It was another of those things that had unexpected uses once it was put into production. Mostly the emergency services used them, for finding bodies hidden in collapsed buildings or locating fleeing suspects on dark nights. Nobody thought the capabilities of infra-red devices would be useful in the sphere of relationships until I did.
I started off with a small unit in a tent, much like a Punch and Judy show on the end of the peer or a palm reader at a county fair. I usually pitched it myself and nobody ever questioned my right to be wherever I was. At first it was a bit like waiting for a telegram in the war or a police knock on the door but eventually I got used to being accepted.
It worked on men and on women, although it was usually the women who wanted to know. They would often come in as a couple, she giggling and pulling him behind her by the hand, he shrugging and declaring it was all a bit of nonsense. I saw it so many times. Then I would sit them in front of my machine and take an image. Like a Polaroid it would take a few minutes to develop and it’d usually blow on it and flap it held by the corner. That was all for show and made no difference, but at least I felt the theatrical touches justified some of my fee.
Then I would hand them a picture showing them both together, with the heat of their hearts displayed in vivid colour. I never said it was a relative measure of whether they felt the same about each other, who felt the most for whom, but they began to see it as such. When a bright red sat with a green or sometimes even a blue, well you can imagine.
Too many happy couples came in and awkward ones left, especially when I settled into permanent accommodation on the High Street. It was a bit out of the way because the lease was cheap, but they still made their way to see me. And I developed a mobile unit too, so I could do home visits. That was so popular I had to start taking on staff and I eventually sold franchises. It became big business for private investigators, apparently.
Sometimes just one person would come to see me without a partner. They would ask for a diagnosis, as it became known. Then they wanted to know was the reason they were alone because their heart was too cold. And was it ever possible to warm a blue heart to red.