THE CALIBRATION OF A CONSTANT RECORDING ILLUMINOMETER WHEN THE SENSITIVE SAMPLING SURFACE IS HORIZONTAL

A recent issue of Plant Physiology contained a description of a constant recording potentiometer developed by Wallace (3), who subsequently suggested its use in conjunction with a Weston Photronic cell for recording sunlight for routine weather records (4). Arrangement and use of the apparatus was discussed in considerable detail, but no thorough attempt at calibration in terms of customary light units was reported. By arrangement with Dr. Wallace, the instrument was purchased, and has been in intermittent use in this laboratory for considerable periods of time. The object of this brief paper is to report an attempt at calibration of the apparatus in foot-candles by comparison with results obtained with a Macbeth illuminometer and a Weston illuminometer. This empirical comparison indicates the accuracy obtained under the specific conditions described. As described here, the instrument does not provide a complete record of light energy or its effects on plant functions. It does, however, offer possibilities of data for weather records with an accuracy greater than that from mere visual observation or from intermittent readings with light meters. Procedure