Infrared camera selection for electrical inspection by IFOV

Electrical inspection camera choices usually start with resolution, but the number that decides whether your temperatures come out right is the instantaneous field of view, IFOV: the angle one sensor pixel subtends. A lug bolt that should read 80 °C reads low, or not at all, when it covers too few pixels, and no software recovers the value after the fact. This guide gives you the sizing arithmetic for a radiometric infrared camera before you buy: convert a datasheet field of view into millimetres per pixel at your working distance, apply the 3 to 5 pixel rule to your smallest target, and pick camera, lens and stand-off that fit the panel. It is written for anyone running panel audits, connection checks or insulation-evaluation rounds.

The decision in one formula

Buy by IFOV, not by pixel count. Thermography practice treats a reading as measurable once the smallest part you must grade spans 3 pixels, and treats 5 or more pixels as the range where values can be trended and defended between visits. Programs audited to NFPA 70B-style electrical maintenance schedules need exactly that: documented, repeatable numbers. Everything follows from two lines of arithmetic:

The units close because 1 mrad at 1 m is exactly 1 mm. A 1.7 mrad camera at 2 m sees 3.4 mm per pixel, so a 10 mm bolt head spans 10 ÷ 3.4 = 2.9, barely 3 pixels. Below 3, the hot centre mixes with cooler background and the reported value drops; below 1, the target vanishes into the average. Detection still works at 1-2 pixels, which is why wide-angle cameras find hotspots they cannot grade. For the field workflow after purchase, the panel hotspot walkthrough picks up where sizing ends.

What IFOV and spot size mean at the panel

IFOV is an angle: pixel pitch divided by focal length, quoted in milliradians. Spot size — some datasheets write measurement footprint or spot of measurement — is that angle converted to a length at your distance. Peer-reviewed field studies treat spot size as a first-order distance error, because the camera averages whatever fills the pixel: a hot spot covering half the pixel contributes roughly half its excess, and the background supplies the rest.

Two datasheet habits to watch. Some quote a spot size at one fixed distance; convert it yourself rather than compare the quoted numbers. Others quote only field of view and resolution; divide one by the other and you have the IFOV, as the next section does with a real listing. For the measurement consequences at longer stand-off, see the guide on working distance and spot size for safe scans.

Worked example: a 10 mm bolt head at 2 m with a 1.7 mrad camera

Inputs: 2 m working distance, taken as the clear space you normally have in front of a distribution panel; 1.7 mrad camera IFOV, typical of mid-range radiometric imagers; a 10 mm hex bolt head on a cable lug, the smallest part a connection audit grades; and the 3-pixel minimum with 5 pixels preferred.

The rule in words: footprint equals IFOV times distance; pixels equal target divided by footprint; required IFOV equals target divided by pixels times distance. Because 1 mm at 1 m is 1 mrad, the divisions come out in mrad, and the last line in metres:

footprint              = 1.7 mrad × 2 m    = 3.4 mm per pixel
pixels on the bolt     = 10 mm ÷ 3.4 mm    = 2.9 ≈ 3 pixels
IFOV for 5 px at 2 m   = 10 mm ÷ (5 × 2)   = 1.0 mrad
max distance for 5 px  = 10 mm ÷ (5 × 1.7) = 1.2 m

Result: at 2 m this camera sits exactly on the 3-pixel limit for the bolt — enough for a defensible single reading, without margin to trend. The decision rule: 3 pixels means you can report a temperature; below 3, treat the value as low and close the distance. For audit-grade numbers on that bolt, either step in to about 1.2 m, or fit glass that brings IFOV to 1.0 mrad — a lens about 1.7× longer on the same sensor.

Reading the field of view spec on a catalog camera

The LS-IRC-0044 prints its optics the way most thermal datasheets do: 640 × 512 pixels behind an 88.5° × 73.2° field of view, sensitive in the 8-14 μm band, 50 Hz frame rate, GigE output, C-mount, 9-24 V DC supply.

Divide field of view by pixels: 88.5° is 1.545 rad, and 1.545 ÷ 640 = 2.4 mrad across the width; 73.2° is 1.278 rad, and 1.278 ÷ 512 = 2.5 mrad vertically. At 2 m that is 4.8-5.0 mm per pixel, so the 10 mm bolt spans about 2 pixels — detection, not grading. The same wide angle is the point of this class of camera: 2 × 2 m × tan(44.25°) = 3.9 m of panel wall across the frame (3.0 m tall), so a whole switchboard row is covered in a few shots. To grade the bolt with it, the 3-pixel limit pulls you in to 10 ÷ (3 × 2.414) = 1.4 m, and 5 pixels to 0.83 m.

The MV-CI003-GL-N25 takes the narrower default: a 0.3 MP LWIR sensor shipping with a 25 mm EFL lens on an M34x0.75 mount. Focal length divides straight into IFOV, so the same pitch behind 25 mm of glass tightens the footprint roughly 1.5× against a 17 mm lens.

Longer lenses and closer distance: the two levers

Distance scales the footprint linearly; focal length scales IFOV inversely. The relation behind the lens lever:

Typical microbolometer pitches are 12-25 μm. Take a common 17 μm pitch and compute: a 5.6 mm lens gives 3.0 mrad, a 17 mm lens 1.0 mrad, a 25 mm lens 0.7 mrad. A 12 μm sensor scales every result by 12 ÷ 17 = 0.71, so check your pitch before quoting a number.

To run the geometry the other way — from sensor size and the field of view you want to the focal length that delivers it — use the calculator:

The narrow-glass answer in the lens catalog:

It is a 17 mm F1.2 LWIR thermal lens sized for a 1/1.2 inch sensor with an 11-16 mm image circle. Behind a 17 μm pitch sensor, 17 mm of focal length is exactly the 1.0 mrad the worked example asked for: 2 mm pixels at 2 m and 5 pixels on the 10 mm bolt without moving your feet. Match mount and image circle to the camera before ordering — the thermal, SWIR and UV lens category lists both per model, and the lens finder filters by them. LWIR focus also shifts as temperatures swing; if the camera lives beside running switchgear, the athermalization guide explains the compensation.

Which task needs which IFOV

Read the table as inputs to the formula, not thresholds from a standard: change any input and the limit moves with it. The direction never moves — closer stand-off, longer focal length or finer pitch all tighten the footprint, and most audits need the wide view for finding plus the narrow one for grading.

Catalog parts across that range:

The LS-IRC-0026 is a 0.5 MP thermal camera on GigE for a fixed audit post; the LS-IRC-0016 is a 640 × 512 LWIR module over USB3 if the camera goes inside your own instrument. Compare any camera on the page by footprint first: its field of view divided by its pixels, times your working distance.

Checks before you order

• Radiometric pixels. Confirm calibrated temperature per pixel, not just a colorized image, and raw 16-bit export. The write-ups on radiometric JPEGs that keep 16-bit temperature data and exporting high-bitdepth grayscale images show what to ask for.
• IFOV printed or computable. Either the IFOV itself, or field of view plus resolution; if a datasheet gives neither, ask before buying.
• A lens upgrade path. A mount that takes longer LWIR glass and an image circle that covers the sensor; the camera finder filters by mount, the lens finder by image circle.
• The right band. Heat reads in LWIR, 8-14 μm: a 300 K bus bar peaks near 9.7 μm (Wien's law). SWIR cameras, 900-1700 nm, image reflected light and do not read surface temperature; the SWIR band-selection guide covers what they do instead.
• Frame rate you will use. High rates pay off when you pan a switch room; a tripod scan of one panel works at a few frames per second. Spend on IFOV first.
• Windows for closed panels. If the door stays shut, plan an LWIR-rated window; ordinary glass and polycarbonate block the 8-14 μm band. The infrared window material guide compares the candidates.
• Ambient range. A camera riding on or beside the enclosure sees the panel's heat too; the guide on camera operating temperature range explains what drifts outside the rating.

When the usual answer is wrong

Emissivity beats IFOV as the first error on shiny metal. Polished copper and aluminium sit near 0.05-0.1 emissivity, so the pixel reports mostly reflected background and the value can be off by tens of degrees regardless of pixel count. The field fix is simple: matte electrical tape on the joint reads at about 0.95, or paint the pad, or grade an adjacent dull surface. Turning corrected deltas into priorities is the job of the severity classification by delta-T guide.

Sub-pixel emitters. A glowing 0.5 mm fuse element at 3 m covers a tenth of the 5 mm footprint a 1.7 mrad camera has there: the image shows a warm smudge, not a temperature. Detect it, then confirm from closer range or with a different instrument; the thermal camera versus spot pyrometer comparison sets out the trade.

Wide-angle averaging. Seen from the frame side, the same math explains why a wide camera under-reports: at 2 m an 88.5° frame spans about 3.9 m, so a 10 mm hot joint fills roughly a quarter of one percent of the frame width. One faint dot among a wall of breakers is a finding, not a measurement.

The short version

• Footprint in mm equals IFOV in mrad times distance in m; 1 mrad at 1 m is 1 mm.
• Three pixels to report a temperature, five to trend it; under one pixel the target is invisible.
• Required IFOV (mrad) equals target (mm) divided by pixels times distance (m).
• Maximum stand-off (m) equals target (mm) divided by pixels times IFOV (mrad).
• Wide field of view finds, narrow IFOV grades: scan wide, then step in or change glass.
• Radiometric 16-bit output, an emissivity plan for shiny joints, and an LWIR window for closed doors.

Size the camera around your smallest target and the distance you can actually stand at; the rest of the datasheet follows from that. Browse the infrared cameras category, work the chain end to end in the camera configurator, or send your panel dimensions and targets through the contact page and we will run the numbers with you. The cameras topic page collects the other camera guides when you want to go deeper.