No one will deny the safety hazards involved when working with energized electrical equipment. The need to perform more comprehensive infrared inspections, while reducing safety risks, is facilitating the installation of infrared windows.

Note: A technical photogrammetry term that you may come across is the “Principal Distance”.  Strictly, the Principal Distance is the distance mentioned above (i.e. distance from imaging plane to the lens optical sensor), and the focal length is the principal distance when the lens is focused at infinity. See below for more information on focus vs focal length. When PhotoModeler lists focal length for a camera, it is actually the Principal Distance that is shown.

One of the most basic tenants of infrared thermography is: infrared imagers do not see through metal. Because thermal imagers cannot see through solid materials, switchgear panel covers must be opened or removed to afford line-of-sight access to subject components. In most cases, panel cover removal and replacement is the most labor-intensive aspect of an infrared inspection, causing safety, efficiency and manpower difficulties. In some cases, these concerns threatened the continuation of infrared electrical system survey programs.

Learn how to use PhotoModeler with your camera to create detailed digital models: www.photomodeler.com/products/why.html

Ideally, increased safety and reduced risk of injury should be enough reason to install IR windows. However, some facilities will be required to show the Cost Savings. Below is an abbreviated formula to demonstrate savings.

Camera focal lengthchart

When purchasing IR windows, consider bulk purchases. In addition to possible discounts, having more windows available will prevent any delays during installation and possibly allow additional windows to be installed if an opportunity arises.

Originally, all IR Windows consisted of a crystal lens. Today, Germanium, Zinc Selenide, Calcium Fluoride, and Sapphire are common materials used to make lens.

Focal lengthof lens formula

The focal length number tells us how much of the scene is captured in the picture. The lower the number the wider the view, and the more we can see.  The higher the number, the narrower the view, and the less we can see.   This is illustrated below – where the camera is stationary and the focal length (in white numerals) changes:

Telephoto lensfocal length

An infrared window is an engineered optic designed to permit infrared inspections of electrical components located within switchgear enclosures. Infrared windows consist of a metal or plastic frame that supports an infrared transmissive optic. Infrared windows usually have a protective cover for the optic when they are not in use. Depending upon the window, optics may be made from crystal or polymer material.

Modern digital cameras can have imaging chips that are as small as 6mm by 4mm; some Smartphone cameras are even smaller, and then up to full 24mm by 35mm size.  A very common size is the APS-C format at 16mm by 24mm. This smaller size affects what is considered to be a ‘normal’ focal length.

Camera manufacturers sometimes list these equivalents because some photographers are more familiar with 35mm cameras and they want to make it easier to understand. It also gives us a standard of reference for all the different format sizes. They may also list the multiplier factor. For example, the APS-C multiplier is around 1.6x.  So a 32mm lens on an APS-C camera (like the Nikon D3200) would act like a 50mm lens on a 35mm film camera. Does focusing affect the focal length?

Risk Management: The Hierarchy of Control requires engineers and managers to eliminate risk where possible (as opposed to relying on engineering controls or PPE to protect personnel after the accident happens). Use of IR inspection windows eliminates the high risk behaviors involved with open panel inspections and replaces them with a non-intrusive work process.

Cameralens distance chart

All lenses have a stated or specified focal length value (or range of values for a zoom lens). This printed number is actually its nominal length or the principal distance when the lens is focused at infinity. As you focus on objects that are closer to the camera, the principal distance changes.  So for example, a 50mm lens focused on an object a few feet away might have a principal distance of 55mm lens at that time. The most extreme example of this is with a macro setting (a lens setting that allows you to focus on very close, very small objects, under 5″ in size for example). A lens that has a 50mm nominal focal length (so a 50mm principal distance when focused at infinity) might in fact have a 100mm principal distance when focused at a few inches! This is why it is good with photogrammetry (where precise geometry is needed) to calibrate a camera at the distance you will be working with.

Are Ports and Grills Different from IR Windows?Although many people refer to ports and grills as ‘windows’, there are key differences between them. An infrared port consists of a metal or plastic frame that has a single, small diameter opening (< 0.5 inches) to permit viewing of components located within switchgear enclosures. Due to their small diameter opening, ports usually require the use of special wide angle or ‘fisheye’ lenses with a thermal imager designed to accept such lenses. Grills are similar to ports; however, they are usually larger in overall diameter and contain a metal plate containing multiple small diameter openings. The thermal image taken with a grill in place will show the metal plate with the electrical component data visible only through the openings.

Proper placement of infrared windows is a critical consideration that goes beyond visual aesthetics of the final installation. To maximize effectiveness, window placement must take into account several factors including location of components within enclosures, internal obstacles such as arc barriers, and required field of view. Working with a professional installer or thermographer who is familiar with the subject electrical devices will often yield the best results for window placement.

Let’s say you take a picture of an automobile with two cameras, a 35mm film camera and a smartphone camera. You stand in the same spot and take two photos, one with each camera. In both cases you want to take a photo of the automobile that fills the frame. If the 35mm film camera lens has a 50mm focal length, the digital camera’s focal length might be 4mm. So even though they are very different numbers they produce the same result because of the size of the imaging surface. So the “equivalent 35mm focal length” for this smartphone camera at 4mm is 50mm.

Nikoncamera focal length

Cameras can have fixed lenses (sometimes called ‘prime’ lenses) which have just one focal length, or zoom lenses which allow the focal length to be varied (for example between 18mm-55mm, or 55mm-200mm). For high accuracy photogrammetric work in PhotoModeler, a fixed (or prime) wide lens (such as a 20mm lens on an APS-C frame camera) is recommended as the primary option, but different applications may require different focal lengths, and cameras with adjustable zoom lenses can still be used to achieve very good results with some extra procedural care over the focal length.

In the past several years, polymer lens have gained popularity. These may be opaque or clear and with or without reinforcement.

Cost Savings/Increased Efficiency: Using infrared windows has been shown in time studies to eliminate over 90% of the time involved with inspections. Inspections will no longer require multiple trained staff to open and close the panels. And after all… time is money!

Ideally, all electrical enclosures should have an IR window, provided that the installer of the IR window allows adequate access for thermal imaging. Opening lighting panels for an infrared inspection takes time, manpower and has a safety risk; however, facility budgets generally do not allocate funds to install windows everywhere. Deciding which equipment will have IR windows installed should be based on several factors, including the following:

A strict technical definition of focal length is difficult without providing a lot of background in lens theory, so we will use a simplification. You can think of focal length as the distance between the imaging plane (e.g. the image chip in a digital camera) and a point where all light rays intersect inside the lens (the ‘optical center’). So a focal length of 20mm means that the distance from the optical center to the imaging plane is 20mm long (about ¾ of an inch). What does the focal length number mean?

When you buy a digital camera you will often see the specification “equivalent 35mm focal length”. What does this mean? Most digital cameras have imaging chips that cover much less area than a standard 35mm film frame. Since 35mm film cameras were the standard for so long in photography, much of the techniques and methods were developed around them. A 35mm film camera has a negative that is about 36mm wide by 24mm high (the “35” comes from the physical width of the film stock that is exactly 35mm wide).  A ‘normal lens’ (has a field of view that appears ‘natural’ to humans) on a 35mm film camera has a focal length of 50mm.

Most people involved with electrical system maintenance have heard of infrared windows, but many do not have a working knowledge of them. Until there is a solid understanding of the “What, Where, and How” of infrared windows, this valuable asset is under used by facilities. This paper will explain what IR windows are and how they work, outline the different options currently available, and provide direction for a facility looking to start or enhance an existing Infrared Window Program.

In the 1980s, performing an infrared electrical system survey was a “new” concept that required explaining and selling the concept and benefits to plant management before the first thermogram could be taken. Through education, documented payback and industry recognition, infrared electrical system surveys are now accepted, and demanded, parts of a PPM program. With the acceptance of this technology, a new set of challenges must be addressed – data management, efficiency and safety.

Thermographer Cost = reduced by up to 90%This is a simplified version of the calculation that clearly demonstrates the potential savings of installing IR Windows. Of course, the purchase and installation cost must be taken into account but these are one-time costs that could be capitalized.

A camera typically has focal length in a range of 10mm to 500mm.  Different types of camera can have different ranges and speciality lenses can extend outside this range as well.  A 10mm focal length would be a very wide lens (capturing a lot of the scene), and 500mm would be a very narrow lens (capturing only a small part of the scene – giving a large magnification like binoculars or a telescope).

Focal length is a number that is vital to photography and photogrammetry but often misunderstood. What is focal length?

What isfocal lengthof lens

PhotoModeler is one of the leading tools for photogrammetry (the science of generating measurements and accurate 3d data from photography).

Phonecamera focal length

The solution to this problem has been the development of a method to perform the infrared electrical system survey without removing or opening the component covers and keeping the integrity of the data collected. Installed at strategic locations on switchgear enclosures, IR windows permit infrared inspections to be conducted without having to open or remove panel covers, thereby saving time and money and improving safety.

Safety: The possibility of performing an infrared survey of energized electrical equipment without removal of the panels reduces the risk of electrocution or an arc flash incident.Standards Complaint: The equipment is maintained in an enclosed and guarded condition without creating a potential electrical hazard during inspection. Therefore, elevated levels of PPE are not required when using an IR window.

Camera focal lengthcomparison

There is some ability to calibrate a camera (which solves the principal distance) at one focus and execute your photogrammetric project at another focus.  The actual discrepancy that is acceptable depends on your accuracy requirements and how much the focus changes.  Generally a calibration done at 2m/6ft focus distance is acceptable for projects up to infinite focus (again depending on accuracy requirements), but may not be acceptable for a project where the focus distance was 50cm/20in.

If you are starting or continuing an Infrared Window Program, contact your IR window manufacturer or representative. Their resources and expertise, combined with your knowledge of your facility, will ensure a successful program!

Data Management: IR windows provide a systematic approach to data collection during inspections – same target, same location, same angle, and same distance. Consistency in the data collection process leads to more trendable data in any PdM process.

Above we mention that focal length is related to focus distance.  Focal length is the principal distance of a camera when it is focused at infinity. In photogrammetry we are interested in the camera’s internal geometry at the time photos were taken – so it is the principal distance that we want to know precisely in photogrammetry.

In many cases though, the advantages of using focus (i.e. crisp targets and distinct features) with subtle effects on principal distance/focal length, outweigh the advantages of keeping focus and principal distance constant (i.e. potentially causing blur in some photos taken at a different distance).

An IR window is a data collection point installed in the metal cover of the electrical switchgear. The window consists of a backing ring, the lens, and the front ring with a cover to secure the opening. This makeup results in the lens being in a secure housing, forming a barrier between the interior and exterior environments. When selecting the IR window to install, several factors should be considered, including the following:

To begin an IR Window Program, establish a list of all equipment that should have IR windows. Prioritize each piece of equipment that ideally should have a window installed. Plan the installation of the windows starting with the highest priority, taking into account electrical equipment availability (shutdown schedules), frequency of inspections and hazards.