6. OPERATING SUGGESTIONS

Focusing

The optical system of the Compix® PC2000 camera operates in the same way as a photographic camera. One implication of this is that the system's large aperture results in a short depth of field. Near the middle of its focal range (12"-15"), a depth of field around 1/2" would be typical. At two inches, the depth of field may be as less than 1/4"; at 60 inches, about 1 1/4". In any case, focusing is important to thermal image quality as well as temperature readout accuracy.

Frequently, edges in a thermal image may not be as defined as those in a video image. As a result, when first viewing an image it is hard to determine whether it is well focused or not. This will become easier with practice.

The first step in focusing is to measure or estimate the distance from the camera to the subject. This is the distance from the forward edge of the aperture bezel to the subject. If the subject isn't flat, use the average distance, or use the distance to the area of greatest importance. Basically, focusing is accomplished through trial and error. With the focus set to match the camera to object distance take a Single Scan. After the scan is completed, click AutoRange. This will set the Min and Max temperatures so that the range dispalyed just encompasses the temperatures in the scanned area. You should now see an image from which you can fine tune the Focus if necessary. In the Camera Control window set the scan for Continual and start scanning. While the system is scanning make a series of very small focusing adjustments and observe the changes. Look for improvements in the definition of the outlines of small passive components, leads, IC legs, board mounting hardware and mounting holes. The straight horizontal edges of larger components are also good guides. Entering this focus distance in the Correction window will indicate the camera's field of view(FOV) as well as provide a correction factor to compensate for variations introduced when the camera is moved away from the10inch focal length used in calibrating the instrument.

Many users find this process easier with the Grayscale colormap, since black and white images tend to appear more natural. A technique which gives excellent results involves shining an external source of infrared radiation on the unpowered target. To the PC2000 system this is the functional equivalent of an infrared flashbulb. The lamp illuminates the target and the camera displays the reflected infrared energy. The resulting image will look much like a visible light picture. Edges will be sharply defined, components will stand out in sharp relief and fine focusing will be easier.

A 40 or 60 watt incandescent desk lamp is a good source of infrared for this purpose. (Fluorescent bulbs won't work; they produce relatively little IR energy). Put the lamp at about two feet above, and at an angle of about 45° to an unpowered target. Don't let the bulb shine directly into the aperture of the camera. Use Grayscale colormap and take an image. The result should look like a B&W photo image of the target. You may have to experiment with the position of the lamp to get the best result.

When finished focusing, turn off the lamp as high levels of reflected infrared will distort the PC2000's temperature readings.

Framing

Framing encompasses two issues: first, the process of aiming the camera so the target is centered in the image; second, being able to accurately relate specific points in the thermal image to the corresponding locations on the target. The same factors which make it more difficult to focus a thermal image can also make it harder to frame properly. And some of the same techniques will be helpful. Many users find their first thermal images are ambiguous and confusing. But image interpretation is a quickly learned if the user has a working knowledge of the board or object being scanned.

The following table can be used to set the camera-to-subject distance to provide the desired field of view. Knowing what to expect for field of view also helps in interpreting the resulting image.

Remember, it is important to start with a well focused image.

Begin by looking for obvious reference points. Typical hot spots are: power resistors, power transistors, power IC's. Cold or low emissivity components would normally include mounting brackets, metal capacitors and metal crystal cans. Surface mount devices, hybrids, etc. are often framed by the contrasting colors of surrounding heat sinks or substrates. The outline of packaged IC's may be identified by the rows of legs or leads on the sides.

Edges of circuit boards are usually easy to find. In an energized board, conduction through the substrate and ground plane normally creates some visible differential heating to the edge of the board. Providing a good background can make the edges easier to see. A dark, matte finish material such as cardboard or black paper behind the board can improve contrast, particularly compared to the shiny formica top of many workbenches.

Simple markers can also be used to locate spots. Any small non-conductive (for safety reasons) object at room temperature will do. Emissivity Dots, plastic tools, non-metallic rulers, etc. are good examples. Put the marker on the target as a pointer or an edge indicator, then take an image. The marker should show up as a distinct dark area against the warmer background of the board and its components. Objects you take out of your pocket will already be warm and may not show up in the image.

Some users have reported good success with active (hot) markers. A loop of insulated wire with a low current applied to it can be bent to form a pointer. The wire will appear as a distinct hot pattern in the thermal image.

The focusing technique described above of using an incandescent lamp or other infrared source is also useful for framing. Make a reflected IR image using that technique. Then, with the lamp off, make a normal thermal image. Watch the display as the thermal image overlays the reflected IR image and you will see which points correspond. Also, it may be helpful to save a TIFF file of the reflected IR image as a reference.

Temperature Measurement

The Compix PC2000 is a sensitive instrument which has the capability to detect small temperature differences well within the range typically needed by the electronics engineer. However, non-contact temperature measurements are affected by a number of variables which can change the accuracy with which the PC2000 system calculates temperatures. The following techniques will help improve the accuracy of temperature measurements made with the PC2000 system.

1. Be consistent. Often all that's needed is good comparative or relative measurements. The engineer wants to know how one temperature compares with another -- perhaps on the same circuit board, maybe on two different circuit boards or perhaps even the same point on aboard at two points in time. Consistency is the key.

- Try to maintain the same environmental conditions, e.g., ambient temperatures, room lighting, ventilation and the presence of other heat sources in the room.

- Try to maintain the same physical set-up for the system. Put the board in the same orientation -- flat or upright, same distance from the camera and the same relative position on the display.

- Try to maintain the same electrical set-up. Keep the power inputs constant and if the circuit is software driven, use the same program to exercise it. - Timing can also be important. A circuit board may take 10-15 minutes to reach thermal stability. It is not important -- in fact, often not desirable -- to wait for the board to stabilize, but comparative readings between boards will be more accurate if both circuits are at approximately the same point in their warm-up cycles. - Use the same scan rate in both cases.

2. Minimize external sources of infrared energy. The Compix PC2000 measures temperature using emitted infrared energy, i.e. the infrared energy generated by the target. Objects also reflect infrared energy from its surroundings. The PC2000 system (or any other infrared system) can't tell the difference between reflected and emitted energy. Therefore, reflected energy is a potential source of error.

Some amount of reflected infrared is unavoidable and the PC2000 system will automatically compensate for typical levels of external infrared which are uniformly distributed over the target. Nevertheless, it will be helpful to minimize any significant sources of external infrared, particularly those which may not be uniformly distributed.

Avoid nearby desk lampsdirected towards the target, except, of course, when you are intentionally using one for focusing or framing as described in earlier in this section. Other sources to consider would be direct sunlight coming through a window and shining on the target, or heat from nearby electronic devices. A general rule of thumb is that if you can feel from a source near the target, then it will affect the accuracy of the image.

3. Minimize the reflectivity of the target. Another aspect of the problem described above is that some objects naturally reflect more infrared energy than others. Unfortunately, due to the laws of physics, objects which reflect infrared well emit infrared poorly(emissivity<1.00). As a result, these objects produce a high ratio of reflected (undesirable) to emitted (desirable) infrared and their temperature readings will be less accurate.

One solution is to reduce the reflectivity of these objects. Fortunately, surfaces which are reflective to infrared are usually reflective to visible light. So look for bright, shiny or metallic surfaces; they will be the problem.

There are several cheap, easy ways to reduce reflectivity with a minimal effect on the component's thermal performance. A strip of tape on the top surface is one. "Cloudy" cellophane tape or masking tape -- anything with a matte surface -- is a good choice. Or a quick, light buffing with fine emery paper -- just enough to break the surface sheen -- will also help.

Most conformal coatings and solder mask provide a good surface for thermal images.

4. Adjust the Emissivity setting. The default Emissivity setting of the PC2000 system is 1.00. This is appropriate for the highly emissive (low reflectivity) materials which are most common on circuit boards. For components with lower emissivity, temperature accuracy will be improved if the PC2000 system's Emissivity parameter is reset to match the component's. Emissivity tables are available for many of the materials commonly used on printed circuit boards.

The Emissivity factor can be reset through the Menu. Remember, Emissivity is one of the parameters which affect how the PC2000 system scans and stores image data, so a new image should be scanned after the Emissivity parameter is changed.