WE CAN FEEL HEAT AND see its effect on the atmosphere-a mirage, for example. We can also detect hot objects in another way- from the infrared radiation they emit. Most infrared sensors are sophisticated, solid state devices [see "Infrared Video Cameras," by Jerry Silverman, Jonathan M. Mooney and Freeman D. Shepherd, page 78]. Recently I developed a way to build a simple infrared camera using a concave spherical mirror and a liquid crystal. Although my camera is not nearly so sensitive as state-of-the-art devices, it can view the infrared radiation from a hot object that is several feet away.

To test the design of my camera, I started with a heat lamp, a shaving mirror and a liquid crystal from a "mood sensor." I used the mirror to focus the radiation from the lamp onto the liquid crystal. Within a second or so, an image appeared on the liquid crystal. It bloomed rapidly into a large circle. Unfortunately, I could not tell whether the liquid crystal was responding to infrared radiation or to the heat generated when the crystal absorbed the visible light from the lamp.

Nevertheless, the experiment encouraged me to build a more sophisticated camera. I purchased a high-quality mirror and a set of liquid crystals that had good temperature sensitivity. (These items can be ordered from a science supply catalogue.) I also bought hardware to build a rigid support to hold the mirror and the liquid crystals. The materials cost a total of about $250. For comparison, commercial infrared cameras sell for thousands of dollars.

The most expensive part of my apparatus is the mirror (about $150). Its quality and size ultimately determined the resolution and sensitivity of the camera. I settled on a gold-coated mirror that was six inches in diameter and had a focal length of 12 inches. The gold coating reflects much of the infrared radiation while absorbing some of the visible light.

I chose the size and focal length of the mirror by applying basic principles of optics. I first calculated where the image would be focused in relation to the mirror. The image distance is equal to the following expression:

Hence, a mirror with a focal length of 12 inches would focus an object 36 inches away to a point 18 inches in front of the mirror.

Next I figured out the magnification of the mirror, which is equal to the image distance divided by the object distance. When the object distance is 36 inches and the image distance is 18 inches, the magnification is 0.5-that is, the image will be 50 percent smaller than the object. I should also point out that the image will appear upside down.

The six liquid-crystal sheets, which together cost about $30, were each six inches wide, 12 inches long and 0.008 inch thick. Each sheet was sensitive to a different range of temperatures (as low as 20 to 25 degrees Celsius and as high as 40 to 45 degrees C). Eventually I realized that the sheets sensitive to low temperatures work best in a cool room, whereas those sensitive to higher temperatures are more appropriate for warm conditions.

All the liquid-crystal sheets are made of Mylar coated with two layers of ink. The inner layer is liquid-crystal ink, and the outer layer is black ink. The liquid-crystal ink is a mixture of three different compounds that contain cholesterol [see "Liquid Crystals," by James L. Fergason; SCIENTIFIC AMERICAN, August 1964]. The liquid-crystal ink is encapsulated so that the sheet does not leak ink when it is cut.

To make the support structure, known as an optical bench, I first joined two boards at an angle of 90 degrees, thereby creating a wide track [see Figure 1]. At one end of the track, I then attached a small board to support the spherical mirror. Next I cut a triangular block that could slide along the track. In the top of the block,

I drilled a hole to accommodate a threaded insert. The insert mated with a threaded brass post, which I had attached to a half-inch length of copper pipe one inch in diameter. Finally, I cut a piece of the liquid-crystal sheet to fit on the end of the pipe.

By sliding the block within the track, I could move the liquid crystal in relation to the mirror and therefore control the focus of the camera. The height o the liquid crystal could also be adjusted slightly by screwing the pipe assembly more or less brightly into the threaded insert. (The coupler is used as a lock-nut when the desired height is achieved.)

I originally designed the apparatus so that infrared radiation would reflect off the mirror and directly illuminate the liquid-crystal ink. I discovered that I could obtain much better results by directing the radiation onto the black surface of the liquid crystal. The black ink presumably absorbs the infrared energy and heats up the liquid-crystal ink. As the ink is heated, it changes color.

To test the apparatus, I searched my home for sources of infrared radiation. I tried a hot tea-kettle, a 250-watt heat lamp, the burners on an electric stove and a bright flashlight. (I should warn all who attempt these experiments that they can burn themselves severely if they mishandle various sources of infrared radiation.)

The liquid crystal did not register the teakettle with boiling water in it. When exposed to the heat lamp, the camera displayed several hot spots that bloomed rapidly. A red-hot burner swamped the crystal instantly. The liquid crystal did detect the radiation from the flashlight, and as I swung the light around, I could draw patterns on the crystal.

Although I was excited by these results, I worried that the liquid crystal might be detecting visible light instead of infrared radiation. To prove that this was not the case, I decided to use a piece of silicon, which absorbs visible light but only attenuates the intensity of infrared radiation. I placed a silicon chip that was four inches in diameter and 0.02 inch thick in front of the aperture of the camera. With this setup, I tested several sources and found that the images still appeared and, as expected, were somewhat dimmer.

Eventually I discovered that a steam iron was the ideal source of infrared radiation for my experiments. I could set the temperature of the iron fairly accurately, within a range of 50 to 250 degrees C. (The temperature of the iron could be measured adequately with a deep-fry kitchen thermometer.) The iron was a good source for another reason: its bottom was made of Teflon, which when heated readily emits infrared radiation.

I had a few problems with the steam iron. When tilted for an extended period, it turned off automatically, and other features did not work at all. After convincing my wife we should buy a new iron, I dismantled the old one and modified it as a test target. I spent some time experimenting with the iron and my infrared camera. The liquid crystal displayed a clear image, 0.75 inch high, when the iron was 12 feet away.

I then set out to determine the resolving power of my camera. In front of the steam iron, I positioned a plastic spatula that had four slots, each four millimeters wide. I reasoned that the spatula would block some of the infrared radiation and therefore form an image consisting of several bars. It took quite some effort to produce this image.

I discovered I needed to focus the image very precisely. To do this, I illuminated the spatula from behind with a flashlight. I could then observe the visible light from the flashlight on the black side of the liquid crystal. I focused the image so that I could clearly see the shadow of the spatula. (Because the infrared camera is a reflecting system, the visual and infrared radiation come to the same geometric focus.)

After I replaced the flashlight with the steam iron, I put a piece of cardboard in front of the infrared camera to block the radiation. I then heated the liquid crystal with the flashlight and allowed it to cool. The liquid crystal is most sensitive at the instant the crystal turns black. At that point, therefore, I removed the cardboard, and the image of the spatula's shadow could be seen for a second or two.

To record the images produced on the liquid crystal, I used a conventional camera. But I had some difficulty bringing the camera lens close to the liquid crystal. For this reason, I mounted a mirror on the support structure so that the image on the liquid crystal could be seen from the side of the apparatus. This arrangement allowed me to position the camera a few inches away from the image.

I have no doubt that other amateur scientists will find ways to improve the sensitivity and range of the infrared camera. I hope they will enjoy, as I have, observing the world in a new light.

For more information, please write to: The Amateur Scientist, Infrared Camera, Scientific American, 415 Madison Avenue, New York, NY 10017-1111.

Bibliography

OPTICS AND OPTICAL INSTRUMENTS: AN INTRODUCTION WITH SPECIAL REFERENCE TO PRACTICAL APPLICATIONS. B. K. Johnson. Dover Publications, 1960.

BASIC OPTICS AND OPTICAL INSTRUMENTS. U.S. Navy Bureau of Naval Personnel. Dover Publications, 1969.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.