Calibration Heimann radiation thermometers W. Kohsiek De Bilt,

In the past years radiation thermometers have been employed by KNMI to measure the sky
temperature at various locations in the Netherlands and the surface temperature at the
Cabauw research station. The instruments used are of the type Heimann KT15-85D. These
instruments measure the radiation from on object at a wavelength around 10 μm and,
assuming and emissivity of 1, convert this to a temperature. The sky looking instruments
have a K6 lens with a narrow field of view of 3o and are sensitive to the wavelength band 9.5-
11.6 μm. The surface temperature is measured from two different heights, 4 m and 213 m.
The lower instrument has a M6 lens that has a field of view of 40o and is sensitive to the 8-
14 μm band, whereas the upper instrument is like the sky looking ones. The instruments are
mounted inside a box that is held at a temperature of approximately 35 C.
In an earlier study (unpublished internal note) questions were raised on the calibration of the
instruments and on their sensitivity to changes in body temperature. Although the
temperature of the box is controlled, changes of several degrees Celsius may occur due to
solar heating of the box. In order to get more insight in their behaviour, two instruments
were sent back to the local representative for inspection. The representative stated that the
problems were caused by moisture that had affected the electronics. The instruments were
repaired and again calibrated by the manufacturer. Their behaviour was again checked at the
KNMI by procedures described below. Also other instruments were successively recalled
from the field and checked. This note explains the calibration procedures that were used and
summarizes the characteristics of the checked instruments.