What Is Field of View in Thermal Imaging?

Field of view (FOV) is the angular extent of the scene that a thermal camera can capture, measured in degrees horizontally and vertically. It is determined by the lens and detector size combination. A wider FOV (e.g., 24° or 25°) captures more of the scene at once, ideal for scanning large equipment. A narrower FOV (e.g., 12° or 7°) magnifies distant targets, ideal for inspecting small components from a distance.

The relationship between FOV, distance, and target size is governed by the Instantaneous Field of View (IFOV), which determines the smallest hot spot the camera can resolve at a given distance. A camera with a 25° lens and 384×288 detector has an IFOV of approximately 1.13 mrad, meaning it can resolve a 1.13 cm target at 10 meters distance. The EasIR Series with its standard 25° lens offers a good balance for general electrical inspection, covering a 4.4m x 3.3m area at 10 meters.

The key concept to understand is that FOV and working distance are inversely related. A wide FOV lets you see more, but at the expense of spatial resolution on distant targets. A narrow FOV gives you more detail on targets farther away, but you see less of the overall scene. This is why Hammer II Series with its dual FOV lens is so valuable for utility work: it gives you both options in a single camera.

"The single biggest mistake we see new thermographers make is trying to inspect an entire substation with a wide-angle lens. They can see everything at once, but at 20 meters the IFOV is too coarse to detect hot spots on individual bushing connections. A dual FOV camera like the Hammer II solves this by letting them scan wide and then switch to narrow for detailed inspection."

— Unitech Tools Training Department

Standard, Dual FOV, and TwinView: Types Explained

FOV Type	How It Works	Advantage	Disadvantage	Unitech Tools Example
Standard Fixed FOV	Single lens with one angular field	Simple, rugged, lower cost	Cannot switch between wide area and detail	EasIR (25° fixed)
Dual FOV (Motorized)	Motor-driven lens shifts between two focal lengths	Two FOVs in one camera without lens change	Slightly higher cost, moving parts	Hammer II (24° wide / 12° narrow)
TwinView DFOV	Simultaneous wide and narrow FOV image fusion	See both context and detail at the same time	Highest cost, advanced processing needed	PT II (TwinView DFOV)

The PT II Series TwinView DFOV technology is a significant advancement for critical asset inspection. Instead of choosing between wide and narrow, TwinView captures both simultaneously and allows the operator to navigate between a contextual wide-angle view and a detailed telephoto view without any lens movement. This is particularly valuable for forensic analysis and commissioning inspections where every detail must be documented.

FOV Requirements for Switchgear and Panel Inspection

For indoor medium-voltage switchgear and distribution panel inspection, the typical working distance is 1-3 meters. At these distances, a standard 25° FOV covers a 0.44m x 0.33m area at 1 meter. This is sufficient for most switchgear compartments, which typically have individual cell widths of 0.6-1.0 meters.

The EasIR Series with its 25° standard lens is ideal for this application. It provides a natural viewing angle that matches the inspector's visual scanning pattern, making it easy to pan across switchgear lineup compartments and identify anomalies. At 1.5 meters, the field covers approximately 0.66m x 0.5m, matching a standard 12kV switchgear cell width.

For confined spaces common in older Latin American substations (e.g., in Lima's historic downtown or Bogotá's central substations), a wider FOV would be helpful but is rarely necessary. What matters more is the camera's close-focus capability. The EasIR's minimum focus distance of 0.3 meters allows inspectors to get detailed thermograms of individual cable lugs and fuse holders at very close range, effectively compensating for the fixed FOV.

FOV Requirements for Transformer and Substation Surveys

Outdoor substation thermography is where FOV selection becomes critical. A power transformer at a typical 15-meter safe working distance requires different FOV capabilities than a distribution panel at 1 meter.

Consider a typical 30 MVA power transformer with dimensions of approximately 6m x 4m (radiator bank). To capture the entire transformer in one thermal image from 15 meters away, you need a horizontal FOV of at least 23°. The Hammer II's 24° wide mode covers this. However, to inspect a single bushing (approximately 15cm diameter) at the same distance, you need the 12° narrow mode, which provides 3.2x magnification compared to the wide setting and resolves details as small as 2.5cm at 15 meters.

For Latin American utilities managing HV transmission lines across challenging terrain, the ability to switch FOV modes without changing lenses is a significant safety advantage. Inspectors can scan a transmission tower from a safe distance with the wide 24° FOV, then zoom to 12° narrow to inspect individual insulator discs, grading rings, and conductor connections without repositioning.

The PT II's TwinView DFOV takes this further. For critical assets like large power transformers (100 MVA+) in major Brazilian and Colombian transmission substations, TwinView lets the operator view the full transformer bank in the context window while simultaneously seeing detailed bushing and cooling tube temperatures in the detail window. This eliminates the need to switch modes and ensures no hot spot is missed.

Working Distance and Safety Considerations

In Latin America, electrical safety standards (NOM-029-STPS in Mexico, NR-10 in Brazil, RETIE in Colombia) specify minimum approach distances for live-front equipment. For medium-voltage switchgear (11-36 kV), the minimum approach distance ranges from 0.7 to 1.1 meters depending on the standard. For HV equipment (69 kV+), distances extend to 3-6 meters. For EHV transmission (230 kV+), safe working distances can exceed 6 meters.

These safety distances directly impact your thermal camera FOV requirements:

• MV switchgear (1m safe distance): A 25° FOV covering 0.44m x 0.33m is adequate. The EasIR with its tool-like form factor (small, lightweight) is particularly suited for this range since inspectors can hold it steady at arm's length
• HV substation (3-5m safe distance): A 24° wide FOV covers 1.27m x 0.95m at 3 meters. A 12° narrow FOV covers 0.63m x 0.47m at the same distance. The Hammer II's 12° narrow mode allows detailed inspection of specific components behind safety barriers
• EHV transmission (6-15m safe distance): At 10 meters, a 24° FOV covers 4.2m x 3.2m. The 12° narrow mode covers 2.1m x 1.6m with 2x better spatial resolution. For 230 kV and 500 kV transmission line connections in the Colombian and Peruvian highlands, the Hammer II's dual FOV is the practical minimum

"For electrical inspectors performing outdoor substation surveys, we strongly recommend a dual FOV camera like the Hammer II. The ability to scan a transformer bank at 24° and then zoom to 12° for detailed bushing inspection without moving closer to energized equipment is both a productivity and a safety advantage."

— Unitech Tools Application Engineering Team

Product Comparison: EasIR vs Hammer II vs PT II

Specification	EasIR Series	Hammer II Series	PT II Series
Field of View	25° x 19° (fixed)	24° x 18° wide / 12° x 9° narrow	TwinView DFOV (simultaneous wide + narrow)
FOV Type	Single fixed lens	Motorized dual FOV	Simultaneous image fusion
IFOV	1.13 mrad	0.65 mrad (wide) / 0.32 mrad (narrow)	0.32 mrad
Smallest Target at 10m	~1.1 cm	~0.65 cm (wide) / ~0.32 cm (narrow)	~0.32 cm
Temperature Range	-20°C to 650°C	-40°C to 2000°C	-40°C to 2500°C
IR Resolution	384 × 288	640 × 480	1280 × 1024
Best For	Panel & switchgear inspections (1-3m)	Substation & transformer surveys (3-15m)	R&D, forensics, critical assets
Price Range	$	$$	$$$

How to Choose the Right FOV for Your Team

Here is a practical guide for selecting the right FOV based on your typical inspection scenarios:

• Industrial plant electrical maintenance (single FOV sufficient): If your team inspects motor control centers, distribution panels, and cable trays at close range (1-3 meters), the EasIR Series with its fixed 25° FOV provides everything you need at an accessible price point
• Substation and transmission surveys (dual FOV recommended): For utilities inspecting outdoor substations, power transformers, and transmission lines at varying distances (2-20 meters), the Hammer II Series with motorized dual FOV is the standard recommendation. It handles both wide-area scanning and detailed spot inspection without lens changes
• Critical asset and forensic analysis (TwinView DFOV): For high-value transformer acceptance testing, new substation commissioning, and forensic failure analysis, the PT II Series TwinView DFOV provides simultaneous wide and narrow views for complete documentation

For most Latin American power utilities, we recommend the Hammer II Series as the primary thermal camera. Its dual FOV lens covers the full range of inspection scenarios from switchgear rooms to transmission lines. The ability to inspect from safe distances without sacrificing image detail makes it the most versatile thermal camera for electrical inspection professionals.