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3. How to Select a Focal Distance
When the field-of-view (FOV) is filled by a uniformly-heated target, all Williamson pyrometers are distance-independent (beyond 6 inches for traditional style models) because they are fixed-focus. Pyrometers with an adjustable focus, as offered by others, are accurate only within the focal plane. Adjustable focus in some ways is preferred, but they are also easier to misuse because they are not appropriate for a target that moves significantly and are inaccurate when out of focus. Williamson prefers the fixed-focus approach because it is accurate at all distances.

## What is FOV?

The field-of-view (FOV) of a pyrometer determines its area of measurement or spot size in relation to its target. This can be further described as the ratio between the pyrometer’s optical resolution and its focal distance. For example a pyrometer with a focal distance of 40 inches and an optical resolution of 100:1 or D/100 would render a spot size with a 0.4 inch diameter from a 100 inch distance or FOV = 0.4in@40in.

The temperature value measured is unchanged as the distance between the pyrometer and the measured target changes, regardless of the focal distance, as long as the field-of-view is filled by a uniformly-heated target.

## Alignment Considerations

It is best to focus the pyrometer so that the field-of-view is most likely to avoid any possible optical obstruction or misalignment. In some cases, misalignment to a small or wandering target is a significant concern; it is best to focus at the distance to the measured target because this produces the smallest spot size at this distance and permits the most significant deviation from alignment to the measured target.

So, for all applications where the field-of-view is full, the focal distance influences only the shape of the field-of-view (the diameter-vs-distance relationship) and nothing more. However, when there is a partial optical obstruction or when the measured target is smaller than the diameter of the field-of-view, then the focal distance becomes more important.

## Considerations for Ratio Pyrometers

For most applications, the measured target fills the pyrometer’s field of view, but with some applications involving a small or moving target, it is possible to have a partially filled viewing area. For these applications, it is possible to use a ratio pyrometer (Two-Color or Dual-Wavelength) because Williamson TC and DW pyrometers do not require a full field-of-view, there are some additional special considerations pertaining to these models to address:

### Edge Effect

Any optical obstruction or misalignment influencing the outer fifth of the field of view may influence the pyrometer reading; this influence is greatest when the obstruction or misalignment is significantly far from the lens or the focal distance. For this reason, the pyrometer should be focused at or near the distance where this optical obstruction or misalignment is most likely to occur.

### Non-Uniformly-Heated Surfaces

For the pyrometer to best differentiate between hot spots and cold spots (steel with scale, for example), they must appear in focus. When the surface is not uniformly heated, then it is best for the pyrometer to be in focus at the distance of use.

### Targets Significantly Smaller than the Field-of-View

For the pyrometer to clearly view a small target (such as a wire or rod) that is filling only a small percent of the field-of-view, it must appear in focus. When the target is small, then it is best for the pyrometer to be in focus at the distance of use.

## Depth of Field

Williamson pyrometers are in focus at the focal distance, and they are also in focus for some distance closer to and farther from the focal distance. This is known as the Depth of Field, or the Effective Focus. The greater the focal distance the greater the depth of field; a pyrometer focused only a few inches from its target has a very shallow depth of field whereas a pyrometer focused further away has a deeper depth of field. Please see the charts below.

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