Why is Field of View (FOV) Critical for Sizing Infrared Inspection Windows?

Companies are purchasing new equipment and replacement panels to comply with the new guidance for reducing risk while performing maintenance tasks in NFPA 70E (2018 Edition). You are receiving more requests from consultants and end user customers to install Infrared Inspection Windows into your switchgear. To understand the size of inspection window needed, you are researching Field of View (FOV) capabilities.

There are two FOV parameters that must be determined when choosing the size of infrared inspection window needed in a piece of switchgear:

1. Infrared Camera’s Field of View

The camera’s FOV, in degrees, for any given distance from the object can be calculated using this formula:

Camera FOV = {(tangent ½ viewing angle) x distance} x 2

* Distance is defined as distance from the panel cover to the target to be measured

* Viewing Angle of the Camera is obtained from the manufacturer’s specification

A typical FOV is 22 degrees horizontally and 16 degrees vertically for the average camera lens. The calculated values should be used for estimation purposes to determine size of windows needed. Note that many IR cameras have interchangeable lenses with various viewing angles and a change in lens type in the camera requires the FOV to be recalculated.

A Practical Field of View Test

2. Window Field of View (WFOV)

The following equation is used to calculate the WFOV:

Window Size (W & H) = Target Size (W&H) – {(2 Tangent(CVA/2)) x DCT x 3]

* W = Width

* H = Height

* CVA = Camera Lens Viewing Angle from manufacturer’s specifications

* DCT = Distance from Cover to Target

* 3 = Maximum Viewing Angle Multiplier

Example of Window Width Calculation:

The distance from cover to target is 8 inches and overall target width is 18 inches and the camera’s viewing angle is 22 degrees. What is the minimum window width required?

Ws = 18 – {(2 tan (22/2)) x 8 x3}

Ws = 18 – (0.389 x 8 x 3) = 18 – 9.34 = 8.66 inches

One More Important Parameter For Window Sizing:

There is one other parameter to consider when determining the size of infrared window needed: the outside diameter of the infrared camera lens. To accurately measure temperature, the infrared camera’s lens must “see” the full view of the target objects being measured. Any obstruction, even a partial obstruction, will introduce measurement errors that cannot be compensated by a setting change. A Window’s IR optic must be larger than the outside diameter of the infrared camera lens.


Collecting accurate infrared temperature data is critical in maintenance inspections of energized electrical equipment. The accuracy of the data is dependent upon the infrared camera’s interaction with the infrared inspection window. Any obstruction will cause temperature measurement errors. Three elements are required for proper Window sizing: the window’s infrared optic diameter, the field of view of the camera and the field of view of the window.


What Standards and Certifications Apply To Infrared (IR) Viewing Panes?

This question is frequently asked by clients as they begin to research the utility of infrared inspection windows for their specific application needs. There are many standards and certifications that can apply to infrared viewing panes so, this communication will help to identify and explain the most prominent ones.

UL Recognized Certifications:

* UL 50V: This is the only standard, but more of a classification, that is specifically applied to infrared windows. It states: Infrared viewports are a fixed aperture, consisting of one or more openings or a solid infrared transmitting media, surrounded by a mounting bezel or frame, that provide a means for the passage of infrared radiation. Infrared viewports are intended for factory installation in doors or walls of electrical enclosures for installation in ordinary (non-hazardous) locations to allow the use of IR scanners for monitoring temperatures of the enclosed equipment on which the viewport is installed, without compromising the integrity of the enclosure with respect to access to live parts.

This classification is applied to two different product categories: Infrared Windows and Infrared Ports. Infrared Windows provide a safety barrier that separates the thermographer from the target environment. In contrast, an Infrared Port is a hole and when opened, it removes the barrier between the thermographer and the target thus increasing the risk of an accident. Knowing the difference between a Window and a Port is essential when determining the use of Personal Protective Equipment.

Certifications Relevant to Infrared Windows

UL 50E: This standard applies to enclosures for electrical equipment intended to be installed and used in non-hazardous locations in accordance with the Canadian Electrical Code, Part I, CSA C22.1, the provisions of the National Electrical Code, NFPA 70, and the provisions of Mexico’s Electrical Installations, NOM-001-SEDE, as follows:

a) Enclosures for indoor locations, Types 1, 2, 5, 12, 12K, and 13; and

b) Enclosures for indoor or outdoor locations, Types 3, 3X, 3R, 3RX, 3S, 3SX, 4, 4X, 6, and 6P

This standard covers additional environmental construction and performance requirements for enclosures. The general requirements for enclosures are contained in CSA C22.2 No. 94.1, UL 50, and NMX-J-235/1-ANCE (See Annex b, Ref. No. 10) or the end-use product standards that are to be used in conjunction with this standard. This standard does not cover the requirements for protection of devices against conditions such as condensation, icing, corrosion, or contamination that may occur within the enclosure or that may enter via conduit or unsealed openings. Where an individual product standard contains requirements that are at variance with those of this standard, the requirements of the individual product standard take precedence.

* UL 746C: These requirements set the impact and flammability standards for polymeric materials used in electrical equipment up to 1500 volts. Any plastic or polymer, as a part of an infrared window, must

pass flammability tests at room temperature, and must remain intact during an impact test performed at 0°C (32°F).

* UL 1558: These requirements cover metal-enclosed low-voltage power circuit breaker switchgear assemblies containing but not limited to such devices as low-voltage power circuit breakers, other interrupting devices, switches, control, instrumentation and metering, protective and regulating equipment. UL 1558 specifies static load and impact testing requirements for infrared windows utilized in these assemblies. These requirements cover equipment intended for use in ordinary locations in accordance with the National Electrical Code. These requirements are intended to supplement and be used in conjunction with the Standard for Metal-Enclosed Low Voltage Power Circuit Breaker Switchgear, ANSI C37.20.1, and the Standard for Conformance Testing of Metal-Enclosed Low-Voltage AC Power Circuit Breaker Switchgear Assemblies, ANSI C37.51. These requirements cover equipment rated 1000 V ac or less nominal.

Ingress Protection

* The Ingress Protection rating system is a classification system showing the degrees of protection from solid objects like dust or liquids coming in contact within the enclosure. The IP rating of an IR window should be the same or higher than the equipment into which it will be installed and ties directly to the UL50E ratings.

Canadian Standards Association (CSA)

CSA is accredited by the Standards Council of Canada, a crown corporation which promotes efficient and effective standardization in Canada as a standards development organization and as a certification body. The CSA registered mark shows that a product has been independently tested and certified to meet recognized standards for safety or performance.

Lloyd’s of London Register

Lloyd’s Register provides independent, 3rd-party approval certificates attesting to a product’s conformity with specific standards or specifications. It also verifies the manufacturer’s production quality system through a combination of design reviews and type testing.

American Bureau of Shipping

ABS Rules form the basis for assessing the design and construction of new vessels and the integrity of existing vessels and marine structures

Arc Resistance

The Arc Rating applies only to switchgear, not to its individual components. Even though infrared windows have passed arc tests, the rating is assigned to the switchgear assembly. Despite what you may have read elsewhere, there is NO SUCH THING as an Arc Resistant Infrared Window in of itself.


Infrared viewing panes may have many certifications depending on the specific application or global location where they will be used. The most common certifications are UL Recognized, CSA recognized and Lloyd’s of London. Partner with your inspection window manufacturer to determine the best solution for your specific application.


What Is The Proper Way To Install A Square Maintenance Inspection Window?

As the manager of your company’s reliability maintenance team. you are responsible for all of the pre-planning, training and post installation processes when a new piece of equipment is installed, or an existing piece of equipment is upgraded. Your company recently purchased maintenance inspection windows for use on several critical electrical assets that will be inspected routinely to record asset health and determine when an asset may be failing. Can the plant’s reliability team perform the window installation correctly? The answer is YES!

Your company partnered with a highly respected maintenance window manufacturer to insure the products purchased are of the highest quality and performance. Proper installation of the windows is critical to the long term use and performance. The company provides detailed instructions for the proper installation of its inspection windows.

Installation Instructions

Installation Video

Pre-Installation: Read the installation instructions thoroughly making sure you understand each step. If you have questions, contact your maintenance inspection window provider for clarification. If necessary, schedule the equipment shutdown and insure that the team understands the company’s internal procedures for a safe shutdown and is following all Lock Out Tag Out (LOTO) procedures.

Getting Started: Gather the tools and equipment required as per the installation instructions.

Verify the Contents: Check that you received the window, a fitting template and a label

Record Transmission Rate: Record the transmission rate of the window on the window’s label

Field of View: Know the Field of View of each window that will be installed

Using the Template: Apply the fitting template to the selected location on the panel

Center Punch Holes: Using a center punch, mark all of the fixing holes on the template

Drill Fixing Holes: Drill the center punched holes

Cut Out: Using a grinder or nibbler, cut the required hole. Alternately, a hydraulic punch set and specific dies recommended by the window manufacturer may be used. De-burr the rough edges and remove the fitting template. Apply a protective anti-corrosion coating to the bare metal edges of the hole.

Fit the Window: Place the window, fit and tighten the hardware

Affix the Label: This label contains critical information for the thermographer: # of targets, emissivity of the targets and the transmission rates of the viewing pane with different Infrared cameras. If Intelligent Asset Tags are being used, now is the time to program them as well.

Post Installation: Post-installation, the thermographer should conduct a benchmark inspection insuring that the window is properly installed allowing the IR camera to “see” all of the targets. Record the baseline data to the Intelligent Asset Tags if applicable.


Maintenance inspection window installation is easy provided the manufacturer’s installation instructions and internal company procedures are followed. Proper installation of the window insures long term use and performance. A post-installation inspection must be performed to establish a baseline and confirm that the installation was properly performed. Maintenance inspection windows are one of the many tools available to conduct condition based maintenance inspections of critical electrical assets.


How Important Are Infrared (IR) Inspection Windows at a Geothermal Energy Generation Facility?

Power generation plants convert heat into electricity – usually by burning fossil fuels to create steam to operate a turbine. A geothermal power generation plant uses heat from the earth to generate electricity. The energy is accessed by drilling water or steam wells deep in the earth. The hot, pressurized geothermal fluid rapidly expands to produce steam which turns the turbine blades. Energy from the turning turbine blades spins magnets inside a large coil to product electric current. A generator sends the electrical current to transformers where voltage is increased and finally transmitted over power lines to homes, buildings and businesses. Like other power generation plants, a geothermal power plant utilizes critical electrical components such as turbines, generators and transformers. Can the plant’s reliability team predict when one of these critical electrical components is deteriorating and fix it before it fails? The answer is YES!

A failure of a critical electrical component at a power plant would severely impact operations and cause major inconvenience and safety risks to the plant’s customers and community in general. Businesses such as water treatment plants, hospitals, airports, industrial plants and our homes would all be impacted by a loss of power.

Geothermal Energy Generation Facility Embraces Infrared Windows

Every geothermal power generation plant should conduct a power distribution asset condition assessment to understand the relative health of their essential infrastructure. Some plants probably have backup systems; however, the backup assets need to be inspected and maintained to insure proper functionality should the need arise and may not protect from all types of equipment failure.

These industries recognize the criticality of establishing and performing Condition Based Maintenance programs on their electrical assets. Routine inspections enable personnel to monitor the relative health of critical electrical components and systems. Innovative products and services, called Electrical Maintenance Safety Devices (EMSDs), enable personnel to perform routine electrical inspections of energized assets both safely and efficiently.

Common types of EMSDs include Maintenance Inspection Windows with Infrared or Infrared and Ultrasound capabilities, Ultrasound Ports and handheld measuring devices, Wireless Temperature Monitoring Systems and Intelligent Asset Tagging Systems. Utilizing these tools within a condition based maintenance program allows the reliability team to routinely and safely perform inspections, collect data, monitor data over time and determine if an electrical asset is starting to deteriorate. These programs allow companies to schedule downtime to fix the asset versus experiencing an unplanned outage and disrupting the lives and safety of thousands of people.


Power generation plants cannot tolerate an internal electrical failure that impacts the safety and comfort of their personnel or their end-user customer base. Improved operational reliability and productivity can be achieved

by implementing a condition based maintenance program using EMSDs to monitor, maintain and anticipate problems on their generators or on-site power distribution and power generation systems before an actual electrical component fails.