A FLIR ONE ® infrared imaging attachment for my Android 6S cell phone arrived at my door the other day. The price was initially a bit high, $350, but had recently dropped to a more attractive price of $249, so I pulled the trigger. The online transaction on the FLIR website was seamless and the delivery time was less than one week. At the time the FLIR ONE® was offered on Amazon for the same price.
The unit has two imaging sensors arranged horizontally side-by-side and one centimeter apart: One optical sensor and one Lepton IR thermal imager detecting in the 8-14 micron wavelength range. According to Optics.org, the Lepton is an “uncooled long-wave infrared (LWIR) microbolometer focal plane array”. The FLIR has its own battery which must be charged separately. It will not energize from your phone’s battery.
The unit arrives nearly ready for plug and play. Before it can be operated the user must download an app from FLIR. This process went smoothly and in a short time I had the unit operating. The compact FLIR unit connects to the Android via the micro USB connector on the phone.
About the imaging. The FLIR ONE superimposes the IR image atop an optical image that consists primarily of edge lines defined through high contrast. This is a useful feature because it improves the image sharpness and helps set the context of the IR image. In a darkened space the optical image is lost and only the IR image will be visible (second image). The IR image itself is relatively low resolution owing to the limited number of pixels from the IR detector. At close range a significant parallax effect occurs, appearing as shifted overlap of the optical and IR images.
The image above is an example of a false color image captured from the FLIR ONE. The shot of this north-facing door was taken during late afternoon on a sunny day in Colorado. The internal air temperature was ~68 °F and the outside air temp was ~35 °F in the shade. As is customary, the coolest temperatures are indicated in blue and warmer temperatures are indicated by a gradient from red to yellow to white. The IR sensor seems to saturate fairly easily, but the automatic exposure control will get a handle on the image, though not instantaneously. I have found that the best images are had by limiting the frame to avoid including overly IR-bright features. This allows the exposure control to bring out thermal subtleties in the image much as any auto exposure feature would in the optical range.
The second image shows a basement gas hot water heater and the hot water output line directed upwards to the floor joists. The hot water lines are insulated with closed cell polymer foam insulation from the local hardware store. The water heater has nothing more than the factory equipped insulation.
The FLIR ONE indicates infrared temperatures by way of false color images and spot temperature readings. But temperature readings from IR thermometry are not the whole story when it comes to understanding fugitive heat losses, radiative or otherwise.
An IR image shows surface temperatures based on assumptions on average emissivity and scaling through the Stefan-Boltzmann law. The amount of radiant energy emitted by a black body is defined by the Stefan-Boltzmann law. A plot is shown here. Emissivity is the quotient of emitted energy from a surface divided by that emitted by a black body radiator at the same temperature. Every surface has a characteristic emissivity based on its composition. According to the linked emissivity table, polished aluminum has an emissivity of 0.095; concrete 0.95; mercury, 0.12; sanded spruce, 0.82; and white lacquer, 0.95. All these values are at 100 °C.
In the third photo, a 1 ft x 1 ft piece of aluminum foil was wrapped around a stretch of the insulated hot water feed line above the heater, as shown in the photo. The foil is in thermal contact with the foam insulation on the 3/4″ copper pipe. Hot water was run for a few minutes to draw heated water into the plumbing. Caution should be taken in that IR radiation does reflect off of surfaces which may lead to inaccurate conclusions about heat flow in the system in question. Above, the aluminum foil is reflecting some IR from another source. Up close and from another angle the foil appears much cooler than it is.
Plainly the emissivity of the highly heat conductive aluminum is different from the foam insulated pipe. The foil is in thermal contact with the foam and should be near the temperature of the foam surface, but the false color image suggests that the foil temperature is lower in temperature. Because of its much lower emissivity (ca. 10 % of foam) the foil only appears to be cooler. The foil is less radiant than the foam which has an emissivity of ~0.90.
Polished aluminum has high thermal conductivity but low IR emissivity. Foam, which has high IR emissivity (see images), is known for it’s insulating properties. And by that we mean, foam is a poor conductor of heat. What aluminum lacks in emissivity, it more than makes up for in conductivity. And while foam lacks in conductivity, it appears to be an efficient emitter of IR.
It is useful to mention the meaning of “insulation“. A material that conducts thermal power poorly can be said to have insulating properties. Thermal power (dq/dt) is the flow rate of thermal energy (q) per second. Thermal power is the rate of flow in Joules per second. For reference, one Joule per second is one Watt. The valuable attribute of a thermal insulator is that it can resist the quantity of power (Watts) flowing through a unit area such as a square meter. The amount of thermal power moving across a unit area, like a surface, is called heat flux and is in units of W/m^2. It is common to express thermal resistance through a material by the R-value. An R-value is the ratio of the temperature drop (ΔT) across the insulating material to the heat flux through it, Q: R = ΔT/Q. So, as the heat flux gets smaller for a given ΔT, R grows larger in magnitude. In practical terms, a large R-value is desirable for insulation.
Looking at the radiant stretch of emissive insulated pipe rising from the water heater, we might initially guess that the IR image shows the whole thermal picture. But really, this guess is muddied by details. A warm pipe will be radiating energy as well as losing heat by conduction to whatever it is in contact with and by air convection.
IR radiation thermometry is useful when measuring a surface temperature is not practical. Accuracy, however, will depend on the emissivity of the surfaces of interest. The FLIR ONE is an economical imaging device for capturing IR images of large areas. The spot temperature feature is useful for recording the temperature of desired objects. Image files are easily downloaded from the phone and manipulated as jpeg files. Users will find many good applications for this affordable and easy to use IR imaging system.
Easy and cheap is great, but it is advisable for those wanting to do commercial work with IR thermography to take credible coursework and obtain some credentials. There are a few subtleties to thermography and it is best to be a little overqualified than not. Thermography courses can be found on the internet.