miCos GmbH - CA-1120 Radio- and Photometry

Measurements and Handling

1.) Calibration of the thermopile detector

The thermopile detector, measuring the radiant flux, has to be calibrated with the incandescent lamp of known luminous intensity. This calibration builds the connection from radiometric to photometric units. The quantity given out of the multimeter of the thermopile is a voltage. To convert this voltage to radiant flux in Watt the responsivity of the thermopile has to be taken into account. At a given distance of the detector from the lamp and given lamp paramters the corresponding luminous intensity can be read from the calibration table. These values are shown in the graph below, together with the radiant flux of the thermopile. learn more ...

2) The lock-in amplifier

For measurements at low radiant flux and for backlight filtering a lock-in amplifier is provided. A proper operation of the system for suitable signal detection is trained. A signal chopped with a frequency low enough for a good signal resolution is shown in the lower curve. The upper curve shows a chopped signal with a frequency beyond the reasonable range.

3) Measurement of the 1/r² relatio

Quantities like the radiant flux scale inversely with the square of the distance to the radiation source. Series of measurements as a function of the thermopile position, of the lamp current and of the iris aperture size can be performed. As an example the series of measurement of the radiant flux at a given lamp current and iris diameter is shown here.

4) Theoretical 1/r² curve

However, since the incandescent lamp is not a point source the 1/r2 relation is valid only in a distance which is large compared to the dimension of the lamp’s filament. When calculating the corresponding 1/r2 curve based on these data points one gets the solid curve of the following figure

5) Correction for finite filament dimension

A discrepancy of the measured data points from the theoretical curve in the previous graph is obvious. The reason for this discrepancy is the finite dimension of the filament. For correction, instead of the real radius r from the lamp to the detector a corrected radius r’ from a virtual point source has to be derived. This leads to the corrected data points (squares) which are nicely coincident with the theoretical curve.