The Linearpyrometer LP5 is distinguished by a highly linear relation between a photocurrent as the detector output signal and the measurement radiance of a target as input signal. This is achieved by means of specially selected InGaAs or Silicon photodiodes as radiation detector in connection with a specially developed high resolution electronics. This special feature of the Linearpyrometer LP5 also enables to realise the International Temperature Scale (ITS 90) when three or better more freezing point black bodies are available as reference temperatures. A specially developed evaluation procedure allows calculation of an optimum approximated calibration curve even without using of a narrow band wavelength filter.A special feature of the LP5 is its compactness combined with high signal stability even during measurement of very high temperatures because of temperature stabilisation of all important components within the instrument. The basing principle was developed by Physikalisch-Technische Bundesanstalt, PTB and the LP2 was the first realisation of this measurement principle in form of a compact, handy and flexible instrument useful for many applications in the field of temperature measurement 1). In the improved version LP3 an advanced electronics for the photocurrent signal processing is implemented allowing measurements with lower time constants, measure¬ment of higher temperatures without neutral density filter, internal calculation of the radiance temperature, and displaying of the values at the measuring head. Extensive tests2) lead to excellent results important specifications like noise equivalent temperature difference long term stability, size-of-source effect, and linearity of the measurement signal.

Apochromate, f = 143 mm, free aperture = 38 mm. Target field stop with 0.2 mm and aperture stop with 9 mm diameter. The corresponding target diameter is 0.8 mm at 600 mm or 1 mm at 1000 mm distance. Fixed measurement distance ranges from 650 mm to 2500 mm. Size-of-Source effect: If the diameter of the radiation source is reduced from the 10-fold value of the measurement field diameter to the 1.4-fold value, the measured signal is reduced from 100% to not less than 99.9% . Polarisation: Very low sensitivity to the orientation of polarised light. The signal variation with rotation of the orientation of an 100% linearly polarised light source is less than 2% typically.

Standard equipment: 1 interference filter: 650 nm, 10 nm HBW or 1300 nm, 15 nm HBW Our standard filters for high temperature application are: 650 nm, 800 nm, 900 nm. Filters for other wavelengths have to be manufactured individually on demand. The interference filters have a broadband blocking of better than 10-6 in order to avoid the influence of sidebands on the effective wavelength. At wavelengths below 700 nm the blocking is better than 10-8 at wavelengths from 90 nm above the central wavelength of the filter up to the sensitivity limit of the Si-photodiode at about 1150 nm.