MICRO-EPSILON TIM 400 Manual de usuario descargar pdf (Pagina 72)

72

Block diagram of an infrared thermometer

Construction and operation of infra-

red thermometers

The illustration shows the general cons-

truction of an infrared thermometer. With

the help of input optics the emitted ob-

ject radiation is focused onto an infrared

detector. The detector generates a cor-

responding electrical signal which then

is amplified and may be used for further

processing. Digital signal processing

transforms the signal into an output

value proportional to the object tempe-

rature. The temperature result is either

shown on a display or may be used as

analog signal for further processing. In

order to compensate influences from the

surroundings a second detector catches

With our eyes we see the world in visi-

ble light. Whereas visible light fills only

a small part of the radiation spectrum,

the invisible light covers most of the re-

maining spectral range. The radiation of

invisible light carries much more additio-

nal information.

The infrared temperature

measurement System

Each body with a temperature above

the absolute zero (-273.15°C = 0 Kel-

vin) emits an electromagnetic radiation

from its surface, which is proportional

to its intrinsic temperature. A part of this

so-called intrinsic radiation is infrared ra-

diation, which can be used to measure

a body’s temperature. This radiation pe-

netrates the atmosphere. With the help

of a lens (input optics) the beams are

The advantages of non-contact

temperature measurement

- Temperature measurements of moving

or overheated objects and of objects

in hazardous surroundings

- Very fast response and exposure

times

- Measurement without interreaction, no

influence on the measuring object

- Non-destructive measurement

- Long lasting measurement, no me-

chanical wear

The grey body

Only few bodies meet the ideal of the

black body. Many bodies emit far less

radiation at the same temperature. The

emissivity ε defines the relation of the

radiation value in real and of the black

body. It is between zero and one. The

infrared sensor receives the emitted ra-

diation from the object surface, but also

reflected radiation from the surroundings

and perhaps penetrated infrared radiati-

on from the measuring object:

ε + ϕ + τ = 1

ε emissivity

ϕ reflection

τ transmissivity

Most bodies do not show transmissivity

in infrared, therefore the following ap-

plies:

ε + ϕ = 1

This fact is very helpful as it is much

easier to measure the reflection than to

measure the emissivity.

the temperature of the measuring device

and of his optical channel, respectively.

Consequently, the temperature of the

measuring object is mainly generated in

three steps:

1. Transformation of the received infra-

red radiation into an electrical signal

2. Compensation of background radiati-

on from thermometer and object

3. Linearization and output of tempera-

ture information.

focused on a detector element, which

generates an electrical signal proporti-

onal to the radiation. The signal is am-

plified and, using successive digital si-

gnal processing, is transformed into an

output signal proportional to the object

temperature. The measuring value may

be shown in a display or released as

analog output signal, which supports an

easy connection to control systems of

the process management.

Physical Basics

Optics Sensor Electronics Display

197.5°C

387.5°F

Target

IR

detector

AMP ADC

4...20mA

processor

Digital

Interface

DAC

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