Heat-seeking smartphones see in the dark

I glow in the dark, but then, everybody does. We can't help it; as long as we live, our bodies throw off a constant infrared glow that's invisible to human eyes.

But not to our smartphones. New thermal imaging cameras turn iPhones or Androids into heat-seeking sensors that can spot warm bodies or warm objects, in broad daylight or total darkness — anything from a missing puppy to an intruder lurking in a parking lot.

Heat-seeking sensors can spot warm bodies or warm objects.SeekwareBlendModeNone

I've tried out two of them. FLIR One, from Oregon-based FLIR Systems, is a $350 gadget for the iPhone 5 or 5s. The other, priced at $199, is made by a California startup called Seek Thermal and is available for iPhones and Android devices.

If you've ever seen a "Predator" movie, you'll recognize the images these cameras generate. Anything warm glows brightly on the phone's screen, while cooler objects fade into the background.

You can also use the cameras as long-range thermometers: They estimate the temperature of whatever they're aimed at.

And they do record still and video images.

Both of these cameras work well, but neither comes cheap. And who needs them? More of us than you might think. Hunters can use them to find game, plumbers to find leaks, electricians to identify wires that are overheating, homeowners to spot energy leaks around windows and doors.

FLIR, the granddaddy of thermal imaging companies, started building products for the military and police agencies in the 1970s. Early models cost hundreds of thousands of dollars and had to be chilled with supercold liquid nitrogen. But in the 1990s, FLIR found a way to build thermal cameras that did not need artificial cooling. Today, they're small enough to attach to a smartphone.

The FLIR One comes in the form of a protective case that snaps around the iPhone. It fits only the iPhone 5 and 5s models, and FLIR hasn't decided whether to produce a version for the new iPhone 6. FLIR One turns the sleek iPhone into a wallet-size hulk, largely because of the rechargeable battery; the battery doesn't provide backup power for the phone, so you must recharge each device separately.

The camera from Seek Thermal comes in a compact dongle that plugs into the Lightning port of late-model iPhones, or the micro-USB port on Android phones. On my HTC One Android phone, the USB jack faces in the wrong direction, so I could only take thermal selfies. So I tested it on a Motorola Moto X phone that aimed the camera toward the world.

The Seek lacks a battery of its own; it sponges off the one in the phone.

It makes a constant clicking noise as it runs. That's the sound of the thermal imager automatically recalibrating itself. It's a smart approach. On the FLIR One, recalibration is done manually, at the prompting of an on-screen icon that you can easily fail to notice.

The FLIR has two cameras, thermal and standard optical, and shoots in both modes at the same time. I found the resulting photos easier to view than those from the Seek. In addition, sliding a finger across a FLIR photo lets you see the same image in regular and infrared modes. With the Seek, you can activate the phone's own camera and get a split-screen image from both cameras.

Each thermal camera lets you see the world in shades of gray, with warm spots showing up as especially bright. Or you can select from a variety of color filters. Sometimes these filters make it easier to see a target, and they give the images an amusing sci-fi-movie look.

The FLIR One and the Seek also act as remote thermometers. They'll display crosshairs in the middle of the screen. Point the phone at a target — a baby's bottle, for instance — and you'll instantly see if the formula is too hot or too cold.

Or you could just splash the milk on your wrist. That's the big problem with these gadgets. We can do without them, and at today's prices most of us will. But the FLIR One and the Seek are first-generation gear, aimed at early adopters. When these devices fall below $100, or better yet when they're built into our phones, lots of us will want to see in the dark.