Pre-Flight Measurements Necessary for a Correct Data Interpretation

Joachim Köppen DF3GJ Kiel/Strasbourg/Illkirch Summer 2004

• pairing of LDRs of similar photometric properties: Since the LDRs of each sensor array should have nearly the same properties, i.e. the slope gamma of the sensivity curve, one should select matching LDRs. Only then the readings e.g. blue/red measure the colour independent of the brightness.
  procedure clamp a test sensor under a light source. Measure for each LDR the resistance under this light as well as with a grey filter held in front of the sensor. The grey filter should be fairly dark, say the resistance should increase by a factor of ten. Determine for each LDR the ratio of the two resistances, and keep a record of this. Select triplets (and one pair) of LDRs of nearly the same resistance ratio. The interpreting person must have the recorded data as well as the grey filter.
• currents through each LDR: is needed to convert the voltage readings into resistances.
  procedure cover the entrance aperture of each sensor, hold a current meter parallel to the LDR, and measure the current flowing past the LDR. Record the value for each sensor with proper identification.
• overall photometric calibration : serves as an overall check of the sensor-and-electrconics system.
  procedure take readings of all the sensors, using an arbitrary light source, such as a sheet of white paper held in front of the sensors and lit by the sun or daylight. Hold a grey filter in front of the sensors' apertures, and take another set of readings. The interpreting person must have the recorded data as well as the grey filter.
• overall colour calibration : ensures that the interpreting person can link the data with real colours. Any local calibration with the colour enlarger could be helpful, but as this cannot provide colour calibration reproducible at other places:
  procedure if possible, use the sunlight on a cloudless day (the only natural standard light source globally available). Hold the prepared coloured cards - or any other homogeneously coloured cards - in front of each sensor array, so that all three (two) sensors are equally illuminated. Record these voltage readings for all arrays. If the sun cannot be used, use other source (daylight, tungsten bulb), and provide precise description of the source (cloud cover, weather conditons, helpful details about the lamp). The interpreting person must have the recorded data, the colour cards, as well as samples of the colour filter material.

• if the above procedures cannot be done before flight, one could attempt to do this after the flight. This requires that the complete sensor-electronics payload is available for the analysis.
  procedure return sensors+electronics to ISU Illkirch

• temperature of sensors: Since the LDRs' resistance may depend on temperature - so far I have not ben able to get any data on our devices - the calibration may differ from the lab and the actual flight.
  procedure To get an estimate about the importance, it would be good to compare the resistances obtained with a single LDR at room temperature and chilled for some time in a fridge ... [I shall be able to do this late next week (after 15 July) - J.K.]
  It is hoped that we can use one i-Button from Jim Burke's experiment to sample the temperature in the vicinity of the sensors. It is not expected that the temperature change during flight will be important, but the difference from the lab conditions might be. Given this temperature information as well as the availability of the sensors for the post-flight analysis, it would then be possible to perform the correct calibrations under the ambient temperature after flight.