A pyrometer measures the temperature of an object over the size of its measuring spot. This is a beneficial characteristic, as the temperature can be measured in a precise location on the object. For some applications, it is important to get an average temperature over a larger area; this is where a large spot can be useful.
For other applications, it may be essential to measure the temperature at an exact and specific location. This is often the case on small or intricate objects, where a smaller spot size is needed.
Explore Our Range of Impac Pyrometers at LIMAB UK
Definition of Spot Size
Spot size is the size of the measuring spot on the surface when using a pyrometer. A pyrometer measures the average temperature of the surface over the area of the spot.
The datasheet for the pyrometer will specify the spot diameter at a certain measuring distance.
For a single-colour pyrometer, the full area of the spot size must be covered by the object being measured. If the spot is larger than the object being measured, the temperature will not be measured correctly.
However, a two-colour pyrometer is not affected in the same way, and it will only need approximately 10% of the object covering the spot to measure the temperature correctly.
Factors That Impact Pyrometer Spot Size
The field of view (FOV) of the optics fitted to the pyrometer determines the spot size for any given distance from the pyrometer. The datasheet for the pyrometer will show the spot diameter for a specified measuring distance. The spot size at positions other than the focal distance can be calculated using the formula below.
Where:
a = measuring distance
M = spot size
D = aperture
The Different Types of Pyrometer Spot Sizes
Fixed Optic Pyrometers
These types of pyrometers have fixed, non-adjustable optics and the minimum spot diameter is at a fixed distance from the pyrometer. It is possible to use the pyrometer outside this focus distance, but the spot size will be bigger than it is at the focus distance.
Variable Focus Pyrometers
Some types of pyrometers have variable focus optics, which means the focus point can be adjusted to suit the application. This is helpful, as the distance from the pyrometer to where the minimum spot size occurs can be adjusted without repositioning the pyrometer.
Line Shaped Optic Pyrometers
Some models, such as the Impac® ISR 6 Advanced dual-colour pyrometer, can be equipped with optional line-shaped optics. These optics transform the normal round measuring spot into a line or rectangle. This is very helpful in maintaining alignment when measuring the temperature of wires or hot pouring streams that may move or vibrate during measurement. As this model is a dual-colour pyrometer, only part of the line needs to be on the object for correct temperature measurement.
Aligning the Spot
There are a number of methods to align the pyrometer measuring spot with the object being measured.
No Alignment
Basic pyrometers do not have built-in tools for precise alignment, so the alignment of the pyrometer is done visually. Monitoring the temperature reading during alignment can help in this process.
Through-Lens
Some pyrometers have through-lens sighting, meaning that the exact position of the measurement spot is visible through an ocular lens or eyepiece. This type of alignment method can only be used where it is possible to get physical access to the ocular fitted to the pyrometer.
Laser or LED Visible Guide Spot
Some pyrometers are fitted with a visible laser or LED, which marks the location of the spot centre. However, this is not exactly the same size as the measurement spot.
Intensity-Based
Pyrometer alignment can also be achieved by looking at the intensity signal. This does not show the spot, but it helps find the hottest area.
Image
Some hybrid pyrometers have a built-in video or thermal imaging camera. With this type of pyrometer, the image and spot location can be viewed on a monitor.
Learn More about Pyrometers
Calculating the spot size for your type of pyrometer is crucial for ensuring accurate temperature measurement. Different pyrometers have different spot sizes, which enables you to choose the right pyrometer for your use case, while the spot size on some pyrometers needs to be covered fully or only partially to deliver an accurate reading.
You can learn more about pyrometers by using the links below:
- Learn more about what a pyrometer is used for
- Learn more about how a pyrometer works
- Learn more about the different types of pyrometers
- Learn more about the advantages of a pyrometer compared to contact measurement
- Learn more about how to choose the right pyrometer
Visit our temperature measurement and thermal processing page to learn more about LIMAB UK’s temperature measurement products, including pyrometers. To speak to a product specialist to find out how a pyrometer could benefit your company, call +44 (0)1823 668633.
