we covered some of the basics of camera modeling, and examined the pinhole projection model of a camera. There, we looked at some of the historical context behind how affinity in the image plane has been traditionally modeled, and why different models are preferable.

The color of laser light is usually described in terms of the wavelength of the laser radiation. The most common unit used for the wavelength of laser is a nanometer (nm - one billionth of a metre). Light from other sources is made up of combination of colours at various wavelengths.

Typically when we think of distortion, we try to break down the components into their constituent parts to aid our understanding. However, most lens systems in the real world will have what is often referred to as compound distortion. There's no tricks here, it's simply an aggregate effect of all the previous types of distortions in some combination. This kind of distortion is especially prevalent in cameras with compound lenses, or very complicated lens assemblies.

Symmetric radial distortions are what are typically imagined when discussing image distortion. Often, this type of distortion will be characterized depending on if it is positive (pincushion) or negative (barrel) distortion.

Asymmetric radial distortions are radial distortion effects much like the above, but unlike symmetric radial distortion, asymmetric radial distortion characterizes distortion effects that are dependent both on the distance from the image centre as well as how far away the object being imaged is. Asymmetric radial effects are most pronounced in two scenarios: